Aircraft power system

Abstract

A power system 4 for an aircraft 1 comprises one or more gas turbine engines 10, 44 arranged to burn fuel to provide power to the aircraft 1; a plurality of fuel tanks 50, 52, 53, each arranged to contain fuel to be used to provide power to the aircraft 1, wherein at least two of the plurality of fuel tanks 50, 52, 53 contain different fuels, the different fuels having different proportions of a sustainable aviation fuel; and a fuel management device 214. The fuel management device 214 is arranged to store information about the fuel contained in each fuel tank 50, 52, 53; and to control the fuel supply so as to select a specific fuel accordingly to power at least the majority of ground operations.The fuel management device 214 can further identify which tank 52 contains the fuel with the highest proportion of sustainable aviation fuel; and this fuel can be used to power at least the majority of ground operations. Figure 14.

Description

Description Title of the invention: Aircraft fuel system The present disclosure relates to aircraft propulsion systems and methods of operating aircraft involving the management of fuels of various types, including the detection of fuel properties and actions taken to improve aircraft performance based on the acquired data, and to methods of modifying aircraft so as to enable the implementation of such methods. The aviation industry expects the trend to be toward the use of fuels other than the traditional kerosene-based jet fuels commonly used today. These fuels may have different fuel characteristics, such as lower aromatics and lower sulfur content, compared to petroleum-based hydrocarbon fuels. Thus, it is necessary to take into account fuel properties in light of the increased possibility of variation, and to adapt the control and management of aircraft propulsion systems and fuel supplies to these new fuels. According to a first aspect, a method of identifying a fuel contained in a fuel tank of an aircraft and arranged to supply a gas turbine engine of the aircraft is provided, the method being carried out by processing circuitry of the aircraft and comprising: obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling; determining one or more fuel characteristics of a fuel added to the fuel tank during refueling; and calculating one or more fuel characteristics of the resulting fuel in the fuel tank after refueling. This approach may be referred to as an active infinite summation approach, since steps are taken to create and continuously update a record of the fuel on board an aircraft. The method may be performed for each of the aircraft's multiple fuel tanks separately, or for all fuel on board the aircraft, regardless of which tank it is in. Obtaining the one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may include obtaining these characteristics from computer storage, directly sensing these characteristics, or determining these characteristics from other sensed parameters. The step of obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may include detecting one or more characteristics of the composition of the fuel already present in the fuel tank. The step of obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may comprise obtaining the result of a prior determination made using the method of identifying a fuel described above for this first aspect. The step of obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may include maintaining current fuel characteristics data by updating the fuel characteristics of the fuel present in the fuel tank after each refueling of the aircraft. The step of determining one or more fuel characteristics of the fuel added to the fuel tank during refueling may include reading a barcode associated with the fuel being supplied. The fuel characteristics may be or include parameters of a hydrocarbon distribution of the fuel. The fuel characteristics may be or include: 1. the percentage of sustainable aviation fuel in the fuel; li. the aromatic hydrocarbon content of the fuel; lii, the multi-aromatic hydrocarbon content of the fuel; iv. the percentage of nitrogen-containing species in the fuel; v. the presence or percentage of a trace species or trace element in the fuel (e.g., a substance in the form of a trace element inherently present in the fuel which may vary from fuel to fuel and therefore be used to identify a fuel, and / or a substance deliberately added to act as a tracer); vi. the hydrogen to carbon (H / C) ratio of the fuel; vii. the distribution of fuel hydrocarbons; viii. the level of non-volatile particulate matter (nvPM) emissions during combustion (e.g. during combustion for a given combustion chamber design, under given operating conditions); ix, the naphthalene content of the fuel; x. the sulfur content of the fuel; Xi, the cycloparaffin content of the fuel; xii. the oxygen content of the fuel; xili. the thermal stability of the fuel (e.g. thermal degradation temperature); xiv. the level of coking of the fuel: xv. an indication that the fuel is a fossil fuel, e.g. fossil kerosene; and xvi, one or more properties such as density, viscosity, calorific value and / or heat capacity. The method may further comprise chemically or physically detecting one or more parameters of the resulting fuel in the fuel tank after refueling, and verifying one or more of the calculated fuel characteristics based on the one or more detected parameters. The detected parameters may be fuel characteristics, or may be used to calculate or derive fuel characteristics - for example, the detected parameters may be a shaft speed and a mass flow rate of the fuel, from which a calorific value (a fuel characteristic) may be determined, or the detected parameters may be fuel density and / or the presence of a tracer, both of which are fuel characteristics. Obtaining the one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may include obtaining stored fuel characteristic data. The method may further include chemically or physically detecting one or more parameters of any fuel already present in the fuel tank prior to refueling, and verifying the input to the calculating step based on the one or more detected parameters. The method may further include chemically and / or physically determining one or more parameters of the fuel in the fuel tank, and using the determined values ​​to override stored fuel characteristics for the fuel in the fuel tank. The chemical and / or physical determination of one or more fuel parameters in the fuel tank can be carried out by extracting a sample of the fuel from the fuel tank for workshop testing. The chemical and / or physical determination of one or more parameters of the fuel in the fuel tank and the use of the determined values ​​to override the stored fuel characteristics for the fuel in the fuel tank may be performed in response to a triggering event, such as: 1, a threshold time elapsed since a previous chemical and / or physical determination of the one or more fuel parameters in the fuel tank; li. a threshold number of refueling events and / or flights reached since a previous determination of the one or more fuel parameters in the fuel tank: and / or iii. a deviation between one or more of the calculated characteristics and a detected parameter exceeding a threshold. The method may further include controlling the propulsion system based on the one or more calculated fuel characteristics of the resulting fuel in the fuel tank after refueling, for example as described below with respect to the fourth and fifth aspects. The method may further include proposing or initiating a change to the flight profile based on the one or more fuel characteristics of the resulting fuel in the fuel tank after refueling, for example as described below with respect to the sixth and seventh aspects. According to a second aspect, a method of controlling the propulsion system of an aircraft is provided, the propulsion system comprising a gas turbine engine and a fuel tank arranged to supply fuel to the gas turbine engine, the method comprising: obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling; determining one or more fuel characteristics of a fuel added to the fuel tank during refueling; and calculating one or more fuel characteristics of the resulting fuel in the fuel tank after refueling; and controlling the propulsion system based on the one or more calculated fuel characteristics of the resulting fuel in the fuel tank after refueling. Obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling may include: (i) the detection of one or more characteristics of the composition of the fuel already present in the fuel tank; or (ii) obtaining the result of a previous determination made using the method of the first aspect. According to a third aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a fuel tank arranged to contain fuel to power the engine gas turbine; and a fuel composition monitoring device arranged to: storing current fuel characteristics data, the fuel characteristics data comprising one or more fuel characteristics of the fuel present in the fuel tank; obtaining one or more fuel characteristics of a fuel added to the fuel tank during refueling; and calculate updated values ​​for the one or more fuel characteristics of the fuel in the fuel tank after refueling. The updated values ​​can then take the place of the stored values, for use in future iterations of the steps performed by the fuel composition tracking device. The fuel characteristic data may be fuel composition data, comprising one or more parameters of a hydrocarbon distribution of the fuel. According to another aspect, a non-transitory computer-readable medium is provided having stored thereon instructions which, when executed by a processor, cause the processor to perform the method of the first and / or second aspects. The processor may be or may be part of an Electronic Engine Governor of the aircraft. According to a fourth aspect, a method of operating an aircraft comprising a gas turbine engine and a fuel tank arranged to supply fuel to the gas turbine engine is provided, the method comprising: determining one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine; and proposing or initiating a change to an aircraft flight profile based on the fuel characteristic(s). Implementations of this aspect may therefore enable the achievement of environmental benefits (e.g., reduced or adapted condensation trail formation) and / or operational benefits (e.g., improved fuel consumption efficiency) based on knowledge of the fuel burned. In some examples, the method may include automatically initiating the change to the flight profile based on the determined characteristics. In some examples, the method may include informing a pilot of the suggested change to the flight profile based on the determined characteristics and providing the pilot with the opportunity to confirm or cancel the change. In some implementations, one and / or the other example may be implemented depending on the nature of the change. The one or more fuel characteristics of the fuel may include at least one of the fuel characteristics listed above. Changing the flight profile based on fuel characteristics may include at least one of the following: (i) a change in the expected altitude; and (ii) a change in the planned itinerary. Determining one or more fuel characteristics of the fuel may comprise carrying out the method of the first aspect, in particular by: obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling; determining one or more fuel characteristics of a fuel added to the fuel tank during refueling; and calculating one or more fuel characteristics of the resulting fuel in the fuel tank after refueling. The method may further include receiving forecast weather conditions for a planned route of the aircraft. The received forecast weather conditions may be used to influence planned route and / or altitude changes. The determination of the one or more fuel characteristics may be performed based on the detection of one or more fuel properties. The detection may be performed on-site. The determination of the one or more fuel characteristics may be performed based on received fuel composition data, e.g., data sent electronically to the aircraft by a third party or entered using a user interface on board the aircraft. The fuel composition data may be provided to the aircraft during refueling. The one or more fuel characteristics may be determined for a fuel in one or more fuel tanks of the aircraft. One or more of the fuel characteristics, e.g., a calorific value, may be derived from the performance of the gas turbine engine during at least one of engine warm-up, taxiing, takeoff, and aircraft climb. The planned flight profile during cruise may be modified / the flight profile may be updated based on the one or more derived fuel characteristics. According to a fifth aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a fuel tank arranged to contain fuel for powering the gas turbine engine; and a fuel composition determination module designed to: determining one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine; and a flight profile adjustment device arranged for: propose or initiate a change to an aircraft flight profile based on the fuel characteristic(s) of the fuel. The flight profile adjustment device may be arranged to initiate or propose at least one of the following based on fuel characteristics: (i) a change in the expected altitude; and (ii) a change in the planned itinerary. The propulsion system may be arranged to perform the method as described in relation to the fourth aspect. According to a sixth aspect, a method of operating an aircraft comprising a propulsion system is provided, the propulsion system comprising a gas turbine engine and a fuel tank arranged to supply fuel to the gas turbine engine, the method comprising: determining one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine; and controlling the propulsion system based on the one or more fuel characteristics. Implementations of this aspect may therefore enable the achievement of environmental benefits (e.g., reduced or adapted condensation trail formation) and / or operational benefits (e.g., improved fuel consumption efficiency) based on knowledge of the fuel burned. In some examples, the method may include controlling the propulsion system based on the determined characteristics, without requiring pilot input or approval. In some examples, the method may include informing a pilot of the suggested change in propulsion system control, based on the determined characteristics, and providing the pilot with the opportunity to confirm or cancel the change. In some implementations, one and / or the other example(s) may be implemented depending on the nature of the change. Thus, control may be implemented directly, or after ve- verification. The method may be performed iteratively in flight, for example, due to changes in the fuel supplied to the gas turbine engine and / or changes in conditions and stage of flight. The fuel characteristic(s) of the fuel may include one or more of the characteristics listed above. The method may further include receiving weather data relating to weather conditions around the aircraft or along a planned route of the aircraft. The received weather data may be used to influence control of the propulsion system. The method may further include detecting weather conditions around the aircraft in flight. The detected weather conditions may be used to influence control of the propulsion system. Control of the propulsion system based on fuel characteristics may include changing, in flight, one or more of the following: * An operating parameter of an aircraft thermal management system (e.g., a fuel-oil heat exchanger) may be changed, or the temperature of fuel supplied to an engine combustion chamber may be changed. * When more than one fuel is stored on board an aircraft, the selection of which fuel to use for which operation (e.g., for ground operations versus flight, for low temperature start-up, or for operations with different thrust demands) can be made on the basis of fuel characteristics such as % Sustainable Aviation Fuel (SAF), non-volatile particulate matter (nvPM) generation potential, viscosity, and calorific value. A fuel delivery system can therefore be appropriately controlled based on the fuel characteristics. “One or more control surfaces of the aircraft can be adjusted to change route and / or altitude based on fuel availability. * The percentage of spillage of a fuel pump (i.e. the proportion of pumped fuel recirculated instead of being passed to the combustion chamber) based on the % SAF of the fuel. The pump and / or one or more valves can therefore be controlled appropriately based on the fuel characteristics. “Changes to the variable inlet guide vane (VIGV) schedule can be made based on fuel characteristics. VIGVs can therefore be moved, or a VIGV move can be overridden, as appropriate based on fuel characteristics. These options can be referred to as control examples, as they are examples of ways in which the propulsion system can be controlled based on fuel characteristics. (The percentage of sustainable aviation fuel (7% SAF) in a fuel can be gravimetric or volumetric - it will be appreciated that there are often - usually small - differences in density between SAF and traditional jet fuels such as Jet A.) The determination of the one or more fuel characteristics may be performed based on the detection of one or more fuel properties. The detection may be performed on-site. The determination of the one or more fuel characteristics may be performed based on received fuel composition data. The fuel composition data may be provided to the aircraft during refueling. The one or more fuel characteristics may be determined for a fuel in one or more fuel tanks of the aircraft. The one or more fuel characteristics may be determined for each of a plurality of fuels stored on board the aircraft. The one or more fuel characteristics may be determined for a fuel immediately prior to entry into a combustion chamber of the gas turbine engine. Determining the one or more fuel characteristics of the fuel immediately prior to entry into a combustion chamber of the gas turbine engine may be performed multiple times during flight to account for changes in fuel composition. The one or more fuel characteristics may be derived from the performance of the gas turbine engine during at least one of engine warm-up, taxi, takeoff, and aircraft climb. The propulsion system may be controlled during cruise based on the one or more derived fuel characteristics. Determining the one or more fuel characteristics of the fuel may comprise carrying out the method of the first aspect, in particular: obtaining one or more fuel characteristics of any fuel already present in the fuel tank prior to refueling; determining one or more fuel characteristics of a fuel added to the fuel tank during refueling; and calculating one or more fuel characteristics of the resulting fuel in the fuel tank after refueling. According to a seventh aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a fuel tank arranged to contain fuel for powering the gas turbine engine; a fuel composition determination module arranged to determine one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine; and a propulsion system control device arranged to control the propulsion system based on the one or more fuel characteristics of the fuel. The fuel composition determination module may comprise a receiver arranged to receive data relating to a fuel composition. The fuel composition determination module may be arranged to determine one or more fuel characteristics based on the received data. The fuel composition determination module may comprise one or more sensors arranged to provide data relating to one or more fuel characteristics. The fuel composition determination module may be arranged to determine one or more fuel characteristics based on the sensor data. The propulsion system may include a plurality of fuel tanks arranged to contain different fuels for powering the gas turbine engine. The fuel composition determination module may be arranged to determine at least one fuel characteristic of each different fuel. The propulsion system may be arranged to perform the method as described in relation to the sixth aspect. According to an eighth aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a plurality of fuel tanks, each arranged to contain a different fuel for use in powering the gas turbine engine, wherein the fuels have different calorific values; and a fuel management device arranged to store information about the fuel contained in each fuel tank and to control the supply of fuel to the gas turbine engine in operation (optionally in flight only) by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks based on a thrust demand of the gas turbine engine such that a fuel having a lower heating value is supplied to the gas turbine engine in response to a lower thrust demand. It will be understood that the propulsion system may include additional fuel tanks containing the same fuels in addition to a plurality of fuel tanks containing different fuels; a minimum of two different fuels are provided on board the aircraft for implementations of this aspect. Varying fuel heating value corresponding to a thrust demand may facilitate maintaining a more constant fuel flow rate during at least a portion of a flight (e.g., in cruise, or for a portion of cruise at constant altitude), and / or more uniform fuel pump operation and discharge in flight during at least a portion of a flight. In addition, lowest possible fuel flow rate at key points (e.g., very low thrust operating points) may be increased by switching to a lower heating value fuel, thereby increasing an overall minimum flow rate level and maintaining the flow rate within a narrower range across the flight envelope. Implementations of this aspect may therefore allow a higher fuel mass flow rate to be maintained at a lower thrust demand than if a fuel were not selected on the basis of heating value, thereby facilitating the use of the fuel as a heat transfer fluid (provided that the lower heating value fuel in question does not have a correspondingly lower heat capacity), and / or improving lubrication and / or reducing the risk of fuel overheating. This may be particularly useful during low idle thrust operation. Similarly, the use of a fuel having a higher heating value at a higher thrust demand may facilitate the satisfaction of this demand without the stress of the fuel flow management system.An implementation of this aspect may therefore mean that when a thrust demand is reduced, the fuel flow rate need not be reduced as much as it would otherwise be. Each fuel tank may be arranged to contain fuel having a different type or proportion of a sustainable aviation fuel. A first fuel tank of the plurality of fuel tanks may be arranged to contain only a fuel that is a sustainable aviation fuel. The sustainable aviation fuel in the first fuel tank may be selected such that the propulsion system can operate only with that fuel. The fuel management device may be arranged to implement different control for ground operations versus flight. For example, a sustainable aviation fuel in the first fuel tank, or a high SAF% blend, may be used to fuel the aircraft when the aircraft is performing at least the majority of ground operations, regardless of the thrust demand or heating value of that fuel. The fuel management device may be arranged such that all fuel used for ground operations is taken from the first fuel tank, and / or such that all fuel used for ground operations is SAF or the highest % SAF blend available to the aircraft. The fuel management device may be arranged such that fuel having a lower calorific value is supplied to the gas turbine engine during cruise than during climb. The fuel management device may be arranged such that fuel having a lower heating value is supplied to the gas turbine engine at low idle than at high idle. A first fuel tank of the plurality of fuel tanks may have a higher proportion of sustainable aviation fuel (e.g., 100%) than a second fuel tank of the plurality of fuel tanks. In some cases, more fuel from the second fuel tank may be used in cruise and more fuel from the first tank is used at operating points in response to higher power demands. The higher % SAF fuel may have a higher heating value. First and second fuel tanks of the plurality of fuel tanks may contain sustainable aviation fuels of different compositions. The blower can have a diameter of at least 330 cm. According to a ninth aspect, a method of operating an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: the arrangement of each fuel tank of the plurality of fuel tanks fuel to contain a different fuel to be used to power the gas turbine engine, wherein the fuels have different calorific values; storing information about the fuel contained in each fuel tank; and controlling the supply of fuel to the gas turbine engine in operation (optionally in flight only) by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks based on a thrust demand of the gas turbine engine such that a fuel having a lower heating value is supplied to the gas turbine engine at a lower thrust demand. Arranging each fuel tank to contain a different fuel may include supplying each fuel tank with a different sustainable aviation fuel, and / or a blended fuel having a different type or proportion of a sustainable aviation fuel. Control of fuel delivery to the gas turbine engine based on thrust demand can be performed only in flight. Fuel delivery can be controlled differently for ground operations. According to a tenth aspect, a method of modifying an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: the arrangement of each fuel tank to contain a different fuel for use in powering the gas turbine engine, wherein the fuels have different calorific values; and providing a fuel management device arranged to store information about the fuel contained in each fuel tank and to control the supply of fuel to the gas turbine engine in operation (optionally in flight only) by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks based on a thrust demand of the gas turbine engine such that a fuel having a lower heating value is supplied to the gas turbine engine in response to a lower thrust demand. Arranging each fuel tank to contain a different fuel may include supplying each fuel tank with a different sustainable aviation fuel, and / or a blended fuel having a different type or proportion of a sustainable aviation fuel. Arranging each fuel tank to contain a different fuel may include adjusting at least one valve to fluidly isolate two or more fuel tanks separated from each other to provide separate containment for different fuels. The fuel management device may be arranged to implement different control for ground operations versus flight. Control of fuel delivery to the gas turbine engine based on thrust demand may be performed only in flight. Fuel delivery may therefore be controlled differently for ground operations, for example, selecting a SAF (or a higher SAF% blend) regardless of the calorific value if the choice is between it and a fossil fuel (or a lower SAF% blend). According to an eleventh aspect, a power system for an aircraft is provided, which system comprises: a gas turbine engine arranged to burn fuel to provide power to the aircraft; a plurality of fuel tanks, each arranged to contain a fuel for use in providing power to the aircraft, wherein at least two of the plurality of fuel tanks contain different fuels, and wherein one or more of the plurality of fuel tanks are arranged to contain only a fuel that is a sustainable aviation fuel; and a fuel management device arranged to store information about the fuel contained in each fuel tank and to control the fuel supply so as to draw only sustainable aviation fuel to power at least the majority of ground operations. In some examples, only sustainable aviation fuel may be used to power aircraft ground operations, such that all ground operations are fueled using the sustainable aviation fuel. In other examples, most, but not all, of the fuel used for ground operations is sustainable aviation fuel, with only small amounts from other sources being used (e.g., less than 10% or less than 5% of fuel usage and / or ground operations operating time). In some examples, particularly in examples where the sustainable aviation fuel has a higher viscosity, at a given temperature, than the fuel in another fuel tank, the fuel in the other fuel tank may be used for starting the engine, and the fuel source may then be switched to the sustainable aviation fuel once the engine is running, e.g., once a certain temperature is reached. The fuel in the tank used for starting may be optimized for initial use at low temperatures, and / or for other characteristics of the starting operation. In such examples, sustainable aviation fuel may be used for all ground operations except engine starting if the fuel in the starting tank is not also SAF, and SAF (possibly different SAFs) may be used for all ground operations if it is. One of the gas turbine engine(s) may be a gas turbine engine of an Auxiliary Power Unit - APU. The APU may be arranged to be primarily active or active only during ground operations. A first fuel tank of the one or more tanks may be arranged to contain sustainable aviation fuel and may be exclusively dedicated to the APU such that the sustainable aviation fuel of the first fuel tank is not arranged to be supplied to any other gas turbine engine of the aircraft. Fuel not certified for use in flight propulsion may therefore be stored in the first fuel tank. A first fuel tank of the one or more tanks may be arranged to contain a sustainable aviation fuel, and may be arranged to provide fuel to the APU when performing ground operations, and to serve as a trim tank in flight. The APU may not be arranged to provide any propulsive power to the aircraft. The gas turbine engine may be arranged to provide propulsive power to the aircraft, and may comprise: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. According to a twelfth aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a plurality of fuel tanks, each arranged to contain a fuel for use in providing power to the aircraft, wherein at least two of the plurality of fuel tanks contain fuels different, and wherein one or more of the plurality of fuel tanks are arranged to contain only a fuel that is a sustainable aviation fuel; and a fuel management device arranged to store information about the fuel contained in each fuel tank and to control the supply of fuel to the gas turbine engine so as to use only sustainable aviation fuel when the aircraft is performing at least the majority of ground operations. Each fuel tank may be arranged to contain a fuel having a different type of sustainable aviation fuel and / or a different proportion of a sustainable aviation fuel. In some implementations, two or more tanks may contain the same fuel. The sustainable aviation fuel in at least a first fuel tank of the one or more tanks arranged to contain sustainable aviation fuel may be selected such that the propulsion system can operate solely on that fuel. The fuel management device may be arranged to control the supply of fuel to the gas turbine engine in flight by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks. The fuel in the one or more tanks arranged to contain a sustainable aviation fuel for use in ground operations may have a lower heating value than any fuel stored in another fuel tank of the plurality of fuel tanks. The fuel in the one or more tanks arranged to contain a sustainable aviation fuel for use in ground operations may produce lower nvPM emissions than any fuel stored in another fuel tank of the plurality of fuel tanks. The fuel selected for use for ground operations may be optimized for ground operations, some or all of which may have a relatively low power demand compared to an average flight operation, and some or all of which may be required by regulation to meet more stringent emissions criteria, A first fuel tank of the one or more tanks arranged to contain a sustainable aviation fuel may be smaller than the one or more other fuel tanks. The first fuel tank may be arranged to be used exclusively for ground operations of the aircraft. This arrangement may be as described in relation to the sixteenth to twentieth aspects, below. The fuel in the first fuel tank may be selected to have a lower heating value than any fuel stored in another fuel tank. fuel from the plurality of fuel tanks. Sustainable aviation fuel can be used to power all aircraft ground operations. According to a thirteenth aspect, a method of operating an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging at least two fuel tanks of the plurality of fuel tanks for storing different fuels, wherein one or more of the plurality of fuel tanks are arranged to contain only a fuel that is a sustainable aviation fuel; controlling the fuel supply so that only sustainable aviation fuel is used when the aircraft is conducting at least the majority of ground operations. The method may further include storing information about the fuel contained in each fuel tank. Control may be performed based on the stored information. The gas turbine engine may be arranged to provide propulsive power to the aircraft, and may comprise: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The gas turbine engine may be a gas turbine engine of an Auxiliary Power Unit - APU - of the aircraft. A first fuel tank of the one or more tanks arranged to contain sustainable aviation fuel may be a trim tank of the aircraft. The sustainable aviation fuel in the first fuel tank may be arranged to be (at least substantially) depleted during the performance of ground operations such that the first fuel tank is at least substantially empty and available to receive fuel pumped therein in flight. According to a fourteenth aspect, a method of modifying an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging at least two of the plurality of fuel tanks to each store a different fuel, wherein one or more re- tanks of the plurality of fuel tanks are arranged to contain only a fuel that is a sustainable aviation fuel; and the provision of a fuel management device arranged to control the fuel supply so as to use only sustainable aviation fuel when the aircraft is carrying out at least the majority of ground operations. The fuel management device may be arranged to store information about the fuel contained in each fuel tank. The fuel management device may be arranged to perform fuel supply control based on the stored information. The gas turbine engine may be arranged to provide propulsive power to the aircraft, and may comprise: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The gas turbine engine may be a gas turbine engine of an Auxiliary Power Unit - APU - of the aircraft. According to a fifteenth aspect, a power system for an aircraft is provided, which system comprises: an Auxiliary Power Unit - APU - comprising a gas turbine engine arranged to burn fuel to provide power to the aircraft; and one or more fuel tanks arranged to contain only fuel that is a sustainable aviation fuel; and in which all fuel used by the APU is sustainable aviation fuel. According to a sixteenth aspect, a power system for an aircraft is provided, which system comprises: a gas turbine engine arranged to burn fuel to provide power to the aircraft; one or more first fuel tanks arranged to be used to fuel a ground operation of the aircraft; one or more secondary fuel tanks, each arranged to contain fuel for use in powering the aircraft in flight; and a fuel management device arranged to control the fuel supply so as to draw fuel only from the one or more first fuel tanks to fuel at least the majority of ground operations. Benefits can therefore be obtained by filling the first tank(s) of fuel of a fuel optimized for use in ground operations, for example for more efficient engine operation and / or for reduced emissions. Having one or more first fuel tanks arranged and possibly dedicated for this purpose can facilitate refueling and operation. In some examples, only fuel from the first fuel tank(s) may be used to fuel aircraft ground operations, such that all ground operations are fueled using fuel from the first fuel tank(s). In other examples, most, but not all, of the fuel used for ground operations is taken from the first fuel tank, with only small amounts from other sources being used (e.g., less than 10% or less than 5% of the fuel usage and / or ground operations operating time). In many examples, only one first fuel tank is provided. However, it will be understood that, although the discussion below often refers to a single first fuel tank, the disclosure is not limited in this manner. In some examples, particularly in examples where the fuel in the first fuel tank has a higher viscosity, at a given temperature, than the fuel in another fuel tank, fuel from another fuel tank may be used for engine starting, and the fuel source may then be switched to the first fuel tank once the engine is running, e.g., once a certain temperature is reached. In such examples, fuel from the first fuel tank may be used for all ground operations except engine starting. The fuel management device may further be arranged to draw fuel only from the one or more secondary fuel tanks for at least the majority of other operations (e.g., climb and cruise). It will be appreciated that any fuel remaining in the first fuel tank may be totally consumed in flight (alone or as part of a mixture); either in the early stages following ground operations or thereafter. The fuel management device may be arranged to draw fuel from the one or more secondary fuel tanks for at least the majority of other operations. The fuel management device may be arranged to draw fuel only from the first fuel tank to supply all ground operations. A gas turbine engine of the one or more gas turbine engines may be a gas turbine engine of an Auxiliary Power Unit - APU. The APU can be arranged to be active only during ground operations. The first fuel tank may be arranged to supply fuel to the APU when performing ground operations, and to serve as a trim tank in flight. The APU may not be arranged to provide any propulsive power to the aircraft. A gas turbine engine of the one or more gas turbine engines may be a gas turbine engine arranged to provide propulsive power to the aircraft. The gas turbine engine may comprise: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The fuel management device may be arranged to supply fuel only from the one or more secondary fuel tanks to the gas turbine engine in flight, such that the first fuel tank is not used to supply fuel to an engine in flight. According to a seventeenth aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; one or more first fuel tanks arranged to be used to fuel a ground operation of the aircraft; one or more secondary fuel tanks, each arranged to contain fuel for use in powering the gas turbine engine in flight; and a fuel management device arranged to control the supply of fuel to the gas turbine engine so as to draw fuel only from the one or more first fuel tanks to power at least the majority of ground operations. The fuel management device may further be arranged to draw fuel only from the one or more secondary fuel tanks for at least the majority of other operations (e.g., climb and cruise). The first fuel tank may be arranged to contain only a fuel that is a sustainable aviation fuel, In examples with a single first fuel tank, the first fuel tank may be smaller than the one or more secondary fuel tanks. In examples with multiple first fuel tanks, the total volume of the first fuel tanks may be less than the total volume of the secondary fuel tanks, and possibly less than the volume of each secondary fuel tank individually. The propulsion system may include a plurality of secondary fuel tanks. The fuel management device may be arranged to be capable of mixing fuels from the secondary fuel tanks to power the gas turbine engine in flight, but may not be capable of mixing a fuel from the first fuel tank with a fuel from the secondary fuel tanks. The fuel in the first fuel tank may have a lower heating value and / or may generate lower nvPM emission levels than any fuel stored in the one or more secondary fuel tanks. According to an eighteenth aspect, a method of operating an aircraft is provided, which method comprises: a gas turbine engine arranged to burn fuel to provide power to the aircraft; one or more first fuel tanks arranged to be used to fuel a ground operation of the aircraft; and one or more secondary fuel tanks, each arranged to contain fuel for use in powering the aircraft in flight, the method comprising: controlling the fuel supply so as to draw fuel only from the first one or more fuel tanks when the aircraft is fueling at least the majority of ground operations. The method may further include drawing fuel only from the one or more secondary fuel tanks for at least the majority of other operations. In some examples, only one fuel from the one or more secondary fuel tanks may be used for other operations, such that the first fuel tank is used exclusively for ground operations. The gas turbine engine may be arranged to provide propulsive power to the aircraft, and may comprise: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The gas turbine engine may be a gas turbine engine of an Auxiliary Power Unit - APU - of the aircraft. The first fuel tank may be a trim tank of the aircraft. The fuel in the first fuel tank, which may be a sustainable aviation fuel, may be arranged to be depleted during the performance of ground operations such that the first fuel tank is ultimately substantially empty and available to receive fuel pumped therein in flight. According to a nineteenth aspect, a method of modifying an aircraft comprising one or more gas turbine engines and a plurality of fuel tanks is provided, the method comprising: providing one or more first fuel tanks which are fluidically isolated from the other (secondary) fuel tanks of the plurality of fuel tanks; and the provision of a fuel management device arranged to control the supply of fuel to the one or more gas turbine engines so as to draw fuel only from the one or more first fuel tanks to power at least the majority of ground operations. The fuel management device may further be arranged to draw fuel only from the one or more secondary fuel tanks for at least the majority of other operations. The first fuel tank(s) may be permanently fluidically isolated from other fuel tanks, or may be reversibly isolated from other fuel tanks, for example by means of one or more pumps and / or valves. In some examples, only one fuel from the one or more secondary fuel tanks may be used for other operations, such that the first fuel tank is used exclusively for ground operations. According to a twentieth aspect, a power system for an aircraft is provided, which system comprises: an Auxiliary Power Unit - APU - comprising a gas turbine engine arranged to burn fuel to provide power to the aircraft; and one or more first fuel tanks which are fluidically isolated from any other fuel tank in the fuel system; and wherein the first fuel tank(s) are dedicated to the APU, such that all fuel used by the APU is taken from the first fuel tank(s) (in normal operation). It will be appreciated that an aircraft is generally arranged so that the APU can also be supplied with fuel from one or more other tanks, for example in the event that the APU needs to start and operate in an emergency during flight, for example for non-propulsive purposes such as powering aircraft control surfaces after a main engine flameout and / or providing power to restart the main engines. According to a twenty-first aspect, a power system for an aircraft is provided, which system comprises: a gas turbine engine arranged to burn fuel in a combustion chamber to provide power to the aircraft; a plurality of fuel tanks arranged to contain a different fuel for use in providing power to the aircraft, wherein a first fuel tank of the plurality of fuel tanks is arranged to contain a first fuel, and a second tank of the plurality of fuel tanks is arranged to contain a second fuel having a different composition from the first fuel; and a fuel management device arranged to store information about the fuel contained in each fuel tank and to control the fuel supply so as to draw fuel from the second tank for engine starting, before switching to the first fuel tank. The second fuel may be selected for its improved starting properties; for example, having a lower viscosity, at a given temperature, than that of the fuel in the first fuel tank, so as to facilitate cold starting of an engine. Fuel selected for its improved starting properties may have a lower viscosity, at a given temperature, than the fuel in the first tank. The second fuel can be fossil / petroleum based. The fuel management device may be arranged to control the fuel supply so as to switch from drawing fuel from the second fuel tank to drawing fuel from the first fuel tank when at least one of the following conditions is met: (i) the fuel reaches a temperature of 60°C at the inlet of the combustion chamber; (ii) the gas turbine engine has been running for a period of 30 seconds; and (iii) the gas turbine engine has reached idle operation. The first tank may be arranged to contain sustainable aviation fuel. The second tank may be arranged to contain a fossil hydrocarbon-based fuel. The gas turbine engine may be an Auxiliary Power Unit (APU) gas turbine engine. The APU may be arranged to be active only during ground operations, at least in normal operation. The first fuel tank may be dedicated exclusively to the APU such that fuel from the first fuel tank is not arranged to be supplied to any other gas turbine engine of the aircraft. The first fuel tank may be arranged to supply fuel to the APU when performing ground operations, and to serve as a trim tank in flight. The APU can be arranged to provide no propulsive power to the aircraft. Alternatively, the gas turbine engine may be arranged to provide propulsive power to the aircraft. The gas turbine engine may comprise an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The first fuel in the first fuel tank can be selected so that the gas turbine engine can operate only on that fuel. The second fuel in the second fuel tank can be selected so that the gas turbine engine can operate only on that fuel in flight, as well as for starting. Each fuel tank may be arranged to contain fuel having a different type or proportion of sustainable aviation fuel (SAF). The fuel management device may be arranged to control the supply of fuel to the gas turbine engine in flight by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks. The first fuel may be SAF or a high % SAF blend, and the fuel management device may be arranged to control the fuel supply so as to draw fuel from the first fuel tank for the majority of ground operations - start-up may be the only exception. According to a twenty-second aspect, a method of operating an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging at least two of the fuel tanks to store a different fuel, wherein a first fuel tank of the plurality of fuel tanks is arranged to contain a first fuel, and a second tank of the plurality of fuel tanks is arranged to contain a second fuel having a different composition from the first fuel; and controlling the fuel supply so as to draw fuel from the second tank for engine starting, before switching to the first fuel tank. The method may further include storing information about the fuel contained in each fuel tank. Control may be performed based on the stored information. The gas turbine engine may be arranged to provide propulsive power to the aircraft. The gas turbine engine may comprise an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The first fuel tank may be a trim tank of the aircraft. The first fuel may be a sustainable aviation fuel (SAF) or a high % SAF blend and the fuel in the first fuel tank may be arranged to be depleted during the performance of ground operations (after start-up) such that the first fuel tank is at least substantially empty at the end of climb, if not at take-off, and available to receive fuel pumped therein in flight. According to a twenty-third aspect, a method of modifying an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging a first fuel tank of the plurality of fuel tanks to contain a first fuel, and a second tank of the plurality of fuel tanks to contain a second fuel having a different composition from the first fuel; and the provision of a fuel management device arranged to control the fuel supply so as to draw fuel from the second tank for engine starting, prior to switching to the first tank of fuel. The fuel management device may further be arranged to store information about the fuel contained in each fuel tank. Control may be performed based on the stored information. The gas turbine engine may be arranged to provide propulsive power to the aircraft, and may comprise an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The gas turbine engine may be a gas turbine engine of an Auxiliary Power Unit - APU - of the aircraft. According to a twenty-fourth aspect, a method of operating an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to supply fuel to the gas turbine engine is provided, wherein at least two of the fuel tanks contain fuels having different fuel characteristics, the method being carried out by processing circuitry and comprising: obtaining a flight profile for a flight of the aircraft; and determining a refueling schedule for the flight based on the flight profile and fuel characteristics, the refueling schedule governing / dictating the variation over time of the amount of fuel consumed in each tank. The refueling schedule lists an expected variation over time in the amount of fuel consumed in each tank and is intended to be used to instruct a refueling management device to supply fuel to the gas turbine engine accordingly. The refueling schedule can therefore be described as governing, dictating, or directing the use of fuel for the flight (possibly for the aircraft in flight only, or also for ground operations). The method may be performed on-site, for example by a refueling schedule determination module of the aircraft, which may be part of an electronic engine governor (EEC) of the aircraft. Alternatively, the method may be performed in a workshop, and the refueling schedule is provided to the aircraft for implementation. The fuel characteristics of the fuel include at least one of: i. the percentage of sustainable aviation fuel in the fuel; li. the aromatic hydrocarbon content of the fuel; iii, the multi-aromatic hydrocarbon content of the fuel; iv. the percentage of nitrogen-containing species in the fuel; v. the presence or percentage of a trace species or trace element in the fuel; vi, the hydrogen to carbon ratio of the fuel; vi. the distribution of fuel hydrocarbons; vii, the level of non-volatile particulate matter (nvPM) emissions during combustion; ix, the naphthalene content of the fuel; X. the sulfur content of the fuel: Xi, the cycloparaffin content of the fuel; xil. the oxygen content of the fuel; xiii, thermal stability of the fuel; xiv. the level of coking of the fuel: XXV. an indication that the fuel is a fossil fuel; and xVvi, at least one of density, viscosity, calorific value and heat capacity. The refueling schedule may be determined using flight profile information including at least one of: (i) an expected altitude; and (ii) a planned route. The method may further include receiving forecast weather conditions for a planned route of the aircraft defined in the flight profile, and the received forecast weather conditions may be used to influence the refueling schedule. Determining the refueling schedule may include determining the amount of sustainable aviation fuel - SAF - available to the aircraft, and / or tanks containing SAF or a high % SAF blend, and preferably scheduling the use of SAF (alone or as part of a blend) for aircraft ground operations. Determining the refueling schedule may include determining a calorific value of each fuel on board the aircraft, and preferably scheduling the use of a lower calorific value fuel for periods of lower thrust demand. The method may further include controlling the supply of fuel to the operating gas turbine engine in accordance with the refueling schedule. The obtaining and determining steps may be performed in a workshop. The method may further include providing the refueling schedule to the aircraft prior to the ordering step. According to a twenty-fifth aspect, a propulsion system for an aircraft is provided, which system comprises: a gas turbine engine, the gas turbine engine optionally comprising: an engine core comprising a turbine, a compressor and a main shaft connecting the turbine to the compressor; and a fan located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft; a plurality of fuel tanks arranged to contain a fuel for powering the gas turbine engine, wherein at least two of the fuel tanks contain fuels having different fuel characteristics; and a refueling program determination module designed for: obtain a flight profile for a flight of the aircraft; and determining a refueling schedule for the flight based on the flight profile and fuel characteristics, the refueling schedule governing the variation over time of the amount of fuel drawn from each tank during the flight. The fuel characteristics of the fuel may include one or more of the fuel characteristics listed above for the twenty-fourth aspect. The refueling schedule determination module may be arranged to determine the refueling schedule using flight profile information comprising at least one of: (i) an expected altitude; and (li) a planned route. The propulsion system may further comprise a receiver arranged to receive data regarding forecast weather conditions for a planned route of the aircraft, the route being defined in the flight profile. The received forecast weather conditions may be used to influence the refueling schedule. The refueling schedule determination module may be arranged to determine the refueling schedule based on determining the amount of sustainable aviation fuel - SAF - available to the aircraft, and to preferentially schedule the use of SAF for ground operations of the aircraft. The refueling schedule determination module may be arranged to determine the refueling schedule based on determining a calorific value of each fuel on board the aircraft, and to preferentially schedule the use of a lower calorific value fuel for periods of lower thrust demand. The refueling schedule determination module may be arranged to control the supply of fuel to the operating gas turbine engine in accordance with the refueling schedule. According to a twenty-sixth aspect, a non-transitory computer-readable medium is provided having stored thereon instructions which, when executed by a processor, cause the processor to: determining a refueling schedule for a flight of an aircraft, the aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to supply fuel to the gas turbine engine, wherein at least two of the fuel tanks contain fuels having different fuel characteristics. The refueling schedule is determined based on a flight profile for the flight of the aircraft and fuel characteristics of fuels available to the aircraft. The refueling schedule records / directs the variation over time of the amount of fuel consumed in each tank during the flight. The instructions may further be arranged to cause the processor to control the delivery of fuel to the operating gas turbine engine in accordance with the refueling schedule. The processor may include or consist of an on-board refueling schedule determination module, and may be or may be part of an electronic engine governor. The instructions may further be arranged to cause the processor to provide the refueling schedule to the aircraft for implementation. The processor may include or consist of a shop refueling schedule determination module. According to a twenty-seventh aspect, a power system for an aircraft is provided, which system comprises: one or more gas turbine engines arranged to burn fuel to provide power to the aircraft; a plurality of fuel tanks each arranged to contain a fuel for use in providing power to the aircraft, wherein at least two of the plurality of fuel tanks contain different fuels, the different fuels each having a different proportion of a sustainable aviation fuel; and a fuel management device arranged to: store information about the fuel contained in each fuel tank; identify which tank contains the fuel with the highest proportion of sustainable aviation fuel; and control the fuel supply so as to draw fuel only from the tank containing the fuel having the highest proportion of a sustainable aviation fuel to power at least the majority of ground operations. The different proportion of sustainable aviation fuel (SAF) can range from 0% SAF to 100% SAF. The fuel with the highest proportion of sustainable aviation fuel can contain more than 50% SAF. One of the gas turbine engine(s) may be a gas turbine engine of an Auxiliary Power Unit - APU. A first fuel tank of the plurality of fuel tanks may be arranged to contain the fuel having the highest proportion of a sustainable aviation fuel, and optionally the fuel may be a sustainable aviation fuel (i.e., a fuel for which the proportion of SAF is 100%). The first fuel tank may be exclusively dedicated to the APU such that fuel from the first fuel tank is not arranged to be supplied to any other gas turbine engine of the aircraft. If a plurality of fuel tanks contain fuel having the same highest proportion of a sustainable aviation fuel, the tank to be used may be selected based on a comparison of at least one of the following: (i) the levels of non-volatile particulate matter emissions during fuel combustion; and (ii) hydrogen to carbon ratios of fuels. One or more other air quality parameters may also be compared to select the fuel that is likely to provide the best air quality results. Environmental factors (e.g., airport altitude and humidity) may also be considered in this assessment. The power system may be a propulsion system of the aircraft, and the gas turbine engine (at least one of the one or more gas turbine engines) may be arranged to provide propulsive power to the aircraft. The fuel with the highest proportion of sustainable aviation fuel can be selected so that the propulsion system can operate only on that fuel. The fuel with the highest proportion of sustainable aviation fuel — SAF — may contain more than 50% SAF and, optionally, may contain at least 55% SAF. The fuel management device may be arranged to control the supply of fuel to the gas turbine engine in flight by selecting a specific fuel or a combination of fuels from one or more of the plurality of fuel tanks. A first fuel tank of the plurality of fuel tanks may be arranged to contain fuel having the highest proportion of a sustainable aviation fuel, and may be smaller than the one or more other fuel tanks. The first fuel tank may be arranged to be used exclusively for ground operations of the aircraft. The fuel having the highest proportion of a sustainable aviation fuel, for use in ground operations, may have a lower heating value than any fuel stored in another fuel tank of the plurality of fuel tanks. The fuel with the highest proportion of sustainable aviation fuel can be used to power all aircraft ground operations. A first fuel tank of the plurality of fuel tanks may be arranged to contain fuel having the highest proportion of a sustainable aviation fuel and may be arranged to supply fuel to the gas turbine engine when performing ground operations, and to serve as an in-flight trim tank. The fuel with the highest proportion of sustainable aviation fuel - SAF - can contain 100% SAF. According to a twenty-eighth aspect, a power system for an aircraft is provided, which system comprises: a gas turbine engine arranged to burn fuel to provide power to the aircraft; a plurality of fuel tanks each arranged to contain a fuel for use in providing power to the aircraft, wherein at least two of the plurality of fuel tanks contain different fuels, a first tank containing a fuel that is more than 50% sustainable aviation fuel and a second tank containing a fuel that is less than 50% sustainable aviation fuel; and a fuel management device arranged to: store information about the fuel contained in each fuel tank; and control the fuel supply so that only fuel that is more than 50% sustainable aviation fuel is used to power at least the majority of ground operations. The first tank may contain fuel that is sustainable aviation fuel (i.e. 100% SAF). According to a twenty-ninth aspect, a method of operating an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging two or more of the plurality of fuel tanks for storing different fuels, the different fuels each having a different proportion of a sustainable aviation fuel; the identification of the tank containing the fuel having the highest proportion of sustainable aviation fuel; and controlling the fuel supply so as to draw fuel only from the tank containing the fuel having the highest proportion of a sustainable aviation fuel when the aircraft is conducting at least the majority of ground operations. The method may further include storing information about the fuel contained in each fuel tank. Control may be performed based on the stored information. The gas turbine engine may be arranged to provide propulsive power to the aircraft. The gas turbine engine may be a gas turbine engine of an Auxiliary Power Unit - APU - of the aircraft. A first fuel tank of the one or more tanks may be arranged to contain fuel having the highest proportion of a sustainable aviation fuel. This first fuel tank may be arranged to function as a trim tank of the aircraft - the fuel in the first fuel tank may therefore be arranged to be (at least substantially) depleted when performing ground operations such that the first fuel tank is at least substantially empty and available to receive fuel pumped therein in flight. According to a thirtieth aspect, a method of modifying an aircraft comprising a gas turbine engine and a plurality of fuel tanks arranged to store fuel for powering the gas turbine engine is provided, the method comprising: arranging two or more of the plurality of fuel tanks for storing different fuels, the different fuels each having a different proportion of a sustainable aviation fuel; and the provision of a fuel management device arranged for: identify which tank contains the fuel with the highest proportion of sustainable aviation fuel; and control the fuel supply so as to draw fuel only from the tank containing the fuel having the highest proportion of a sustainable aviation fuel when the aircraft is conducting at least the majority of ground operations. The fuel management device may be arranged to store information about the fuel contained in each fuel tank. The fuel management device may be arranged to perform tank identification and fuel supply control based on the stored information. As noted elsewhere herein, the present disclosure may relate to a gas turbine engine. Such a gas turbine engine may include an engine core comprising a turbine, a combustor, a compressor, and a main shaft connecting the turbine to the compressor. Such a gas turbine engine may include a fan (having fan blades) located upstream of the engine core. Alternatively, in some examples, the gas turbine engine may include a fan located downstream of the engine core. Thus, the gas turbine engine may be an open rotor engine or a turboprop. In the case where the gas turbine engine is an open rotor engine or a turboprop, the gas turbine engine may comprise two stages of counter-rotating propellers attached to and driven by a free turbine via a shaft. The propellers may rotate in opposite directions such that one rotates clockwise and the other counterclockwise about the engine's rotational axis. Alternatively, the gas turbine engine may comprise a propeller stage and a guide vane stage configured downstream of the propeller stage. The guide vane stage may have a variable pitch. Accordingly, the high-pressure, intermediate-pressure, and free turbines may, respectively, drive high- and medium-pressure compressors and propellers via suitable interconnecting shafts. Thus, the propellers may provide the majority of the propulsive thrust. Where the gas turbine engine is an open rotor engine or a turboprop engine, one or more of the propeller stages may be driven by a gearbox of the type described. The arrangements of the present disclosure may be particularly, but not exclusively, advantageous for fans that are driven by a gearbox. Accordingly, the gas turbine engine may include a gearbox that receives an input from the main shaft and outputs a drive to the fan so as to drive the fan at a rotational speed lower than that of the main shaft. The input to the gearbox may be directly from the main shaft, or indirectly from the main shaft, by example via a straight shaft and / or a gear. The main shaft can rigidly connect the turbine and compressor, so that the turbine and compressor rotate at the same speed (with the fan rotating at a lower speed). The gas turbine engine as described and / or claimed herein may have any suitable general architecture. For example, the gas turbine engine may have any desired number of shafts connecting turbines and compressors, for example, one, two, or three shafts. For illustrative purposes only, the turbine connected to the main shaft may be a first turbine, the compressor connected to the main shaft may be a first compressor, and the main shaft may be a first main shaft. The engine core may further comprise a second turbine, a second compressor, and a second main shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second main shaft may be arranged to rotate at a higher rotational speed than the first main shaft. In such an arrangement, the second compressor may be positioned axially downstream of the first compressor. The second compressor may be arranged to receive (e.g., directly receive, e.g., via a generally annular duct) a flow from the first compressor. The gearbox may be arranged to be driven by the main shaft which is configured to rotate (e.g. in use) at the lowest rotational speed (e.g. the first main shaft in the example above). For example, the gearbox may be arranged to be driven only by the main shaft which is configured to rotate (e.g. in use) at the lowest rotational speed (e.g. being only the first main shaft, and not the second main shaft, in the example above). Alternatively, the gearbox may be arranged to be driven by any one or more shafts, e.g. the first and / or second shaft(s) in the example above. The gearbox may be a reduction gear (in the sense that the output to the blower is a lower rotational speed than the input to the main shaft). Any type of gearbox may be used. For example, the gearbox may be a "planetary" or "star" gearbox, as described in more detail elsewhere herein. The gearbox may have any desired gear ratio (defined as the rotational speed of the input shaft divided by the rotational speed of the output shaft), for example, greater than 2.5, for example in the range 3 to 4.2, or 3.2 to 3.8, for example in the order of or at least 3, 3.1, 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 or 4.2. The gear ratio can be, for example, between any two values ​​of the values ​​of the preceding sentence. For illustrative purposes only, the gearbox may be a "star" gearbox having a ratio in the range of 3.1 or 3.2 to 3.8. In some arrangements, the gear ratio may be outside these ranges. In any gas turbine engine as described and / or claimed herein, a fuel of a given composition or mixture is supplied to a combustor, which may be provided axially downstream of the fan and the compressor(s). For example, the combustor may be directly downstream (e.g., at the outlet) of the second compressor, in the case where a second compressor is provided. As a further example, the flow at the outlet to the combustor may be supplied to the inlet of the second turbine, in the case where a second turbine is provided. The combustor may be provided upstream of the turbine(s). The or each compressor (e.g., the first compressor and the second compressor as described above) may comprise any number of stages, e.g., multiple stages. Each stage may comprise a row of rotor blades and a row of stator vanes, which may be variable stator vanes (in the sense that their angle of incidence may be variable). The row of rotor blades and the row of stator vanes may be axially offset from each other. The or each turbine (e.g., the first turbine and the second turbine as described above) may comprise any number of stages, e.g., multiple stages. Each stage may comprise a row of rotor blades and a row of stator vanes. The row of rotor blades and the row of stator vanes may be axially offset from each other. Each fan blade may be defined as having a radial span extending from a root (or hub) at a radially inner gas-washed location, or 0% span position, to a tip at a 100% span position. The ratio of the fan blade radius at the hub to the fan blade radius at the tip may be less than (or in the order of) one of the following values: 0.4, 0.39, 0.38, 0.37, 0.36, 0.35, 0.34, 0.33, 0.32, 0.31, 0.3, 0.29, 0.28, 0.27, 0.26, or 0.25. The ratio of the fan blade radius at the hub to the fan blade radius at the tip may be within an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, within the range 0.28 to 0.32. These ratios may be commonly referred to as the hub-to-tip ratio.Both the radius at the hub and the radius at the tip can be measured at the leading edge (or axially foremost) portion of the blade. The hub-to-tip ratio refers, of course, to the gas-washed portion of the fan blade, i.e., the portion radially outboard of any platform. The fan radius can be measured between the engine centerline and the tip of a fan blade at its leading edge. The blower diameter (which may simply be twice the blower radius) may be larger than (or of the order of) one of the following: 220 cm, 230 cm, 240 cm, 250 cm (about 100 inches), 260 cm, 270 cm (about 105 inches), 280 cm (about 110 inches), 290 cm (about 115 inches), 300 cm (about 120 inches), 310 cm, 320 cm (about 125 inches), 330 cm (about 130 inches), 340 cm (about 135 inches), 350 cm, 360 cm (about 140 inches), 370 cm (about 145 inches), 380 cm (about 150 inches), 390 cm (about 155 inches), 400 cm, 410 cm (approximately 160 inches) or 420 cm (approximately 165 inches).The fan diameter may be in an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range from 240 cm to 280 cm or from 330 cm to 380 cm. The rotational speed of the blower may vary during use. In general, the rotational speed is lower for blowers with a larger diameter. As a purely non-limiting example, the rotational speed of the blower under cruising conditions may be lower than 2500 rpm, for example lower than 2300 rpm. As a further purely non-limiting example, the rotational speed of the fan under cruising conditions for an engine having a fan diameter in the range of 220 cm to 300 cm (e.g. 240 cm to 280 cm or 250 cm to 270 cm) may be in the range of 1700 rpm to 2500 rpm, for example in the range of 1800 rpm to 2300 rpm, for example in the range of 1900 rpm to 2100 rpm.As a further purely non-limiting example, the rotational speed of the fan under cruising conditions for an engine having a fan diameter in the range of 330 cm to 380 cm may be in the range of 1200 rpm to 2000 rpm, for example in the range of 1300 rpm to 1800 rpm, for example in the range of 1400 rpm to 1800 rpm. When operating the gas turbine engine, the fan (with associated fan blades) rotates about an axis of rotation. This rotation causes the fan blade tip to move with a velocity Utip. The work done by the fan blades 13 on the flow results in an enthalpy increase dH of the (lux. A fan tip load can be defined as dH / Utip?, where dH is the enthalpy increase (e.g., the 1-D average enthalpy increase) across the fan and Utip is the (translational) velocity of the fan tip, e.g., at the leading edge of the tip (which can be defined as the fan tip radius at the leading edge multiplied by the angular velocity). The fan tip load in cruise conditions may be greater than (or of the order of) any of the following values: 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39 and 0.4 (all values ​​being dimensionless). The fan peak load may be in an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range from 0.28 to 0.31, or from 0.29 to 0.3. The gas turbine engines according to the present disclosure may have any desired bypass ratio, where the bypass ratio is defined as the ratio of the mass flow rate of the flow through the bypass duct to the mass flow rate of the flow through the core at cruise conditions. In some arrangements, the bypass ratio may be greater than (or in the order of) one of the following values: 10, 10.5, 11, 11.5, 12, 12.5, 13, 13.5, 14, 14.5, 15, 15.5, 16, 16.5, 17, 17.5, 18, 18.5, 19, 19.5, and 20. The bypass ratio may be within an inclusive range bounded by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range 12 to 16, 13 to 15, or 13 to 14. The bypass duct may be substantially annular. The bypass duct may be radially outboard of the base engine.The radially outer surface of the bypass duct may be defined by a nacelle and / or a fan casing. The overall pressure ratio of a gas turbine engine as described and / or claimed herein may be defined as the ratio of the stagnation pressure upstream of the fan to the stagnation pressure at the compressor outlet at the highest pressure (before entering the combustion chamber). By way of non-limiting example, the overall pressure ratio of a gas turbine engine as described and / or claimed herein at cruise may be greater than (or in the order of) one of the following values: 35, 40, 45, 50, 55, 60, 65, 70, 75. The overall pressure ratio may be within an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range from 50 to 70. The specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow rate through the engine. In some examples, the specific thrust may depend, for a given thrust condition, on the specific composition of the fuel supplied to the combustion chamber. Under cruising conditions, the specific thrust of an engine described and / or claimed herein may be less than (or on the order of) one of the following values: 110 Nkg"'s, 105 Nkg “s, 100 Nkg-*s, 95 Nkg-'s, 90 Nkg's, 85 Nkg'!s, or 80 Nkg"s. The specific thrust may be within an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form limits su- higher or lower), for example in the range from 80 Nkg"s to 100 Nkg!s, or from 85 Nkg-s to 95 Nkg-s. Such engines can be particularly efficient compared to conventional gas turbine engines.A gas turbine engine as described and / or claimed herein may have any desired maximum thrust. By way of purely non-limiting example, a gas turbine engine as described and / or claimed herein may be capable of producing a maximum thrust of at least (or on the order of) one of the following values: 160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN, 450 kN, 500 kN, and 550 kN. The maximum thrust may be within an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits). For illustrative purposes only, a gas turbine as described and / or claimed herein may be capable of producing a maximum thrust in the range of 330 kN to 420 KN, for example 350 kN to 400 kN.The above mentioned thrust may be the maximum net thrust under normal atmospheric conditions at sea level plus 15 degrees C (ambient pressure 101.3 kPa, temperature 30 degrees C), with the engine static. In use, the flow temperature at the inlet to the high-pressure turbine may be particularly high. This temperature, which may be referred to as the TET, may be measured at the outlet to the combustion chamber, for example immediately upstream of the first turbine blade, which may itself be referred to as the turbine nozzle blade. In some examples, the TET temperature may depend, for a given thrust condition, on the specific composition of the fuel supplied to the combustion chamber. In cruise, the TET temperature may be at least (or of the order of) one of the following values: 1400K, 1450K, 1500K, 1550K, 1600K, and 1650K. The TET temperature in cruise may be within an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits).The maximum TET temperature during engine operation may be, for example, at least (or in the order of) one of the following values: 1700K, 1750K, 1800K, 1850K, 1900K, 1950K and 2000K. The maximum TET temperature may be in an inclusive range limited by any two of the values ​​in the preceding sentence (i.e., the values ​​may form upper or lower limits), for example, in the range from 1800K to 1950K. The maximum TET temperature may occur, for example, in a high thrust condition, for example, in a maximum takeoff (MTO) condition. A fan blade and / or an airfoil portion of a fan blade described and / or claimed herein may be manufactured from any suitable material or combination of materials. For example, At least a portion of the fan blade and / or airfoil may be made at least in part from a composite, for example, a metal matrix composite and / or an organic matrix composite, such as carbon fiber. As a further example, at least a portion of the fan blade and / or airfoil may be made at least in part from a metal, such as a titanium-based metal or an aluminum-based material (such as an aluminum-lithium alloy) or a steel-based material. The fan blade may include at least two regions made using different materials. For example, the fan blade may have a protective leading edge, which may be made using a material that is more capable of resisting impacts (e.g., with birds, ice, or other material) than the remainder of the blade.Such a leading edge can for example be manufactured using titanium or a titanium-based alloy. Thus, purely for information purposes, the fan blade can have a body made of carbon fiber or aluminum (such as an alloy . aluminum-lithium) with a titanium leading edge. A fan as described and / or claimed herein may comprise a central portion, from which the fan blades may extend, for example in a radial direction. The fan blades may be attached to the central portion in any desired manner. For example, each fan blade may comprise a fastening element which may engage a corresponding slot in the hub (or a disc). For purely illustrative purposes, such a fastening element may be in the form of a dovetail which may insert into and / or engage a corresponding slot in the hub / disc in order to secure the fan blade to the hub / disc. As a further example, the fan blades may be formed integrally with a central portion. Such an arrangement may be referred to as a bladed disc or a bladed ring.Any suitable method may be used to manufacture such a bladed disc or bladed ring. For example, at least a portion of the fan blades may be machined from a block and / or at least a portion of the fan blades may be attached to the hub / disc by welding, such as linear friction welding. The gas turbine engines described and / or claimed herein may or may not be provided with a variable area nozzle (VAN). Such a variable area nozzle may allow the exit area of ​​the bypass duct to be varied during use. The general principles of the present disclosure may be applied to engines with or without a VAN. The fan of a gas turbine as described and / or claimed herein may have any desired number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26 fan blades. As used herein, the terms idle, taxi, takeoff, climb, cruise, descent, approach, and landing have the conventional meaning and would be readily understood by those skilled in the art. Thus, for a given gas turbine engine for an aircraft, those skilled in the art would immediately recognize each term to refer to a phase of engine operation within a given mission of an aircraft to which the gas turbine engine is designed to be attached. In this regard, ground idling may refer to a phase of engine operation where the aircraft is stationary and in contact with the ground, but where it is necessary for the engine to be running. At idle, the engine may produce between 3% and 9% of available engine thrust. In other examples, the engine may produce between 5% and 8% of available thrust. In still other examples, the engine may produce between 6% and 7% of available thrust. Taxiing may refer to a phase of engine operation where the aircraft is propelled along the ground by the thrust produced by the engine. During taxiing, the engine may produce between 5% and 15% of available thrust. In other examples, the engine may produce between 6% and 12% of available thrust. In still other examples, the engine may produce between 7% and 10% of available thrust.Takeoff may refer to a phase of engine operation where the aircraft is propelled by the thrust produced by the engine. At an initial stage of the takeoff phase, the aircraft may be propelled while the aircraft is in contact with the ground. At a later stage of the takeoff phase, the aircraft may be propelled while the aircraft is not in contact with the ground. During takeoff, the engine may produce between 90% and 100% of available thrust. In other examples, the engine may produce between 95% and 100% of available thrust. In still other examples, the engine may produce 100% of available thrust. Climb may refer to a phase of engine operation where the aircraft is propelled by the thrust produced by the engine. During climb, the engine may produce between 75% and 100% of available thrust. In other examples, the engine may produce between 80% and 95% of available thrust. In still other examples, the engine may produce between 85% and 90% of available thrust. In this regard, climb may refer to a phase of operation in a flight cycle of an aircraft between takeoff and arrival at cruise conditions. Additionally or alternatively, climb may refer to a nominal point in a flight cycle of an aircraft between takeoff and landing, where a relative increase in altitude is required, which may require a demand for additional thrust from the engine. As used herein, cruising conditions have the conventional meaning and would be readily understood by those skilled in the art. Thus, for an engine gas turbine engine for an aircraft, one skilled in the art would immediately recognize cruise conditions as meaning the mid-cruise engine operating point of a given mission (which may be referred to in the industry as the "economic mission") of an aircraft to which the gas turbine engine is designed to be attached. In this regard, mid-cruise is the point in an aircraft flight cycle where 50% of the total fuel burned between the end of climb and the beginning of descent has been burned (which may be approximated by the midpoint - in terms of time and / or distance - between the end of climb and the beginning of descent).Cruise conditions thus define an operating point of the gas turbine engine that provides thrust that would ensure steady-state operation (i.e., maintaining a constant altitude and a constant Mach number) at mid-cruise of an aircraft to which it is designed to be attached, taking into account the number of engines intended for that aircraft. For example, when an engine is designed to be attached to an aircraft equipped with two engines of the same type, under cruise conditions the engine provides half the total thrust that would be necessary for steady-state operation of that aircraft at mid-cruise. In other words, for a given gas turbine engine for an aircraft, cruise conditions are defined as the operating point of the engine that provides a specified thrust (necessary to ensure - in combination with any other engine on the aircraft - the steady-state operation of the aircraft to which it is designed to be attached at a given mid-cruise Mach Number) under mid-cruise atmospheric conditions (defined by the International Type Atmosphere in accordance with ISO 2533 at mid-cruise altitude). For any given gas turbine engine for an aircraft, the mid-cruise thrust, atmospheric conditions and Mach Number are known, and thus the operating point of the engine under cruise conditions is clearly defined. For guidance purposes only, the forward speed in cruise conditions may be any point in the range from Mach 0.7 to 0.9, for example 0.75 to 0.85, for example 0.76 to 0.84, for example 0.77 to 0.83, for example 0.78 to 0.82, for example 0.79 to 0.81, for example of the order of Mach 0.8, of the order of Mach 0.85 or in the range from 0.8 to 0.85. Any single speed within these ranges may be part of the cruise conditions. For some aircraft, the cruise conditions may be outside these ranges, for example below Mach 0.7 or above Mach 0.9. For information purposes only, cruising conditions may correspond to normal atmospheric conditions (according to the International Standard Atmosphere, ISA) at an altitude which is in the range from 10000 m to 15000 m, for example in the range from 10000 m to 12000 m, for example in the range from 10400 m to 11600 m (about 38000 ft), for example in the range from 10500 m to 11500 m, for example in the range from 10600 m to 11400 m, for example in the range from 10700 m (about 35000 ft) to 11300 m, for example in the range from 10800 m to 11200 m, for example in the range from 10900 m to 11100 m, for example in the order of 11000 m. Cruise conditions may correspond to normal atmospheric conditions at any given altitude within these ranges. For illustrative purposes only, cruise conditions may correspond to an engine operating point that provides a known required thrust level (e.g., a value in the range 30 kN to 35 kN) at a Mach number of 0.8 and normal atmospheric conditions (according to the International Type Atmosphere) at an altitude of 38,000 feet (11,582 m). For purely additional purposes, cruise conditions may correspond to an engine operating point that provides a known required thrust level (e.g., a value in the range 50 kN to 65 kN) at a Mach number of 0.85 and normal atmospheric conditions (according to the International Type Atmosphere) at an altitude of 35,000 feet (10,668 m). In use, a gas turbine engine described and / or claimed herein may operate under the cruise conditions defined elsewhere herein. Such cruise conditions may be determined by the cruise conditions (e.g., mid-cruise conditions) of an aircraft on which at least one gas turbine engine (e.g., 2 or 4) may be mounted to provide propulsive thrust. Further, one skilled in the art would readily recognize descent and / or approach to refer to a phase of operation in a flight cycle of an aircraft between cruise and landing of the aircraft. During descent and / or approach, the engine may produce between 20% and 50% of available thrust. In other examples, the engine may produce between 25% and 40% of available thrust. In still other examples, the engine may produce between 30% and 35% of available thrust.Additionally or alternatively, descent may refer to a nominal point in an aircraft flight cycle between takeoff and landing where a relative decrease in altitude is required, and which may require a reduced engine thrust demand. In one aspect, an aircraft comprising a gas turbine engine as described and / or claimed herein is provided. The aircraft in this aspect is the aircraft to which the gas turbine engine has been designed to be attached. Thus, the cruise conditions in this aspect correspond to mid-cruise of the aircraft, as defined elsewhere herein. In one aspect, a method of operating a gas turbine engine as described and / or claimed herein is provided. The operation may be in the cruise conditions as defined elsewhere herein (e.g. in terms of thrust, atmospheric conditions and Mach number). In one aspect, a method of operating an aircraft comprising a gas turbine engine as described and / or claimed herein is provided. Operation in accordance with this aspect may comprise (or may be) mid-cruise operation of the aircraft, as defined elsewhere herein. Those skilled in the art will understand that, unless mutually exclusive, any feature or parameter described in connection with any of the above aspects may be applied to any other aspect. Furthermore, unless mutually exclusive, any feature or parameter described herein may be applied to any aspect and / or combined with any other feature or parameter described herein. Embodiments will now be described by way of example only, with reference to the Figures, in which: [Fig.1] is a sectional side view of a gas turbine engine; [Fig.2] is a close-up sectional side view of an upstream portion of a gas turbine engine; [Fig.3] is a partially sectional view of a gearbox for a gas turbine engine; [Fig.4] is a schematic view of an aircraft including a fuel composition monitoring device; [Fig.5] is a schematic representation of a fuel identification process; [Fig.6] is a schematic view of an aircraft fuel composition monitoring system, in the context of a fuel supply line and an on-board tank, indicating possible use as a fuel composition determination module; [Fig.7] is a schematic view of an aircraft comprising a fuel composition determination module; [Fig.8] is a schematic representation of a method of operating an aircraft; [Fig.9] is a schematic representation of another method of operating an aircraft; [Fig.10] is a schematic view of an aircraft including a fuel management device; [Fig.11] is a schematic representation of another method of operating an aircraft; [Fig.12] is a schematic view of an aircraft fuel distribution system, in the context of a fuel tank and a gas turbine engine; [Fig.13] is a schematic representation of a process for modifying an aircraft; and [Fig.14] is a schematic view of an aircraft with a tank arrangement other than that shown in [Fig.4], [Fig.7] or [Fig.10], including a fuel management device and a trim tank; [Fig.15] is a schematic representation of another method of operating an aircraft; [Fig.16] is a schematic view of an aircraft fuel distribution system, in the context of a fuel tank, an APU and a gas turbine engine; [Fig.17] is a schematic representation of another method of modifying an aircraft; and [Fig.18] is a schematic view of an aircraft with a different tank arrangement from that shown in [Fig.14]; [Fig.19] is a schematic view of an aircraft including a fuel management device and having a tank arrangement different from that shown in [Fig.14]; [Fig.20] is a schematic representation of a method of operating an aircraft; [Fig.21] is a schematic view of an aircraft fuel distribution system, in the context of fuel tanks and a gas turbine engine; |Fig.22] is a schematic representation of a process for modifying an aircraft; [Fig.23] is a schematic representation of a method of operating an aircraft; [Fig.24] is a schematic representation of a process for modifying an aircraft; and [Fig.25] is a schematic representation of an aircraft operating process. [Fig. 1] illustrates a gas turbine engine 10 having a main rotational axis 9. The engine 10 includes an air intake 12 and a propulsion fan 23 which generates two airflows: a main airflow A and a bypass airflow B. The gas turbine engine 10 includes a core 11 which receives the [main airflow A. The engine core 11 includes, in axial flow series, a low pressure compressor 14, a high pressure compressor 15, combustion equipment 16, a high pressure turbine 17, a low pressure turbine 19 and a main exhaust nozzle 20. A nacelle 21 surrounds the gas turbine engine 10 and defines a bypass duct 22 and a bypass exhaust nozzle 18. The bypass airflow rivation B flows through the bypass duct 22. The blower 23 is attached to and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearbox 30. In use, the main air stream A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where further compression takes place. The compressed air discharged from the high pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel F and the mixture is subjected to combustion. The resulting hot combustion products then expand through, and thus drive, the high pressure and low pressure turbines 17, 19 before being discharged through the nozzle 20 to provide some propulsive thrust. The high pressure turbine 17 drives the high pressure compressor 15 by a suitable interconnecting shaft 27. The fan 23 generally provides the majority of the propulsive thrust. The epicyclic gearbox 30 is a reduction gear. An exemplary arrangement for a geared fan gas turbine engine 10 is shown in [Fig. 2]. The low-pressure turbine 19 (see [Fig. 1]) drives the shaft 26, which is coupled to a sun gear, or sun gear, 28 of the epicyclic gear arrangement 30. A plurality of planetary gears 32, which are disposed radially outwardly of the sun gear 28 and mesh therewith, are coupled together by a planet carrier 34. The planet carrier 34 constrains the planetary gears 32 to precess about the sun gear 28 in synchronism while allowing each planetary gear 32 to rotate about its own axis. The planet carrier 34 is coupled via connecting rods 36 to the fan 23 in order to cause it to rotate around the engine axis 9.A ring or crown 38 which is coupled, via connecting rods 40, to a stationary support structure 24 is disposed radially outside the planetary gears 32 and meshes with the latter. It should be noted that the terms "low pressure turbine" and "low pressure compressor" as used herein may be interpreted to mean, respectively, the lowest pressure turbine stages and the lowest pressure compressor stages (i.e., not including the fan 23) and / or the turbine and compressor stages that are interconnected by the interconnecting shaft 26 with the lowest rotational speed in the engine (i.e., not including the output shaft of the gearbox that drives the fan 23). In some publications, the terms "low pressure turbine" and "low pressure compressor" referred to herein may also be referred to as "intermediate pressure turbine" and "intermediate pressure compressor." When such alternative nomenclature is used, the fan 23 may be referred to as the first compression stage or stage lower pressure compression. The epicyclic gearbox 30 is shown in greater detail, and by way of example, in [Fig. 3]. Each of the sun gear 28, the planetary gears 32, and the ring gear 38 includes teeth on its periphery that mesh with those of the other gears. However, for clarity, only exemplary portions of the teeth are illustrated in [Fig. 3]. There are four planetary gears 32 illustrated, although it will be apparent to those skilled in the art that more or fewer planetary gears 32 may be provided within the scope of the claimed invention. Practical applications of an epicyclic planetary gearbox 30 generally include at least three planetary gears 32. The epicyclic gearbox 30 illustrated by way of example in Figures 2 and 3 is of the planetary type, in that the planet carrier 34 is coupled to an output shaft via connecting rods 36, with the ring gear 38 fixed. However, any other suitable type of epicyclic gearbox 30 may be used. As an additional example, the epicyclic gearbox 30 may be a star arrangement, in which the planet carrier 34 is held fixed, with the ring gear (or ring) 38 rotatable. In such an arrangement, the fan 23 is driven by the ring gear 38. As an additional alternative example, the gearbox 30 may be a differential gearbox in which both the ring gear 38 and the planet carrier 34 are rotatable. It will be understood that the arrangement shown in Figures 2 and 3 is given by way of example only, and that various variations are within the scope of the present disclosure. For illustrative purposes only, any suitable arrangement may be used to install the gearbox 30 in the engine 10 and / or to connect the gearbox 30 to the engine 10. As an additional example, the connections (such as the connecting rods 36, 40 in the example of [Fig. 2]) between the gearbox 30 and other parts of the engine 10 (such as the input shaft 26, the output shaft and the fixed structure 24) may have any desired degree of rigidity or flexibility.As an additional example, any suitable arrangement of bearings between rotating and stationary portions of the motor (e.g., between the input and output shafts of the gearbox and stationary structures, such as the gearbox housing) may be used, and the disclosure is not limited to the particular arrangement of [Fig. 2]. For example, where the gearbox 30 has a star arrangement (described above), those skilled in the art will readily understand that the arrangement of the output and support rods and the bearing locations will typically be different from those shown by way of example in [Fig. 2]. Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of gearbox types (e.g., radial or planetary), support structures, input and output shaft arrangements, and bearing locations. Optionally, the gearbox can drive additional and / or alternative components (e.g. the intermediate pressure compressor and / or a precompressor). Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may have an alternative number of compressors and / or turbines and / or an alternative number of interconnecting shafts. As an additional example, the gas turbine engine shown in [Fig. 1] has a split-flow nozzle 18, 20, i.e., the flow through the bypass duct 22 has its own nozzle 18 which is separate from and radially outboard of the main engine nozzle 20. However, this is not limiting, and any aspect of the present disclosure may also be applied to engines in which the flow through the bypass duct 22 and the flow through the core 11 are mixed, or combined, before (or upstream of) a single nozzle, which may be referred to as a mixed-flow nozzle.One or both nozzles (whether a mixed-flow or split-flow nozzle) may have a fixed or variable area. Although the described example relates to a turbofan engine, the disclosure may apply, for example, to any type of gas turbine engine, such as, for example, an unducted rotor engine (whose fan stage is not surrounded by a nacelle) or a turboprop. In some arrangements, the gas turbine engine 10 may not include a gearbox 30. The geometry of the gas turbine engine 10, and its components, is defined by a conventional axis system, comprising an axial direction (which is aligned with the axis of rotation 9), a radial direction (in the bottom-up direction in [Fig. 1]) and a circumferential direction (perpendicular to the page in the view of [Fig. 1]). The axial, radial and circumferential directions are perpendicular to each other. The fuel F supplied to the combustion equipment 16 may comprise a fossil hydrocarbon-based fuel, such as kerosene. Thus, the fuel F may comprise molecules from one or more of the chemical families of n-alkanes, iso-alkanes, cycloalkanes, and aromatic compounds. Additionally or alternatively, the fuel F may comprise renewable hydrocarbons produced from biological or non-biological resources, otherwise known as sustainable aviation fuel (SAF). In each of the examples provided, the fuel F may comprise one or more trace elements including, for example, sulfur, nitrogen, oxygen, inorganic materials, and metals. The functional performance of a given composition, or of a mixture of fuels to be used in the context of a given mission, can be defined, at at least in part, by the fuel's ability to support the Brayton cycle of the gas turbine engine 10. Parameters defining functional performance may include, for example, specific energy; energy density; thermal stability; and particulate matter-containing emissions. A relatively higher specific energy (i.e., energy per unit mass), expressed in MJ / kg, may at least partially reduce takeoff weight, thereby potentially providing a relative improvement in fuel efficiency. A relatively higher energy density (i.e., energy per unit volume), expressed in MJ / L, may at least partially reduce takeoff fuel volume, which may be particularly important for volume-limited missions or military operations involving refueling.Relatively higher thermal stability (i.e., the ability to inhibit fuel degradation or coking under thermal stress) can enable the fuel to withstand high temperatures in the engine and fuel injectors, potentially providing relative improvements in combustion efficiency. Reduced emissions, containing particulate matter, can reduce contrail formation, while reducing the environmental impact of a given mission. Other fuel properties can also be critical to operational performance.For example, a relatively lower freezing point (°C) may allow long-range missions to optimize flight profiles; minimum aromatic hydrocarbon concentrations (%) may ensure sufficient swelling of certain materials used in the construction of O-rings and seals that have been previously exposed to high aromatic fuels; and, a maximum surface tension (MN / m) may ensure sufficient spray breakup and atomization of the fuel. The ratio of the number of hydrogen atoms to the number of carbon atoms in a molecule can influence the specific energy of a given fuel composition or mixture. Fuels with higher ratios of hydrogen atoms to carbon atoms may have higher specific energies in the absence of bonding constraints. For example, fossil hydrocarbon fuels may comprise molecules with approximately 7 to 18 carbons, with a significant portion of a given composition derived from molecules with 9 to 15 carbons, with an average of 12 carbons. ASTM International (ASTM) D7566, Standard Specification for Aviation Turbine Fuel Containing Synthesized Hydrocarbons (ASTM 2019c), approves a number of sustainable aviation fuel blends comprising between 10% and 50% sustainable aviation fuel (the remainder comprising a or several fossil hydrocarbon fuels, such as kerosene), with other compositions pending approval. However, the aviation industry anticipates that sustainable aviation fuel blends comprising up to (including) 100% sustainable aviation fuel (SAF) will eventually be approved for use. Sustainable aviation fuels may comprise one or more of n-alkanes, iso-alkanes, cyclo-alkanes and aromatics, and may be produced, for example, from one or more of synthesis gas (syngas); lipids (e.g. fats, oils and greases); sugars; and alcohols. Thus, sustainable aviation fuels may comprise a lower content of aromatic hydrocarbons and / or sulfur, compared to fossil hydrocarbon-based fuels. Additionally or alternatively, sustainable aviation fuels may comprise a higher content of iso-alkanes and / or cyclo-alkanes, compared to fossil hydrocarbon-based fuels. Thus, in some examples, sustainable aviation fuels may comprise a density of between 90% and 98% of that of kerosene and / or a calorific value of between 101% and 105% of that of kerosene. Due, at least in part, to the molecular structure of sustainable aviation fuels, sustainable aviation fuels may provide benefits including, for example, one or more of higher energy density; higher specific energy; higher specific heat capacity; higher thermal stability; higher lubricity; lower viscosity; lower surface tension; lower freezing point; lower soot emissions; and lower CO2 emissions, compared to fossil hydrocarbon fuels (e.g., upon combustion in combustion equipment 16). Therefore, compared to fossil hydrocarbon fuels, such as kerosene, sustainable aviation fuels may result in a relative decrease in specific fuel consumption and / or a relative decrease in maintenance costs. As shown in Figures 4 and 7, an aircraft 1 may include multiple fuel tanks 50, 53; for example, a larger primary fuel tank 50 located in the aircraft fuselage, and a smaller fuel tank 53a, 53b located in each wing. In other examples, an aircraft 1 may have only one fuel tank 50, and / or the wing fuel tanks 53 may be larger than the center fuel tank 50, or no center fuel tank may be provided (all fuel being stored in this case in the wings of the aircraft) - it will be understood that many different tank configurations are contemplated and that the illustrated examples are provided for ease of description and not are not intended to be limiting. [Fig.4] and [Fig.7] show an aircraft 1 with a propulsion system 2 comprising two gas turbine engines 10. The gas turbine engines 10 are supplied with fuel from a fuel supply system on board the aircraft 1. The fuel supply system of the illustrated examples comprises a single fuel source. For the purposes of this application, the term "fuel source" means either 1) a single fuel tank or 2) a plurality of fuel tanks that are fluidly interconnected. Each fuel source is arranged to provide a separate fuel source, i.e., a first fuel source may contain a first fuel having a different characteristic or characteristics than a second fuel contained in a second fuel source. The first and second fuel sources are therefore not fluidly coupled to each other so as to separate the different fuels (at least under normal operating conditions). In the present examples, the first (and, in these examples, the only) fuel source comprises a center fuel tank 50, located primarily in the fuselage of the aircraft 1 and a plurality of wing fuel tanks 53a, 53b, where at least one wing fuel tank is located in the left wing and at least one wing fuel tank is located in the right wing for balancing. All of the tanks 50, 53 are fluidically interconnected in the illustrated example, thereby forming a single fuel source. Each of the center fuel tank 50 and the wing fuel tanks 53 may comprise a plurality of fluidically interconnected fuel tanks. In another example, the wing fuel tanks 53a, 53b may not be in fluid communication with the center tank 50, thereby forming a second, separate fuel source. For balancing purposes, one or more fuel tanks in the left wing may be in fluid communication with the one or more fuel tanks in the right wing. This may be accomplished via a center fuel tank (if that tank is not part of the other fuel source), or by bypassing the center fuel tank(s), or both (for maximum flexibility and safety). In another example, the first fuel source comprises wing fuel tanks 53 and a center fuel tank 50, while a second fuel source comprises another separate center fuel tank. A fluid interconnection between wing fuel tanks and the center fuel tank of the first fuel source may be provided for balancing the aircraft 1. In some examples, the distribution of fuel tanks 50, 53 available on the aircraft 1 may be constrained such that the first fuel source and the second fuel source are each substantially symmetrical about the aircraft axis. In cases where an asymmetric distribution of fuel tanks is permitted, a suitable means for transferring fuel is generally provided between fuel tanks of the first fuel source and / or between fuel tanks of the second fuel source such that the position of the center of mass of the aircraft can be maintained within acceptable lateral limits throughout the flight. An aircraft 1 may be refueled by connecting a fuel storage container 60, such as one provided by an airport tanker truck, or a permanent pipeline, to a fuel line connection port 62 of the aircraft, via a fuel line 61. A desired amount of fuel may be transferred from the fuel storage container 60 to the one or more tanks 50, 53 of the aircraft 1. Particularly in examples with more than one fuel source, in which different tanks 50, 53 are to be filled with different fuels, multiple fuel line connection ports 62 may be provided instead of one, and / or valves may be used to direct the fuel appropriately. Aircraft typically refuel at several different airports, for example, at the beginning and end of a long-distance flight. While there are standards that all aviation fuels must comply with, as mentioned above, different aviation fuels have different compositions, for example, depending on their source (e.g., different petroleum sources, biofuels, or other synthetic aviation fuels (often described as sustainable aviation fuels - SAFs) and / or blends of petroleum-based fuels and other fuels) and any additives included (e.g., such as antioxidants and metal catalysis inhibitors, biocides, static reducers, icing inhibitors, corrosion inhibitors) and any impurities.In addition to varying from airport to airport and fuel supplier to fuel supplier, the fuel composition of available aviation fuel may vary from batch to batch, even for a given airport or fuel supplier. Furthermore, the fuel tanks 50, 53 of the aircraft 1 are generally not emptied before being refilled for a subsequent flight, resulting in mixtures of different fuels in the tanks - fuel of different composition effectively resulting from the mixture. The inventors appreciated that since different fuels may have different properties, yet still comply with standards, knowledge of the or of the fuel(s) available to an aircraft 1 may enable more efficient and adaptive control of the aircraft 1, and more particularly of the propulsion system 2 of the aircraft (i.e. the one or more gas turbine engines 10 of the aircraft 1, and associated controls and components). Knowledge of the fuel may therefore be used as a tool to improve aircraft performance, so that monitoring the fuel composition may have benefits. In various examples, an active infinite summation approach may be adopted to track the variation in fuel composition of a fuel in a fuel tank 50, 53 over time after multiple refills. For this approach, it is assumed that all aviation fuels are fully miscible, and that a homogeneous mixture is formed in a fuel tank 50, 53 at least during aircraft operation (separation of fuels in the tank due to differences in densities may be observed in static tanks when a less dense fuel is added on top of a denser fuel, but such separation is not expected to persist in flight as tank motion and system vibration will induce mixing). A record may be maintained for each fuel tank in examples where the aircraft 1 has multiple fuel tanks 50, 53. Such an approach includes obtaining one or more fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling. As used herein, the term "fuel characteristics" refers to intrinsic or inherent properties of the fuel such as fuel composition, and not to variable properties such as volume or temperature. Examples of fuel characteristics include one or more of: i the percentage of sustainable aviation fuel (SAF) in the fuel, or an indication that the fuel is a fossil fuel, e.g. fossil kerosene, or that the fuel is pure SAF fuel; li. the parameters of a fuel hydrocarbon distribution, such as: * the aromatic hydrocarbon content of the fuel, and possibly also / alternatively the multi-aromatic hydrocarbon content of the fuel; * the hydrogen to carbon (H / C) ratio of the fuel; » information on the % composition of some or all of the hydrocarbons present; iii, the presence or percentage of a particular element or species, such as: * the percentage of nitrogen-containing species in the fuel; * the presence or percentage of a trace species or trace element in fuel; * the naphthalene content of the fuel; * the sulfur content of the fuel: * the cycloparaffin content of the fuel; * the oxygen content of the fuel; iv, one or more properties of the fuel when used in a gas turbine engine 10, such as: * the level of non-volatile particulate matter (nvPM) emissions or CO2 emissions during combustion; * the fuel coking level: v. one or more properties of the fuel itself, independent of its use in an engine 10 or of its combustion, such as: * the thermal stability of the fuel (e.g. thermal degradation temperature); and “one or more physical properties such as density, viscosity, calorific value, freezing temperature and / or heat capacity. The fuel characteristics to be monitored can be selected based on the fuel properties most relevant to the changes that may be made to the propulsion system 2. Obtaining the fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling may include one or more of: (i) the physical and / or chemical detection of one or more characteristics or parameters of the composition of the fuel already present in the fuel tank 50, 53 (this may allow direct detection of the fuel characteristics, and / or may allow the fuel characteristics to be determined from the detection results), and / or the detection of one or more trace elements or compounds added to the fuel to facilitate its identification (e.g. a dye); (ii) obtaining the result of a prior determination made using an active infinite summation approach as described herein, for example by retrieving one or more fuel characteristic values ​​from a local data store on board the aircraft 1; (iii) receiving data, for example from input provided at a user interface, or from data transmitted to the aircraft 1. In some examples, several different methods may be performed to obtain the fuel characteristics - for example, different methods may be used for different characteristics, and / or different methods may be used for the same characteristic as a control. In some examples, obtaining the one or more fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling may comprise obtaining stored fuel characteristic data, and chemically or physically detecting one or more parameters of any fuel already present in the fuel tank 50, 53 prior to refueling, and comparing these to the stored fuel characteristic data. The input to the calculation step described below may therefore be checked based on the one or more detected parameters. If there is a mismatch between the stored fuel characteristic and the corresponding detected parameter, an alert may be provided. As mentioned above, for this approach, it is generally assumed that the fuels are perfectly miscible, forming a homogeneous mixture in the tanks 50, 53. However, if there is a possibility of imperfect mixing of fuels in the tank 50, 53 (e.g., after a long period of no movement for a fuel mixture known to contain fuels of different densities), the composition of fuel exiting the fuel tank 50, 53 on its way to the engine 10, 44 can be examined.If the measured, calculated, or otherwise determined fuel characteristics of the fuel leaving the tank 50, 53 differ from those of the expected homogeneous mixture in the tank 50, 53, a possible imperfect mixing problem may be flagged in certain scenarios (e.g., if there is a significant density difference between the fuel already in the tank and the newly added fuel, which could result in stratification) instead of, or in addition to, flagging possible misunderstandings of the overall tank contents. Fuel characteristics may be detected in various ways, both directly (e.g., from sensor data corresponding to the fuel characteristic in question) and indirectly (e.g., by inference or calculation from other characteristics or measurements, or by reference to data from a specific detected tracer in the fuel). The characteristics may be determined as relative values ​​with respect to another fuel, or as absolute values. For example, one or more of the following detection methods may be used: » The aromatic hydrocarbon or cycloparaffin content of the fuel can be determined based on measurements of the swelling of a sensor component made from a sealing material such as a nitrile sealing material. + Trace substances or species, either naturally occurring in the fuel or added to act as a tracer, can be used to determine fuel characteristics such as the percentage of sustainable aviation fuel in the fuel or whether the fuel is kerosene. * Measurements of the vibrational mode of a piezoelectric crystal exposed to fuel can be used as a basis for determining various fuel characteristics, including the aromatic content of the fuel, the oxygen content of the fuel, and the thermal stability or coking level of the fuel - for example, by measuring the accumulation of surface deposits on the piezoelectric crystal that will cause a change in vibrational mode. “Various fuel characteristics may be determined by collecting performance parameters of the gas turbine engine 10 during a first operating period (such as during takeoff), and possibly also during a second operating period (e.g., during cruise), and comparing these collected parameters to values ​​expected when using fuel of known properties. “Various fuel characteristics, including the aromatic hydrocarbon content of the fuel, may be determined based on sensor measurements of the presence, absence, or degree of condensation trail formation by the gas turbine 10 during operation. * Fuel characteristics, including aromatic hydrocarbon content, can be determined based on UV-Vis spectroscopy measurement of the fuel. “Various fuel characteristics including sulfur content, naphthalene content, aromatic hydrocarbon content, and hydrogen to carbon ratio can be determined by measuring the substances present in the exhaust gases emitted by the gas turbine engine 10 during its operation. * The calorific value of the fuel may be determined, during operation of the aircraft 1, on the basis of measurements taken as the fuel is burned - for example using the fuel flow rate and shaft speed or the temperature change across the combustion chamber 16. * Various fuel characteristics may be determined by performing an operational change arranged to affect operation of the gas turbine engine 10, detecting a response to the operational change; anddetermining the one or more fuel characteristics of the fuel based on the response to the operational change. » Various fuel characteristics may be determined in relation to fuel characteristics of a first fuel by changing a fuel supplied to the gas turbine engine 10 from the first fuel to a second fuel, and determining the one or more fuel characteristics of the second fuel based on a change in a relationship between T30 and one of T40 and T41 (the relationship indicating the temperature rise across the combustion chamber 16). The ca- characteristics may be determined as relative values ​​with respect to the first fuel, or as absolute values, for example by reference to known values ​​for the first fuel. (As used herein, T30, T40, and T41, and all other numbered pressures and temperatures, are defined using the position numbering listed in SAE AS755, specifically: * T30 = Total Temperature at the High Pressure Compressor (HPC) Outlet; * T40 = Total Temperature at the Combustion Chamber Outlet; * T41 = Total Temperature at the Inlet of the High Pressure Turbine (HPT) Rotor. In the examples currently described, the quantity of fuel present in the tank 50, 53 before refueling (e.g., mass, volume and / or % full) is also noted, e.g., automatically detected and recorded in a computer storage / memory on board the aircraft 1. In addition to obtaining one or more fuel characteristics of any fuel already present in the fuel tank 50, 53 prior to refueling, one or more fuel characteristics of a fuel to be added to the fuel tank 50, 53 upon refueling are also obtained. Obtaining fuel characteristics of the fuel to be added to the fuel tank 50, 53 during refueling may include one or more of: (i) the physical and / or chemical detection of one or more characteristics of the fuel composition (e.g. in a workshop test unit, or when the fuel is transported to an on-site fuel tank, or in use in the gas turbine engine 10), thereby enabling direct detection of the fuel characteristics or providing data from which they can be determined, as mentioned above, and / or (ii) the detection of one or more tracer elements or compounds added to the fuel to facilitate its identification (e.g. a dye); (iii) receiving data, for example from an input provided at a user interface, or data transmitted to the aircraft, for example by scanning a barcode associated with fuel dispensing. With respect to obtaining fuel characteristics of a fuel already present in the tank 50, 53, in some examples, several different methods may be performed to obtain the fuel characteristics of the resulting fuel mixture—for example, different methods may be used for different characteristics, and / or different methods may be used for the same characteristic as a control. Any suitable detection method as mentioned above may be used. Assuming that the uniform mixture assumption applies, the fuel characteristics of a fuel leaving the tank 50, 53 and entering the engine 10 can be determined as a means of validating the 2006 calculation result of the fuel characteristics of the mixture. In the presently described examples, an amount of fuel added to the tank 50, 53 during refueling (e.g., explicitly by added mass, or added volume, and / or implicitly by change in mass, volume or % fill) is also noted (e.g., automatically detected, or provided by a fuel supplier, and recorded in a computer storage / memory on board the aircraft 1). One or more fuel characteristics of the resulting fuel in the fuel tank 50, 53 after refueling are then calculated using the data from any fuel initially in the tank, and the data from the fuel added to the tank. These calculations may be performed for each fuel source separately - for example, a first fuel source may be empty before refueling and therefore contain only the new fuel, and a second fuel source may not be empty before refueling and may contain a mixture of the old and new fuel after refueling.In such cases, burning a fuel from the first fuel source in the gas turbine engine 10 may allow the fuel characteristics of the new fuel to be determined, and the fuel characteristics of the mixture in the second fuel source may then be calculated using the determined characteristics for the new fuel, the mixture percentage, and the data from the old fuel. Current fuel characteristics data of the fuel in the tank may be stored, updating the recorded fuel characteristics of the fuel present in the fuel tank 50, 53 after each refueling of the aircraft 1. Alternatively, a continuous record of the fuel compositions used over time may be maintained; alternatively, only a current fuel composition may be stored. A fuel composition tracking device 202 may be used to record and store fuel composition data, and optionally also to receive the fuel characteristics of the fuel to be added during refueling, and calculate updated fuel composition data. The fuel composition tracking device 202 may be provided as a separate fuel composition tracking unit 202, as shown in Figures 4 and 6, or as a module integrated into the propulsion system 2, and / or as software and / or hardware incorporated into pre-existing aircraft control systems, for example as part of an electronic engine governor (EEC) VA In the example shown, two sensors 204a, 204b are provided, each arranged to physically and / or chemically detect one or more characteristics of the composition of the fuel added to the fuel tank 50, 53 during refueling. The sensors 204 and the fuel composition tracking device 202 may be described together as a fuel composition tracking system 203, as shown in [Fig. 6]. In alternative examples, no such sensor 204 may be provided (e.g., a barcode associated with the fuel storage container 60 may be read and the corresponding fuel data provided to the fuel composition tracking device 202), or more or fewer sensors may be provided. Some examples may further include chemically or physically detecting one or more parameters of the resulting fuel in the fuel tank 50, 53 after refueling. The detected parameters may then be compared to one or more of the calculated fuel characteristics to verify the result. If there is a mismatch between the calculated fuel characteristic and the corresponding detected parameter, an alert may be provided. An active infinite summation approach as described herein could be used continuously throughout the life of an aircraft 1 (or between services during which a fuel tank 50, 53 may be drained). However, it may be advantageous to establish a new baseline for fuel composition data at intervals. Establishing a new baseline may include chemically and / or physically determining one or more parameters of the fuel in the fuel tank 50, 53, and using the determined values ​​to replace the stored fuel characteristics for the fuel in the fuel tank. In some examples, the chemical and / or physical determination of one or more parameters of the fuel in the fuel tank 50, 53 for establishing a new baseline may be performed by extracting a sample of the fuel from the fuel tank for shop testing; for example, it may be sent to a laboratory for analysis or provided to an available ground testing station at an airport. In other examples, on-site and possibly in-situ testing methods may be used. Establishing a new baseline (i.e., chemically and / or physically determining one or more parameters of the fuel in the fuel tank 50, 53 and using the determined values ​​to replace the stored fuel characteristics for the fuel in the fuel tank) may be performed in response to a triggering event. A triggering event may be a threshold time elapsed since a previous (chemical and / or physical) determination of the or more fuel parameters in the fuel tank, or a threshold number of refueling events and / or flights reached since a previous (chemical and / or physical) determination of the one or more fuel parameters in the fuel tank. Additionally or alternatively, a triggering event may be the detection of a deviation between one or more of the calculated characteristics and a detected parameter - for example, when a calculated value differs from a detected value by an amount exceeding a threshold or tolerance value. In some examples, an alert is provided (e.g., an audible and / or visual alarm, and / or a message sent to the pilot or other person) upon detection of such a deviation - a decision may then be made as to whether to establish a new baseline immediately, or to accept that the uncertainty in the fuel composition may mean that it is impossible to operate in the most efficient manner for the next flight(s) until an opportunity to establish a new baseline arises.Intelligent control of the propulsion system 2 based on fuel characteristics can be disabled until the next re-reference event. Once the one or more fuel characteristics of the resulting fuel in the fuel tank 50, 53 after refueling have been determined, the propulsion system 2 may be controlled based on the calculated fuel characteristics. For example : * An operating parameter of an aircraft thermal management system (e.g., a fuel-oil heat exchanger) may be changed, or the temperature of fuel supplied to the combustion chamber 16 of the engine 10 may be changed. * When more than one fuel is stored on board an aircraft 1, the selection of which fuel to use for which operation (e.g., for ground operations as opposed to flight, for low temperature start-up, or for operations with different thrust demands) can be made on the basis of fuel characteristics such as % SAF, nvPM generation potential, viscosity, and calorific value. A fuel delivery system can therefore be appropriately controlled on the basis of the fuel characteristics. * One or more control surfaces of aircraft 1 may be adjusted to change route and / or altitude based on fuel knowledge. The spillage percentage of a fuel pump (i.e., the proportion of pumped fuel recirculated instead of being passed to the fuel chamber) combustion) can be changed, for example based on the SAF% of the fuel. The pump and / or one or more valves can therefore be controlled appropriately based on the fuel characteristics. “Changes to the variable inlet guide vane (VIGV) schedule can be made based on fuel characteristics. VIGVs can therefore be moved, or a VIGV move can be overridden, as appropriate based on fuel characteristics. A propulsion system 2 for an aircraft may therefore comprise a fuel composition monitoring system 203 arranged to: storing 2002 current fuel characteristic data, the fuel characteristic data comprising one or more fuel characteristics of a fuel present in the fuel tank 50, 53; obtaining 2004 one or more fuel characteristics of a fuel added to the fuel tank 50, 53 during refueling; and calculate 2006 updated values ​​for the one or more fuel characteristics of the fuel in the fuel tank 50, 53 after refueling. Obtaining 2004 the one or more fuel characteristics of a fuel added to the fuel tank 50, 53 during refueling may be performed before, during, or after the refueling itself, for example, by using a shop test unit, fuel line sensors, or gas turbine engine performance sensors, respectively, or by receiving data electronically from a third party. [Fig. 6] shows an example of a fuel composition tracking system 203, in the context of a refueling event in which a fuel F is supplied to a fuel tank 50, 53. The arrows with dotted lines in [Fig. 6] indicate fuel flow, while the solid lines indicate electronic communication. The fuel composition tracking system 203 includes a fuel composition tracking device 202. The fuel composition tracking device 202 of the described example includes a memory 202a (which may also be referred to as computer storage) arranged to store the current fuel characteristics data, and processing circuitry 202c arranged to calculate updated values ​​for the one or more fuel characteristics of the fuel in the fuel tank 50, 53 after refueling. The calculated values ​​may then replace the fuel characteristics data previously stored in the memory, and / or may be time-stamped and / or dated and added to the memory. A log of fuel characteristics data over time may therefore be established. The fuel composition monitoring device 202 of the illustrated example also includes a receiver 202b arranged to receive data relating to a fuel composition and / or requests for fuel composition information. The fuel composition monitoring device 202 of the illustrated example is part of, or in communication with, an electronic engine governor (EEC) 42. The EEC 42 may be arranged to issue propulsion system control commands based on the calculated fuel characteristics. It will be appreciated that an EEC 42 may be provided for each gas turbine engine 10 of the aircraft 1, and that the role played by the EEC in or for the fuel composition monitoring device 202 may represent only a small part of the functionality of the EEC.Indeed, the fuel composition tracking device 202 may be provided by the EEC, or may include an EEC module separate from the engine's EEC 42 in various implementations. In alternative examples, the fuel composition tracking device 202 may not include any engine control functionality, and may instead simply provide fuel composition data on demand, to be used as appropriate by another system. Optionally, the fuel composition tracking device 202 may provide a proposed change to the engine control functionality for approval by a pilot; the pilot may then directly implement the proposed change, or approve or reject automatic completion of the proposed change. The executed method 2000 is illustrated in |Fig.5]. In step 2002, current fuel characteristic data, comprising one or more fuel characteristics of a fuel present in the fuel tank 50, 53, is stored, possibly in a memory of a fuel composition tracking device 202. This data may be provided for storage in any suitable manner, e.g., by manual entry, e.g., via a graphical user interface in communication with the fuel composition tracking device 202, communicated electronically to the fuel composition tracking device 202, e.g., by wired or wireless communication from a barcode reader following reading of a barcode (or, equivalently, any other type of optically readable or otherwise readable code, such as a QR code), and / or determined from sensor data.It will be understood that, if the reservoir 50, 53 is currently empty, zero or equivalent values ​​may be stored. In step 2004, the characteristics of a fuel added to the fuel tank 50, 53 during refueling are determined. The determination may be performed by the fuel composition tracking device 202 itself, for example by interpreting sensor data, or determined fuel characteristics elsewhere may be provided to the fuel composition monitoring device 202. In step 2006, updated values ​​for the one or more fuel characteristics of the fuel in the fuel tank 50, 53 after refueling are calculated, optionally by the fuel composition tracking device 202, using the stored fuel characteristics data (if any / if not zero) and the obtained fuel characteristics. Method 2000 may then be iterated on or after each refueling event, returning to step 2002, with the updated values ​​replacing the stored values ​​(or being added to the storage as part of a log), and proceeding accordingly. The method 2000 may further comprise controlling 2008 a propulsion system 2 of an aircraft 1 based on the one or more calculated fuel characteristics of the resulting fuel in the fuel tank 50, 53 after refueling. Updated values ​​may be used to influence this control after each refueling event. The control 2008 may be performed automatically in response to the determination of the fuel properties, or upon approval by a pilot, following notification to the pilot of a proposed change. In some examples, the same method 2000 may include automatically performing certain changes, and requesting others, depending on the nature of the change.In particular, changes that are “transparent” to the pilot—such as internal changes to engine flow rates that do not affect engine power output and would not be noticed by a pilot—may be made automatically, while any changes that the pilot would notice may be notified to the pilot (i.e., the appearance of a notification that indicates the change will occur unless the pilot decides otherwise) or suggested to the pilot (i.e., the change will not occur without positive input from the pilot). In implementations in which a notification or suggestion is provided to a pilot, it may be provided on an aircraft cockpit display and / or as an audible alarm, and / or sent to a separate device such as a handheld tablet or other computing device. In examples in which an aircraft | has multiple fuel tanks 50, 53 that are all fluidly connected such that the fuels in all the tanks 50, 53 are equivalent, a single set of fuel composition data may be stored and updated. In examples in which an aircraft 1 has multiple fuel tanks 50, 53 that are not fluidly connected, such that there may be differences between the fuels in the different tanks 50, 53, a separate set of fuel composition data may be stored and updated for each tank. In such cases, a determination of the fuel supplied to the gas turbine engine 10 may be performed prior to the making or suggestion of any propulsion system control change. The inventors also appreciated that, since different fuels may have different properties, yet still comply with standards, knowledge of the fuel(s) available to an aircraft 1 may allow for more efficient and tailored control of the flight profile. For example, a fuel with a higher hydrogen to carbon ratio may allow for contrail formation at higher threshold temperatures / lower altitudes, and a choice may be made to fly at a slightly higher altitude (e.g., 100 m to 200 m higher), or to move to an adjacent separate flight level (typically separated by a vertical distance of 2000 feet under current policies) to compensate for the otherwise increased contrail formation.Additionally or alternatively, a different route may be selected to travel in slightly warmer or less humid air to reduce contrail formation. Fuel knowledge may therefore be used as a tool to improve aircraft performance, for example by reducing contrail formation. It will be noted that contrail formation is described herein by way of example only, and is not intended to be limiting. Prior knowledge of the fuel for a flight of an aircraft 1 may therefore be used for the prior planning and adaptation of flight profile details, thereby improving environmental outcomes and / or aircraft performance. One or more fuel characteristics of a fuel arranged to be supplied to a gas turbine engine 10 of an aircraft may therefore be determined. The fuel characteristics may include one or more of the fuel characteristics listed above. The determination can be made in many different ways. For example: "a bar code of a fuel to be added to a fuel tank 50, 53 of the aircraft 1 may be scanned to read the fuel data, or a tracer substance (e.g., a dye) is identified and the fuel properties are searched based on this tracer; * data can be entered manually, or transmitted to the aircraft |; * a fuel sample may be extracted for analysis on the ground before takeoff; * fuel properties may be inferred from measurements of propulsion system activity during one or more periods of aircraft operation, e.g. engine warm-up (including any engine use prior to aircraft movement!), taxiing, take-off, climb and / or cruise; and / or “One or more fuel properties may be detected in flight, for example using in-line sensors and / or other measurements. Fuel characteristics can therefore be detected chemically and / or physically, determined from other detected data, or determined otherwise. In some examples, combinations of these techniques may be used to determine and / or verify one or more fuel characteristics, for example, using one or more of the example detection techniques described above. In some examples, such as those shown in Figures 4 and 7 described above, the aircraft 1 may have only a single fuel tank 50 (which may be in the form of a pair of connected wing tanks rather than a center tank), and / or may have multiple fuel tanks 50, 53 that each contain the same fuel, and / or are fluidly connected, or are in fluid communication with the gas turbine engine 10, such that only one type of fuel is supplied to the gas turbine engine 10 between refueling events—i.e., the fuel characteristics may remain constant throughout a flight. In other examples, the aircraft 1 may have a plurality of fuel tanks 50, 53 that contain fuels of different compositions, and the propulsion system 2 may include an adjustable fuel distribution system, allowing selection of which tank(s), and thus which fuel / fuel blend, to use. In such examples, fuel characteristics may vary during a flight, and a specific fuel or fuel blend may be selected to improve operation at certain stages of flight or under certain external conditions. Once one or more fuel characteristics have been determined, a flight profile can be selected, changed, or adjusted based on those fuel characteristics. In many examples, external data—for example, weather data such as humidity and temperature data, and time data such as day and night—can be used in combination with the determined fuel characteristics to select or adjust the flight profile. For example, the implemented method may include receiving forecast weather conditions for a planned route of the aircraft 1. These received forecast weather conditions may be used to effect or influence planned route and / or altitude changes, or used to guide the planning of the route and / or altitude. As used herein, the term "flight profile" refers to the operational characteristics (e.g., height / altitude, power setting, flight path angle, airspeed, etc.) of an aircraft 1 while flying along a flight route, as well as the flight path / route (route) itself. Route changes are therefore included in the term "flight profile" as used herein. In examples where some or all of the fuel characteristics are derived from measurements of propulsion system activity during early stages of aircraft operation, e.g., engine start / warm-up, taxi, takeoff, and climb, or otherwise measured on-site, the flight profile during cruise may be adjusted even if knowledge of the fuel characteristics is not available in time to guide the flight profile in the early stages of operation. In examples where some or all of the fuel characteristics are determined prior to takeoff (e.g., during refueling, or by analysis of the propulsion system 2 during taxi), the flight profile for takeoff and / or climb may also be adjusted; for example, a takeoff time, direction, and / or climb gradient may be selected to avoid regions or periods of high humidity. In any case, the future route and / or operational characteristics of the aircraft 1 can be adjusted based on the determined fuel characteristics — pre-planning of how to control the aircraft 1 based on the available fuel, and in particular a specific flight path (in particular, route and altitude), can therefore be carried out. A propulsion system 2 for an aircraft 1 may therefore comprise a fuel composition determination module 210 arranged to determine 2052 one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine 10; and a flight profile adjustment device 212 arranged to propose or initiate a change to the planned flight profile based on the one or more fuel characteristics of the fuel. In some examples, fuel characteristics may be determined some time before the start of a flight and planned changes to the flight profile may then be proposed to a pilot and / or an air traffic controller or other authority (possibly via an automated system), in order to obtain approval of the changes before takeoff. In other examples, the changes made may be sufficiently minor that they do not require approval from air traffic control, or even the pilot, and may be implemented automatically. An automated notification or proposal of the change may be provided to air traffic control and / or the pilot, as appropriate. The notification or proposal may be provided on a cockpit display of the aircraft 1, and / or sent to a separate device such as a portable tablet or other computing device. If the fuel composition is known sufficiently in advance of the flight, one or more competent authorities with whom a flight plan has been filed may also be informed of a change in the flight plan, or a new flight plan may be filed with them. Examples in which the fuel composition may be known with certainty well in advance of a flight might include cases where an aircraft 1 carries sufficient fuel for its current flight from a first airport to a second, and for its next scheduled flight (or return to the first airport) from the second airport. A decision to carry excess fuel, rather than refuel at the second airport, may be made to allow a quick turnaround at the second airport or to avoid high fuel prices at the second airport, or if an available fuel composition at the second airport is undesirable. Thus, after fuel loading at the first airport, any changes that might be necessary for the second flight from the second airport may be determined before or during the first flight, and a filed flight plan for the second flight may be replaced or adjusted accordingly. Different locations and types of fuel composition determination module 210 may be used depending on when, where, and how a fuel composition is to be determined. For example, a fuel composition may be determined for a fuel when it first enters the aircraft 1, via a fuel line connection port 62, by a fuel composition determination module 210a possibly located along a fuel supply line in the aircraft 1, extending from the fuel line connection port 62 to a tank 50, 53.It will be understood that if fuel composition data is provided to the aircraft 1, manually or electronically, during refueling, no detection or measurement of the incoming fuel will be necessary and the fuel composition determination module 210a may be located at a convenient location and arranged to receive this data. Alternatively (or additionally), a fuel composition may be determined for a fuel in a tank 50, 53, by a fuel composition determination module 210b possibly located in or near a tank 50, 53 within the aircraft 1. Alternatively (or additionally), a fuel composition may be determined for a fuel approaching a combustion chamber 16 of the gas turbine engine 10, by a fuel composition determination module 210c possibly located near a conduit. fuel supply line from the fuel tank 50, 53 to the combustion chamber 16, or using any other approach described above. In some examples, one or more fuel characteristic sensors may be provided integrally with processing circuitry and / or memory of the fuel composition determination module 210. In alternative examples, one or more fuel characteristic sensors may be located remotely from, and in communication with, processing circuitry and / or memory of the fuel composition determination module 210. In alternative or additional examples, one or more fuel characteristics may be communicated to the fuel composition determination module 210, which in such cases may not include one or more sensors.The location of the processing circuitry and / or memory of the fuel composition determination module 210 may vary accordingly. It will be understood that, although [Fig. 7] shows three fuel composition determination modules 210a, b, c, these are intended to demonstrate possible locations only - a single fuel composition determination module 210 may be provided in other examples. A second fuel composition determination module 210 may be provided in some examples, to provide redundant control, and / or to determine a fuel composition of a different fuel source, in examples in which an aircraft 1 includes one or more fuel tanks 50, 53 that are not fluidly connected. In some examples, a fuel composition may be detected when the aircraft 1 is refueled. The fuel composition determination module 210a in such examples may be or include a fuel composition tracking device 202. In such examples, a fuel composition tracking device 202 as shown in [Fig. 6] may be used to record and store fuel composition data, and optionally also to receive the fuel characteristics of the fuel to be added when refueling, and calculate updated fuel composition data as described above. The arrows with dotted lines in [Fig. 6] indicate fuel flow, while the solid lines indicate electronic communication. The active infinite summation approach described above may therefore be used in conjunction with the currently described flight profile adaptation approach, or either approach may be used separately. The flight profile adjustment device 212 shown in [Fig. 6] may not be present in examples in which the currently described flight profile adaptation approach is not implemented. As mentioned above with respect to the previous examples, the fuel composition tracking device 202 may be provided as a separate fuel composition tracking unit 202, as shown in [Fig. 6], or as a module integrated into the propulsion system 2, and / or as software and / or hardware incorporated into pre-existing aircraft control systems, for example the EFEC 42. In the example shown, two sensors 204a, 204b are provided, each arranged to physically and / or chemically detect one or more characteristics of the fuel added to the fuel tank 50, 53 during refueling. The sensors 204 and the fuel composition monitoring device 202 may be described together as a fuel composition monitoring system 203, as shown in [Fig. 6]. The fuel composition tracking system 203 includes a fuel composition tracking device 202. The fuel composition tracking device 202 of the described example includes a memory 202a arranged to store current fuel characteristic data, and processing circuitry 202c arranged to calculate updated values ​​for the one or more fuel characteristics of the fuel in the fuel tank 50, 53 after refueling. The calculated values ​​may then replace the fuel characteristic data previously stored in the memory, and / or may be time-stamped and / or dated and added to the memory. A log of fuel characteristic data over time may therefore be established. The fuel composition tracking device 202 of the example shown also comprises a receiver 202b arranged to receive data relating to a fuel composition and / or requests for fuel composition information. The fuel composition determination module 210 may therefore comprise a receiver 202b arranged to receive data relating to a fuel composition, from which one or more fuel characteristics may be determined (either directly by extraction or by calculation, possibly in conjunction with data from another source). The fuel composition determination module 210 may therefore comprise, or have access to the output of, one or more sensors 204 arranged to provide data relating to one or more fuel characteristics. The sensor data may provide one or more fuel characteristics directly, or may allow one or more fuel characteristics to be obtained by calculation, possibly in conjunction with data from another source. In alternative examples, no such sensor 204 may be provided (e.g., a barcode associated with the fuel storage container 60 may be read and the corresponding fuel data is provided to the fuel composition monitoring device 202), or more or fewer sensors may be provided. Data from the fuel composition tracking device 202 may be used to change the planned flight profile, based on the one or more fuel characteristics. A flight profile adjustment device 212 may be used to change the planned flight profile based on the one or more fuel characteristics of the fuel, based on data provided by the fuel composition tracking device 202 and possibly also other data. The flight profile adjustment device 212 may be provided as a separate flight profile adjustment unit 212 integrated into the propulsion system 2, and / or as software and / or hardware incorporated into pre-existing aircraft control systems, such as the EEC 42. The fuel composition tracking means (e.g., the tracking device 202) may be provided as part of the same unit or assembly. The executed flight profile control method 2050 is illustrated in [Fig. 8]. In step 2052, one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine 10 are determined, possibly using any of the methods described above. In step 2054, the flight profile of the aircraft 1 is changed based on the one or more fuel characteristics. The change in flight profile may be or include a change in one or more of the trajectory, route, angle of attack, and altitude. A flight profile adjustment device 212 may be used to initiate and / or effect the flight profile change. In some examples, the flight profile adjustment device 212 may change the flight profile itself, and, in some implementations, may further control the implementation of this change, for example, by recording a planned change and then providing a command to one or more control surfaces of the aircraft 1 to change altitude at the appropriate time. In other examples, the flight profile adjustment device 212 may request approval of the planned change, and / or may not itself send instructions to cause the flight profile change. The flight profile adjustment device 212 may therefore provide notification or a suggestion of a proposed flight profile change to the pilot and / or other authority regarding the planned change, for approval.A notification or suggestion may be provided to a pilot on a cockpit display of the aircraft, and / or sent to a separate device such as a portable tablet or other computing device, for example. In some examples, the same adjustment device. flight profile 212 may automatically make some changes and request approval for others, depending on the nature of the change (e.g., whether or not the planned change is significant enough to require authorization from air traffic control, or another authority). The flight profile adjustment device 212 may therefore propose and / or initiate a change 2054 to a flight profile of the aircraft | based on the at least one fuel characteristic. The inventors also appreciated that knowledge of the one or more fuel characteristics selected and determined in any of the ways described above can be used to suggest, guide, or make in-flight adjustments to the propulsion system 2, so as to further improve the performance of the aircraft. For example, a fuel with a higher heat capacity can be used to cool the engine more than a fuel with a lower heat capacity, and a fuel with a higher heating value can allow a lower fuel flow rate to be delivered to the combustion chamber for the same power output. Knowledge of the fuel can therefore be used as a tool to improve the performance of the aircraft in flight.With respect to the pre-planning and flight profile changes described above, real-time or near real-time decisions may be made and implemented, and these decisions may only affect the internal operation of the engine 10 rather than changing the route and / or altitude, for example. In some examples, the aeronet 1 may have only one fuel tank 50, and / or may have multiple fuel tanks 50, 53 that each contain the same fuel, and / or are fluidly connected, or are in fluid communication with the gas turbine engine 10, such that only one type of fuel is supplied to the gas turbine engine 10 between refueling events - i.e., the fuel characteristics may remain constant throughout a flight.In such examples, fuel properties do not change during a flight, but external conditions (e.g., weather, altitude) and internal conditions (e.g., thrust demand) do change, and changes may be made (i) initially when the fuel characteristics are first determined or addressed, and / or (ii) based on what is appropriate for that fuel given the changing conditions. In other examples, the aircraft 1 may have a plurality of fuel tanks 50, 53 that contain fuels of different compositions, and the propulsion system 2 may include an adjustable fuel distribution system, allowing the tank(s) 50, 53, and thus which fuel / fuel mixture to use, to be selected. In such examples, the fuel characteristics may vary during a flight, and a specific fuel or fuel mixture fuels may be selected to improve operation at certain stages of flight or under certain external conditions. In such examples, changes to the propulsion system control may also be made when the fuel changes, for example due to a determination that a fuel is nearly depleted, or the selection of a different fuel or fuel mixture. (It will be appreciated that, in general, a fuel system may be arranged to never allow a tank 50, 53 to become completely empty, as this could lead to a flameout of the engine 10 - however, a tank may be allowed to become completely empty if its fuel is supplied as part of a mixture; one or more other fuels in the mixture may have their flow rate increased to ensure that the engine 10 never runs out of fuel.) A fuel change may therefore be a response to a propulsion system control change and may cause one or more other propulsion system control changes. In examples where direct sensing is used for one or more fuel characteristics, or where the fuel characteristics are calculated from sensed parameters, the sensing may be performed in the or each tank SO, 53 (and fuel characteristics for a mixture of fuels resulting from different tanks may then be calculated where appropriate), and / or upon approach to the engine 10, for example in a pipe containing a mixture from multiple tanks. In some examples, the sensing may be performed on the fuel immediately before entering the engine 10, or more specifically the combustion chamber 16, to ensure that the correct fuel / mixture of fuels is identified and that the data is as up-to-date as possible (near real-time). Once one or more fuel characteristics have been determined for a fuel currently supplied to the gas turbine engine 10, control of the propulsion system 2 may be adjusted based on the determined fuel characteristics. Additional data may be used in conjunction with the determined fuel characteristics to adjust the control of the propulsion system 2. For example, data of the current conditions around the aircraft 1 may be received (either from a provider, such as a third-party weather monitoring agency, or from on-board sensors). This received data (e.g., weather data, temperature, humidity, presence of a contrail, etc.) may be used to effect or influence changes in the propulsion system control. Instead of, or in addition to, using “real-time” or near-real-time weather data, the forecast weather data for the aircraft's route can also be used to estimate current conditions. Examples of propulsion system changes that may be made based on fuel characteristics include any or all of the control examples described above, such as adjusting VIGV scheduling. A propulsion system 2 for an aircraft 1 may therefore comprise a fuel composition determination module 210 arranged to determine 2052 one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine 10; and an electronic engine governor 42 arranged to issue propulsion system control commands based on the determined fuel characteristics. The fuel composition monitoring device 202 of the example shown may be part of, or have access to, an electronic engine governor (EEC) 42 arranged to issue propulsion system control commands based on the fuel characteristics. In some cases, the EEC may issue recommendations for pilot approval (or approval by another authority), and may then issue a propulsion system control command subject to that approval.It will be appreciated that an EEC 42 may be provided for each gas turbine engine 10 of the aircraft 1, and that the role played by the EEC for the fuel composition tracking device 202 may represent only a small part of the functionality of the EEC. Indeed, the fuel composition tracking device 202 may be provided by the EEC, or may comprise an EEC module separate from the engine's EEC 42 in various implementations. In alternative examples, the fuel composition tracking device 202 may not comprise any engine control functionality, and may instead simply provide fuel composition data on demand, to be used as appropriate by another system. Different locations and types of fuel composition determination module 210 and associated fuel characteristic sensors may be used depending on when, where, and how the fuel composition is to be determined. For example, as described above with respect to the flight profile control method 2050 shown in [Fig. 8]. In some examples, the fuel composition may be detected when the aircraft 1 is refueled. The fuel composition determination module 210a in such examples may be or include a fuel composition tracking device 202. The active infinite summation approach described above can therefore be used in conjunction with the flight profile adaptation approach described above, and / or with the currently described in-flight adjustment approach, or any one of the three may be used separately. As mentioned above, the flight profile adjustment device 212 shown in [Fig. 6] may not be present in examples in which the flight profile adaptation approach described above is not implemented. The fuel composition tracking device 202 and the fuel composition tracking system 203 may be as described above. Data from the fuel composition tracking device 202 may be used to change the planned flight profile and / or to guide or make in-flight adjustments to the propulsion system 2, based on the one or more fuel characteristics. A propulsion system 2 for an aircraft 1 may therefore comprise a fuel composition tracking device 202, or another fuel composition determining module 210, arranged to record and store fuel composition data, and possibly also to receive the fuel characteristics of the fuel to be added during refueling, and calculate the updated fuel composition data. The fuel composition determining module 210 may be provided as a separate fuel composition tracking unit integrated into the propulsion system, and / or as software and / or hardware incorporated into pre-existing aircraft control systems. Data from the fuel composition determination module 210 may be used to adjust control of the propulsion system 2, based on the one or more fuel characteristics. A propulsion system controller 42, also referred to as an electronic engine governor 42, may be used to adjust the control of the propulsion system 2 based on the one or more fuel characteristics of the fuel, using data provided by the fuel composition determination module 210 and possibly other data. It will be understood that the propulsion system controller 42 may control elements of the propulsion system, which may or may not be considered components of the engine 10 itself, such as one or more control surfaces. The term "electronic engine governor" (EEC) 42 as used herein synonymously is not intended to be limited in this sense.The propulsion system control device 42 may be provided as a separate propulsion system control unit 42 integrated into the propulsion system 2, as part of the fuel composition determination module 210, and / or as software and / or hardware incorporated into pre-existing aircraft control systems. The fuel composition monitoring means may be provided as part of the same unit or of the . same set. The propulsion system controller 42 may make changes to the propulsion system 2 directly or may provide a notification or suggestion to the pilot (or other authority) regarding the change, for approval. A notification or suggestion may be provided to a pilot on a cockpit display of the aircraft, and / or sent to a separate device such as a portable tablet or other computing device. In some examples, the same propulsion system controller 42 may automatically make some changes, and request others, depending on the nature of the change.In particular, as mentioned above, changes that are “transparent” to the pilot—such as internal changes to engine flow rates that do not affect the engine’s power output and would not be noticed by a pilot—may be made automatically, while any changes that the pilot would notice may be notified to the pilot (i.e., the appearance of a notification that indicates the change will occur unless the pilot decides otherwise) or suggested to the pilot (i.e., the change will not occur without positive input from the pilot). The executed method 2051 is illustrated in [Fig. 9]. In step 2052, one or more fuel characteristics of the fuel arranged to be supplied to the gas turbine engine 10 are determined. The fuel characteristics may be or include any of those listed above, and may be selected based on the characteristics that have the most significant effects on optimal control of the propulsion system. In step 2056, the propulsion system 2 of the aircraft 1 is controlled based on the one or more fuel characteristics. The control actions taken may be or include any of those listed above. A propulsion system controller 42 may therefore be used to initiate and / or effect control of the propulsion system 2. In some examples, the propulsion system controller 42 may automatically effect a change, for example, by providing a command to cause a change in position of one or more steerable inlet guide vanes of the aircraft propulsion system 2 in response to an evaluation of fuel characteristics (and possibly other conditions). In other examples, the propulsion system controller 42 may not automatically send instructions to control the propulsion system 2, but rather may provide a proposal to change the propulsion system control for approval, based on one or more fuel characteristics. The inventors also appreciated that since different fuels may have different properties, while still remaining compliant with standards, knowledge of the fuel(s) available for an aircraft 1 may allow for more efficient and tailored control of the propulsion system 2. For example, switching to a fuel with a higher calorific value may allow for a constant fuel feed rate to the combustion chamber 16 while still providing higher power output. Selecting a specific fuel based on the intended or current operations of the aircraft may therefore be used as a tool to improve aircraft performance. In particular, the calorific value of a fuel may be considered. This approach is described below with respect to two aircraft fuel source arrangements different from that shown in [Fig. 4]. It will be understood that each of the approaches described herein can be used with any suitable fuel system, and that the examples illustrated and described in detail are not intended to be limiting. In particular, as shown in [Fig. 10] and [Fig. 14], an aircraft 1 may include multiple fuel tanks 50, 52, 53; for example, a first fuel tank 52 and a second, larger fuel tank 50, each located in the aircraft fuselage, and a smaller fuel tank 53a, 53b located in each wing. In other examples, only two fuel tanks 50, 52, or more fuel tanks, may be provided. The sizes, shapes, and locations of the fuel tanks may vary; for example, all of the fuel may be stored in tanks 53 in the wings. [Fig. 10] shows an aircraft 1 with a propulsion system 2 comprising two gas turbine engines 10. The gas turbine engines 10 are supplied with fuel from a fuel supply system on board the aircraft 1. The fuel supply system of the illustrated example comprises two fuel sources. Each of the fuel sources is arranged to provide a separate fuel source, i.e. they are fluidically isolated and the first fuel source may contain a first fuel having a different characteristic or characteristics from a second fuel contained in the second fuel source. For example, the fuels may have different compositions and / or different origins, for example one being a fuel of fossil origin such as Jet-A, another being a paraffinic SAF, a non-paraffinic SAF, a paraffinic SAF of different composition or a mixture.The first and second fuel sources are therefore not fluidically coupled to each other so as to separate the different fuels (at least under normal operating conditions). A fuel source may be a single tank or may consist of multiple fluidically interconnected tanks, and may be called . fuel tank even when it actually comprises multiple interconnected tanks. In this example, the first fuel source is the first fuel tank 52. In other examples, the first fuel source may include multiple interconnected tanks. In the present example, the second fuel source comprises a center fuel tank 50, located primarily in the fuselage of the aircraft and a plurality of wing fuel tanks 53a, 53b, where at least one wing fuel tank is located in the left wing and at least one wing fuel tank is located in the right wing for balancing. All of the tanks 50, 53, except the first fuel tank 52, are fluidically interconnected in the example shown in [Fig. 10], thereby forming a single second fuel source. Each of the center fuel tank and the wing fuel tanks may comprise a plurality of fluidically interconnected fuel tanks. In another example, the wing fuel tanks 53a, 53b may not be in fluid communication with the center tank 50, thereby forming a third separate fuel source. For balancing purposes, one or more fuel tanks in the left wing may be in fluid communication with one or more fuel tanks in the right wing as described above. In the example of [Fig. 10], however, a fluid interconnection between wing fuel tanks 53 and the center fuel tank 50 of the second fuel source is provided for balancing the aircraft 1. The example shown in [Fig.14] is generally similar to that shown in [Fig.10], but the differences are described below. In the example shown in [Fig. 14], the first fuel tank 52 is smaller than the second fuel tank 50. The first fuel tank 52 of this implementation is located further towards the rear of the fuselage. The first fuel tank 52 may therefore be used as a trim tank 52 in flight - it will be appreciated that, as known in the art, a trim tank 52 may be used to provide adjustment of the center of gravity in the longitudinal axis of the aircraft 1; the aircraft is "trimmed" by pumping fuel into (or out of) a trim tank. One or more sensors, pumps, valves, controls and the like may be provided to control the trim operation.In other implementations, the first fuel tank 52 may not be a trim tank; its connections to other fuel tanks 50, 53 may differ or not be present in such other implementations. A trim tank 52 is therefore in fluid communication with at least one other fuel tank 50, 53 of an aircraft 1 in order to allow a trim to be performed. in-flight compensation, This is a controllable fluid communication, so that the first tank 52 can be isolated from the other tanks 50, 53 and used as a separate fuel source if necessary. In addition to the propulsion system 2 described with respect to [Fig. 10], a power system 4 of the implementation shown in [Fig. 14] includes an auxiliary power unit (APU) 44. The APU 44 is a gas turbine engine smaller than those 10 on the wings of the aircraft 1, and is arranged to provide electrical power to systems of the aircraft |; for example, lighting, heating, air conditioning and / or the like. The APU 44 may be, for example, a Honeywell Series 331 APU, such as the HGT1700 auxiliary power unit (APU). In some implementations, the APU 44 may be certified for flight use; in other implementations, it may be certified for ground use only.An aircraft APU 44 is generally arranged to be started using one or more aircraft batteries to provide electrical power as well as possibly bleed air for air conditioning and for engine starting. The APU 44 of the illustrated implementation is located toward the rear of the fuselage, and is not arranged to provide any propulsive power to the aircraft 1. In alternative implementations, the APU may be located differently (e.g., in a nacelle 21 of the aircraft 1), and / or may provide some propulsive power. In the example shown in [Fig. 14], the first fuel tank 52 is arranged to supply fuel to the APU 44. In this example, the first fuel tank 52 is also arranged to supply fuel directly to the main gas turbine engines 10, although these connections may not be present in some implementations, or, alternatively, the first fuel tank 52 may be arranged to supply fuel directly to the main gas turbine engines 10 and not to the APU 44 in some implementations. In alternative implementations, the first fuel tank 52 may be a dedicated APU fuel tank, and may not be fluidly interconnected to the main gas turbine engines 10, or any other fuel tanks. In some of these examples, a fuel management device 214 as described below may not be used for fuel supply to the APU. In various implementations, a plurality of fuel line connection ports 62 may be provided, possibly to facilitate the supply of different fuels to different fuel tanks / sources 50, 52, 53. Alternatively or additionally, a fuel supply management system may direct incoming fuel from the same port 62 to different tanks, as appropriate. In particular, in the described examples, the first tank 52 may be fueled di- directly from a fuel supply rather than having to be filled by transfer from another fuel tank 50, 53 of the aircraft 1. In [Fig. 14], internal fuel lines from the port(s) 62 to the tanks are not shown for clarity. In [Fig. 10], one internal fuel line is shown (to the larger tank 50); it will be understood that at least a second fuel line to the first fuel tank 52 would generally also be provided but this is not shown for clarity. In examples of the present invention, the aircraft 1 has a plurality of fuel tanks 50, 52, 53, and in particular, at least two different fuel sources and possibly more. At least two of the fuel tanks are arranged to contain different fuels - i.e., fuels having at least one difference in fuel characteristics - and in particular, to contain fuels having different calorific values. One or more fuel characteristics of each fuel stored on board the aircraft 1 may be determined. The determined fuel characteristics may include one or more of the examples of fuel characteristics listed above. The determination can be made in many different ways. For example: "a bar code of a fuel to be added to a fuel tank 50, 53 of the aircraft 1 may be scanned to read the fuel data, or a tracer substance (e.g., a dye) is identified and the fuel properties are searched based on this tracer; * data can be entered manually, or transmitted to the aircraft |; * a fuel sample may be extracted for ground analysis prior to takeoff; * fuel properties may be inferred from measurements of propulsion system activity during one or more periods of aircraft operation, e.g., engine warm-up, taxiing, takeoff, climb, and / or cruise; and / or * one or more fuel properties may be detected in flight, for example using in-line sensors and / or other measurements, and any of the detection approaches described above. Fuel characteristics may therefore be chemically and / or physically detected, or otherwise determined, by any suitable means described herein. In some examples, combinations of these techniques may be used to determine and / or verify one or more fuel characteristics. The calorific values ​​(also called thermal values) of fuels may be directly determined—for example, by measuring or deducing the energy released when a certain volume or mass of the fuel is burned in the gas turbine engine 10—or calculated from other fuel parameters; for example, by examining the hydrocarbon distribution of the fuel and the calorific value of each constituent hydrocarbon type. Alternatively, or additionally to provide verification, the calorific value may be determined using external data, such as a look-up table for a tracer substance in the fuel, or data encoded in a bar code associated with the fuel. In some examples, each fuel tank 50, 52, 53 of the aircraft 1 may be arranged to contain a fuel having a different calorific value; that is, each fuel tank may be a separate fuel source. As used herein, the term "heating value" means the lower thermal value (also referred to as net heating value) of the fuel, unless otherwise specified. Net heating value is defined as the amount of heat released by the combustion of a specified quantity of the fuel, generally in J / kg, assuming that the latent heat of vaporization of water in the reaction products is not recovered (i.e., the water produced remains as water vapor after combustion). In some examples, two or more tanks 50, 52 of the aircraft may be arranged to contain fuels having a different type or proportion of a sustainable aviation fuel, the fuels having different calorific values. The propulsion system 2 includes an adjustable fuel delivery system 220, allowing selection of which source(s) / tank(s) 50, 52, 53, and thus which fuel or fuel mixture to use. In such examples, fuel characteristics may vary during a flight, and a specific fuel or fuel mixture may be selected to improve operation at certain stages of flight or under certain external conditions. A first fuel tank 52 of the plurality of fuel tanks 50, 52, 53 may have a higher proportion of sustainable aviation fuel (SAF) than a second fuel tank 50 of the plurality of fuel tanks and may have a higher calorific value than a fossil fuel, or SAF-fossil fuel blend, in another, second, fuel tank. More fuel from the second tank 50 may be used in cruise and more fuel from the first tank 52 may be used at higher power demand operating points (e.g., takeoff and climb). In other examples, the first fuel tank 52 may contain fuel having a lower heating value than another tank 50. More fuel from the first tank 52 may be used during cruise and more fuel from the second tank 50 may be used at higher power demand operating points (e.g., takeoff and climb). One fuel tank 52 of the plurality of fuel tanks may be arranged to contain only a fuel that is a sustainable aviation fuel (SAF) - this tank may contain 100% pure SAF, or SAF with one or more additives, but contains no fossil fuel. (As used herein, "SAF" means a pure sustainable aviation fuel, containing no fossil / petroleum-derived fuel (but possibly one or more additives, e.g., an icing inhibitor); the term "SAF blend" or "blended fuel" may be used for a mixture comprising both a SAF and a petroleum-derived fuel.) The SAF in this tank 52 may be selected so that the propulsion system 2 may operate only on this fuel (e.g., for ground operations, or in the event of an in-flight emergency, or if / when fuel regulations change so that flight on SAF fuel alone is generally permitted). In such examples, the fuel tank 52 containing only the sustainable aviation fuel may therefore be arranged to be used to fuel the aircraft 1 when the aircraft is performing ground operations. Optionally, all of the fuel, or at least the majority of the fuel, used for ground operations may be arranged to be taken from the fuel tank 52 containing the sustainable aviation fuel, for example to meet airport emissions and / or SAF usage requirements. A high % SAF fuel blend may be used in place of SAF in other implementations. It will be appreciated that the use of SAF (alone or in a blended fuel) can provide a significant reduction in non-volatile particulate matter (nvPM) emissions at idle - the percentage reduction can be greater than 90% under these conditions in some cases. The percentage reduction in nvPM for the use of SAF is estimated to be greater at idle than at high power demands because soot creation (nvPM) is more closely related to the aromatics content of the fuel at these low power conditions compared to higher power demands where other soot formation mechanisms come into play - SAFs generally have lower aromatics content than petroleum-derived aviation fuels.As such, if a total amount of SAF is limited, using SAF for ground operations / operations around the airport rather than elsewhere in a flight cycle can ensure a . increased benefit in terms of reducing nvPM production. In addition, airport air quality can be improved. Similarly, in flight, a greater benefit from using SAF can be found if SAF is used for low-power portions of the flight envelope. For the same reasons, a SAF may be selected for use in an aircraft auxiliary power unit (APU) 1 at an airport gate. However, SAFs often have a higher calorific value than traditional jet fuels - in such cases, a different control of fuel delivery to the gas turbine engine 10 may therefore be used for ground operations (where nvPM reductions may be prioritized) compared to in flight (where matching calorific value to thrust may be prioritized), if only a limited amount of SAF is available. In scenarios where fuels with similar nvPM properties but different calorific values ​​are available on board an aircraft 1, the same control may be used throughout operation; for both ground and flight operations. In examples with only one SAF tank 52, this fuel tank 52 may be smaller than the one or more other fuel tanks 50, 53, for example, the first fuel tank 52 may represent 3% to 20%, and possibly 5% to 10% of the total available tank volume of the aircraft 1. Optionally, this tank 52 may be arranged to be used exclusively for ground operations of the aircraft 1. A selection between the other tanks 50, 53 based on calorific value may then be made in flight based on engine thrust. In implementations such as that shown in [Fig. 14], in which a trim tank 52 is present on the aircraft 1 and is used as the first fuel tank 52, fuel would be drawn from the trim tank 52, the mass of which therefore decreases due to fuel loss, during ground operations, for example, stand operations, warm-up, and possibly during one or more of taxiing and takeoff.In particular, if the trim tank 52 is filled with SAF (or a high SAF% blended fuel), the air quality benefits obtained by using this SAF from the trim tank 52 for the APU 44 at the stand can be obtained, and this SAF can further be used for the main gas turbine engines 10 during warm-up, taxi, takeoff, and possibly part of the climb (possibly in a blend with another fuel), until the trim tank 52 is empty. In some cases, the amount of fuel in the trim tank 52 can be selected so that the trim tank 52 is at least substantially . empty at takeoff, so that it can be (partially) filled during climb, to allow its use to trim the aircraft 1 even during climb. In other examples, it may be used to trim the aircraft 1 only later in the flight. Fuel from a different fuel source 50, 53 may be used thereafter. Such implementations may be particularly useful when the aircraft 1 is flying missions towards the limit of its payload-range capacity; allowing all fuel tanks to be initially filled to their maximum capacity while ensuring trim tank capacity quickly after takeoff / before the usual use of trim tank 52. Thus, at the time when the aircraft 1 is in flight, the first fuel tank 52 would be relatively light, if not completely empty, and does not affect the center of mass significantly. The first fuel tank 52 is therefore available for normal use as a trim tank 52 for at least the cruise portion of the flight - fuel from one or more of the other fuel sources 50, 53 can be pumped into the tank when ready to reduce resistance during cruise, and possibly also during climb (after at least substantially emptying the first fuel tank 52 at the latest halfway through the climb). The amount of fuel initially in the trim tank 52 may be selected to allow this fuel to be fully depleted well before the aircraft 1 reaches its cruising altitude. More particularly, in such implementations, the amount of fuel supplied to the first fuel tank 52 upon refueling may be calculated to be at least substantially depleted by the time the aircraft 1 takes off, and possibly more precisely at the time of the aircraft's pitch-up (the pitch-up being what happens toward the end of the takeoff roll when the nose wheel of the aircraft 1 leaves the ground, but the main landing gear is still on the ground. In the moments following the pitch-up, the aircraft 1 gains speed and then the main landing gear also leaves the ground).The fuel in the first fuel tank 52 of these examples may be selected based on its effects on air quality and pollution, and may or may not have a lower heating value than the fuel(s) in other tanks 50, 53. In examples where the first fuel tank 52 is used as a trim tank 52, an on-board fuel management device 214 may be arranged to detect the fuel level in the first fuel tank 52 and automatically switch supply to a different tank (regardless of the additional amount of nvPM that may be caused) if it detects an imminent drying out of this tank 52, to avoid any interruption of the fuel supply to the engine 10, 44. It will be understood that having a relatively small quantity of fuel remaining in the trim tank 52 does not prevent its use accordingly, and / or that the fuel management device 214 may be arranged to pump fuel out of the trim tank 52 and into a different tank before the start of the takeoff roll, possibly following a negative result of a check that the trim tank 52 is sufficiently empty for the front / rear center of gravity to be within acceptable limits. In some implementations, the fuel management device 214 may be arranged to automatically switch from the first fuel tank 52 to another, possibly larger, tank 50, 53 before commencing the takeoff roll, to eliminate the possibility of the first tank 52 running dry during the takeoff roll and / or climb. In some implementations, the fuel management device 214 may be arranged to request or enforce more engine idle time before commencing a takeoff roll to ensure that the trim tank 52 is sufficiently empty for use, if trimming of the aircraft 1 should be necessary during the takeoff roll and / or if no spare capacity is available for fuel to be pumped out of the trim tank 52. In examples in which sensing is used to determine one or more fuel characteristics, sensing may be performed in each tank 50, 52, 53 (or in one tank of each fuel source), and fuel characteristics for a resulting fuel mixture from different tanks may then be calculated, if applicable, based on the mixture ratios. Alternatively or additionally, sensing may be performed in the approach to the engine 10, for example in a fuel pipe / line that may contain a mixture from multiple tanks. When data is collected for a fuel mixture, for example on approach to engine 10, the data may be recorded as the mixture changes (by drawing more or less fuel from a given tank 50, 52) to determine the fuel composition in each tank 50, 52 and 53 individually. This determination may allow adaptation of the fuel selection or mixture in flight, for example by using different fuels as needed for different parts of a flight envelope, even when the fuel composition is not known at takeoff. Furthermore, in some implementations, such a determination may be made during engine warm-up and / or in the early stages of taxiing, so that the fuel selection for the remainder taxiing can be adjusted appropriately, for example by selecting a fuel or fuel blend with maximum nvPM benefits while the aircraft is still at the airport. In some examples, the detection may be performed on the fuel immediately before entering the engine 10 / combustion chamber 16, possibly to ensure that the correct fuel / fuel mixture is identified (e.g., as a check on the intended composition if this is already known) and that the data on the fuel being burned is as up-to-date as possible (near real-time). Once the heating value of each fuel available for supply to the gas turbine engine 10 has been determined, by any suitable method, a single fuel (from a single tank) or a mixture of fuels (from multiple tanks) may be selected and supplied to the gas turbine engine 10, based on a thrust demand of the gas turbine engine 10. In particular, a fuel (a single fuel or mixture) having a lower heating value may be supplied to the gas turbine engine 10 at a lower thrust demand. Correspondingly, a fuel (a single fuel or mixture) having a higher heating value may be supplied to the gas turbine engine 10 at a higher thrust demand.Fuel control based on calorific value can be performed in flight only in certain scenarios, allowing fuel supply to be controlled differently for ground operations (e.g., prioritizing reduction of nvPM generation rather than matching calorific value to power demand). It will be understood that the thrust demand may be determined using one or more approaches known in the art, for example based on fuel flow and / or cockpit throttle angle, or one or more other pilot settings, and possibly taking into account the outside air density, or a proxy thereof such as altitude, ambient temperature, and / or pressure. Use of fuel flow alone may be insufficient due to differences in fuel flow ranges at altitude relative to the ground. Varying the fuel heating value corresponding to a thrust demand may facilitate maintaining a more constant fuel flow rate, and / or more uniform fuel pump operation and dumping in flight. In general, the fuel mass flow rate varies considerably between different parts of flight for an aircraft 1, such that the differences in fuel heating value are not significant enough to compensate for the differences in fuel energy flow requirements between climb and initial cruise, for example. However, during a level cruise segment where the aircraft 1 is burning fuel at a constant altitude, the thrust requirement may slowly decrease over a extended period, and adjustments in fuel flow heating value may allow a substantially constant flow rate to be maintained. A substantially constant fuel flow rate may therefore be maintained in certain parts of aircraft operation by implementing the approach described herein. In addition, fuel flow maxima may be reduced, and / or fuel flow minima may be increased, by selecting a fuel having an appropriate heating value at the corresponding points of aircraft operation. "More constant fuel flow" may therefore refer to a decrease in the maximum distribution of fuel flows across the engine operating envelope / flight envelope. It will be appreciated that the pump speed is generally related to the shaft speed in some aircraft 1, with a dump rate being adjusted as needed for a given speed, so that a fuel flow rate to the combustion chamber 16 is controlled by a fuel delivery system 220 (e.g., including a hydromechanical unit, HMU) comprising multiple components, rather than by a fuel pump alone. This more uniform operation can be beneficial in terms of providing an appropriate fuel flow rate through the system, e.g., for lubrication, for a fuel system, and for heat transfer, even at very low power demands. Using a lower heating value fuel in the engine 10 at lower power demands can aid thermal management, as the higher flow rate of fuel through the engine provides more heat transfer medium.Additionally, it can be difficult to run large 10 engines smoothly at low idle boost using standard fuels - so being able to switch to a fuel with a lower heating value could improve performance at low idle boost. A method 2060 of operating an aircraft 1 comprising a gas turbine engine 10 and a plurality of fuel tanks 50, 52, 53 arranged to store fuel for powering the gas turbine engine 10 is shown in [Fig.11]. The method 2060 includes arranging 2062 each fuel source / tank 50, 52 of the plurality of fuel tanks to contain a different fuel to be used to power the gas turbine engine 10, where the fuels have different heating values. In some examples, one or more of the fuel tanks 50, 52 may be part of a separate set of interconnected fuel tanks. In other examples, each fuel tank 50, 52 may be a self-contained, single-tank fuel source. The method 2060 further comprises storing 2064 information about the fuel contained in each fuel tank 50, 52, optionally in the memory of an on-board fuel management device 214. The method 2060 further includes controlling 2066 the fuel supply to the gas turbine engine 10 by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks 50, 52. The control 2066 may be performed only in flight, or throughout operation of the aircraft. The selection is performed based on a thrust demand of the gas turbine engine 10 such that a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand (e.g., ground operations (if implemented on the ground), descent, cruise), and a fuel having a higher heating value is supplied to the gas turbine engine 10 at a higher thrust demand (e.g., climb). Even for an example with only two separate fuel sources 50, 52, a range of different fuel heating values ​​can be provided by dynamically blending the two fuels at different levels, depending on the thrust demand. In some arrangements, the method 2060 includes switching between two, three, four, or five fuels and / or predefined mixtures, having determined heating values, depending on a thrust demand. In other arrangements, the mixture can be changed depending on a thrust demand, possibly continuously (within the accuracy limits of the fuel pump and / or other flow regulators). Additional data may be used in conjunction with the determined fuel characteristics to adjust the control of the propulsion system 2. For example, the method may include receiving data of the current conditions around the aircraft 1 (either from a provider, such as a third-party weather monitoring organization, or from on-board sensors). This received data (e.g., weather data, temperature, humidity, the presence of a contrail, etc.) may be used to effect or influence changes in the composition of the fuel supplied to the gas turbine engine 10. Instead of, or in addition to, using "real-time" or near-real-time weather data, forecast weather data for the aircraft's route may also be used to estimate the current conditions. A propulsion system 2 for an aircraft 1 may therefore comprise a fuel management device 214 arranged to store information on the fuel contained in each fuel tank 50, 52 and to control the supply of fuel to the main gas turbine engine(s) 10, and possibly also to an APU 44, in operation. The fuel management device 214 may be provided as part of a fuel distribution system 220 arranged to allow control and adjustment of the fuel supplied to the gas turbine engine 10. The fuel delivery system 220, as shown in [Fig. 12], may include one or more flow regulators 216, such as valves and pumps, arranged to be controlled by the fuel management device 214 to control the supply of fuel to the main gas turbine engine(s) 10, and possibly also to an APU 44. For example, a flow regulator 216a, 216b may be provided between each fuel source and each engine 10. Such arrangements may allow different fuels to be supplied to different engines 10 of the same aircraft propulsion system 2. Optionally, the fuel management device 214 may further receive other data (in addition to the fuel characteristic data) and use this other data and the fuel characteristic data to determine a desired fuel or fuel mixture for the gas turbine engine 10. The fuel management device 214 may be provided as a separate fuel management unit 214 integrated into the propulsion system 2, and / or as software and / or hardware incorporated into pre-existing aircraft control systems (e.g., in the EEC 42).In some examples, the fuel composition data may be stored separately from the circuitry performing fuel management and retrieved as needed - regardless of where the data is stored, that storage may be considered part of the fuel management device 214, whether or not it is an integral part thereof, or physically connected thereto in any way. The more general term "power system" 4 may be used for the propulsion system 2 to ensure that implementations in which fuel is supplied, in addition to or alternatively to an APU 44 are included, as propulsive power may not always be provided by such power systems 4, for example while performing gate operations while the aircraft 1 is stationary (it will also be appreciated that the main gas turbine engines 10 may also be used to provide non-propulsive power in many implementations). The fuel management device 214 may be arranged to select a specific fuel or a combination of fuels from one or more of the plurality of fuel tanks 50, 52, 53 based on a thrust demand of the gas turbine engine 10. In particular, a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa. A fuel having a higher heating value may therefore be used in the high power stages of the flight envelope, such as during takeoff. The fuel heating value can be adjusted linearly based on a % increase or decrease in thrust demand in certain scenarios, within an available range. The fuel management device 214 may be arranged so that a fuel having a lower heating value is supplied to the gas turbine engine 10 during cruise than that supplied during climb. Optionally, a fuel having an even lower heating value may be supplied to the gas turbine engine 10 at ground idle or low idle - this same fuel may also be supplied to the APU 44 in some implementations. It will be understood that the term "low idle" is a generic term generally used for the idle setting for ground or flight idle when the engine is operating at one of its limiters at minimum (e.g., minimum speed, pressure and / or temperature limits), set in EEC 42, with the throttle position in the reverse idle position. The power level at in-flight idle can vary considerably depending on factors such as altitude, power consumption, customer bleed air, and anti-icing demands; therefore, the term "low idle" covers a range of power / thrust demands. Idling operation during ground operation of the aircraft | is referred to as "ground idle" and idling operation during flight operation of the aircraft 1 is referred to as "flight idle". High idle is a more specific term, referring to conditions in which the aircraft 1 is in an approach and landing configuration and the idle is raised above the flight low idle in order to obtain adequate thrust response if necessary for a go-around. While the throttle remains in the reverse idle condition, there is an increased thrust level, and high idle may only be applicable in flight (not for ground operations). In some implementations, a fuel having a higher heating value may be supplied to the gas turbine engine 10 at high idle than at low idle, and a fuel having an even higher heating value may be supplied when the thrust demand exceeds that of high idle. The fuel management device 214 may make changes to the fuel supply directly, or may provide a suggestion or notification to the pilot regarding the change, for approval (e.g., as described above for the propulsion system controller 42, noting that the fuel management device 214 may be part of, or in communication with, the propulsion system controller 42). In some examples, the same fuel management device 214 may perform automatically make some changes and request others, depending on the nature of the change. In some examples, an aircraft 1 may be modified to perform the method 2060 described above, possibly by installing an adjustable fuel delivery system 220. A method 2070 of modifying an aircraft 1 in this manner is shown in [Fig. 13]. The original aircraft 1 comprises a gas turbine engine 10, which, in the example described, comprises an engine core 11 comprising a turbine 19, a compressor 14, and a main shaft 26 connecting the turbine to the compressor. The aircraft 1 also comprises a plurality of fuel tanks 50, 52 and a fan 23 located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The aircraft 1 may also comprise an APU 44. The method 2070 includes arranging 2072 each fuel tank 50, 52, 53 (or at least two fuel tanks of a plurality of fuel tanks) to contain a different fuel for use in powering the gas turbine engine(s) 10, 44, where the fuels have different heating values. In some cases, the aircraft 1 may already include a plurality of fuel tanks 50, 52, 53 arranged to store fuel to power the gas turbine engine 10; in such examples, the step 2072 of arranging the fuel tanks may simply comprise selectively filling the tanks with different fuels. In cases where the aircraft 1 previously had only one fuel tank 50, a new fuel tank 52 may be added to provide a plurality of fuel tanks. In cases where the aircraft | previously had only one fuel source, albeit comprised of multiple tanks, a new fuel tank 52 may be added and / or fuel lines may be adjusted so that the original tanks 50, 53 are no longer all fluidly interconnected, thereby providing at least two separate fuel sources.The arrangement step 2072 may therefore vary depending on the initial configuration of the aircraft. The method 2070 further comprises providing 2074 a fuel management device 214 arranged to store information about the fuel contained in each fuel tank 50, 52 and 53 and to control the supply of fuel to the gas turbine engine 10. The fuel management device 214 may operate only in flight. In examples in which it operates both in flight and during ground operations, the control strategy it employs may differ between flight and ground operations in some examples. In other examples, for example, where all available fuels have suf- particularly low, fuel control based on thrust and calorific value can also be carried out during ground operations. The storage and control functions may be performed by separate entities or by the same entity; it will be understood that the fuel management device 214 may therefore be a distributed system or a single unit or module. The step of providing 2074 the fuel management device 214 may include installing or consist of installing software into existing memory, to be executed using existing systems, in some examples. In other examples, a new physical unit or module may be mounted on the propulsion system 2, possibly including one or more flow regulators 216 and / or replacement fuel line sections, if applicable, to achieve the desired fuel flow and mixture control. The fuel management device 214 is arranged to control the supply of fuel to the gas turbine engine 10 by selecting a specific fuel or a combination of fuels from one or more of the plurality of fuel tanks 50, 52, 53 based on a thrust demand of the gas turbine engine 10 such that a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa. This control may be performed throughout the operation of the aircraft, or only at certain stages (e.g., only in flight, or only during cruise). The inventors also appreciated that current standards mean that a (neat) SAF cannot be used for commercial flights, but that a SAF could be used for ground operations, for example to reduce airport emissions. Similarly, there may be advantages to using such a sustainable aviation fuel for ground operations even when it can also be used in flight, for example to maximize the environmental benefits of a limited amount of available SAF. Furthermore, if fuels that are not neat SAF but instead contain a proportion of SAF are available for aircraft 1, using the fuel(s) with the highest % SAF for ground operations may correspondingly reduce airport emissions and thus improve air quality.Redesigning the aircraft fuel system can therefore achieve technical and environmental benefits from SAF, whether or not the SAF is supplied as part of a blend. If only a relatively small amount of pure SAF or a high SAF percentage blend is available to an aircraft 1 (the remainder of the fuel being either petroleum-based fuel or a low SAF percentage blend), then the greatest benefit of that SAF (or high SAF percentage blend) may be be obtained by using this fuel in and around the airport, where the power demand is relatively low (see discussions on nvPM generation above). For the same reasons, this SAF or high SAF% mixture, possibly stored in a first fuel tank 52 as described above, can be used in the auxiliary power unit (APU) 44 of the aircraft 1, for example at the gate of an airport. Although it is appreciated that a synthetic fuel may be manufactured to exactly mimic a traditional kerosene fuel, one or more fuel characteristics of a SAF stored on board the aircraft 1 may differ from the fuel characteristics of the one or more other fuels stored on board the aircraft 1, in other tanks. The fuel characteristics may include one or more of the fuel characteristics described above, and may be determined using any of the approaches described above, including any of the exemplary detection techniques listed. If two or more different SAFs, or two or more different SAF blends with the same SAF %, are available for aircraft 1, one or more other fuel characteristics, such as the hydrogen to carbon (H / C) ratio of the fuel or the level of non-volatile particulate matter (nvPM) emissions during combustion, may be used to select between two or more fuels with the same SAF %. One or more other parameters that may influence the air quality around an exhaust of aircraft 1 may also be compared to select the fuel that is likely to provide the best air quality results. Environmental factors (e.g., airport altitude and humidity) may also be considered in this assessment. In the present examples, described with respect to Figures 14 and 18, the first fuel source is the first fuel tank 52. In other examples, the first fuel source may comprise multiple interconnected tanks. In the described example, the first fuel tank 52 is arranged to contain only fuel that is pure sustainable aviation fuel (SAF), i.e., 100% sustainably sourced and not kerosene-derived / fossil-based. In other examples, multiple fuel tanks of a plurality of fuel tanks may all contain SAF—so any of the subset of fuel tanks containing SAF may be used to supply SAF; it will be understood that the example of a single fuel tank 52 containing SAF is described herein by way of non-limiting example only. In other examples, the first fuel tank 52 is arranged to contain a fuel, which is a higher SAF % SAF blend than any other fuel tank 50, 53. In other examples, multiple fuel tanks of a plurality of fuel tanks may all contain a SAF blend having the same SAF % SAF, such that there are multiple first fuel tanks 52 - any one of the first fuel tanks may therefore be used to provide fuel for at least the majority of ground operations; it will be understood that the example of a single first fuel tank 52 is described herein by way of non-limiting example only. One or more second fuel tanks 50, 53 contain one or more fuels having a lower SAF % SAF (possibly 0% SAF) and are used for other operations. The example shown in [Fig. 18] is generally similar to that shown in [Fig. 14], but the differences are described below. (In [Fig. 18], an internal fuel line is shown - to the larger tank 50; it will be understood that at least a second fuel line to the first fuel tank 52 would generally also be provided but this is not shown for clarity - similarly, no such fuel line is shown in [Fig. 14] for clarity, but it would generally be present.) In the presently described examples, the aircraft 1 has a plurality of fuel tanks 50, 52, 53, and in particular, at least two separate fuel sources / tanks 50, 52, and possibly more. Each fuel tank 50, 52 is arranged to store a fuel to be used to power one or more gas turbine engines 10, 44 of the aircraft. One of the fuel tanks 52 - referred to as the first fuel tank 52 - is arranged to contain only one fuel, which is a sustainable aviation fuel (SAF), or to contain a high % SAF fuel blend. In some implementations, such as that shown in [Fig. 18], this first fuel tank 52 is arranged to contain only the SAF or the high % SAF mixture, and may still be isolated from the other fuel source(s) 50, 53. In other implementations, such as that shown in [Fig.14], this first fuel tank 52 is arranged to initially contain the SAF or the high % SAF mixture, and to be fluidically isolated from the other fuel source(s) 50, 53 when this fuel is in use (e.g. for ground operations), but may then be fluidically connected to one or more other fuel sources 50, 53 in flight (e.g. by opening one or more valves), and may have a different fuel from a different fuel source 50, 53 pumped into it (e.g. to serve as a trim tank 52). The first fuel tank 52 of these examples is therefore arranged to contain only a fuel, which is a sustainable aviation fuel (SAF), or a high % SAF blend, at least during ground operations. In an implementation in which the fuel is 100% SAF, the SAF in this tank 52 may be selected so that the propulsion system 2 may operate only with this fuel (e.g., for ground operations, in the event of an in-flight emergency, or if fuel regulations change so that flight with 100% SAF is generally permitted), or may be suitable for use in an APU 44 only and not suitable for combustion in a main gas turbine engine 10. The fuel tank 52 containing the sustainable aviation fuel only may therefore be arranged to be used to fuel the aircraft 1 when the aircraft is performing ground operations.Optionally, the first fuel tank 52 may be used to supply SAF to the gas turbine engine 10, but possibly only during ground operations. Alternatively, the first fuel tank 52 may be used to supply SAF to the gas turbine engine 10 as part of an in-flight mix, or may be arranged to supply SAF only to the APU 44. In implementations in which the fuel in the first fuel tank(s) 52 is not pure SAF, more flexible use can be made of this fuel in flight even under the regulations in force at the time of writing. In some implementations, the first fuel tank 52 may be used to supply fuel to both the main gas turbine engine 10 and the APU 44, In some of the described examples, all of the fuel used for ground operations is sustainable aviation fuel or the highest available SAF% blend, and all of the fuel used for ground operations is therefore drawn from the first fuel tank 52 (in examples with multiple first tanks, any one or more of these tanks may be used). In other examples, the majority of the fuel used for ground operations is SAF or the highest available SAF% blend, with only small amounts from other sources being used (e.g., less than 10% or less than 5% of the fuel usage / run time, or using fuel from another source only for initial engine start-up). It will be understood that if a first fuel tank 52 runs out of fuel, a second fuel tank 50, 53 having the highest SAF% mixture among the second fuel tanks may be reclassified as the first fuel tank and used for remaining ground operations. A fuel management device 214 may be arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to draw fuel only from the one or more first fuel tanks 52 when the aircraft 1 is performing at least the majority of ground operations. As used herein, "ground operations" generally refers to operations prior to takeoff and may include one or more of the following: * starting the engine itself; * heating, lighting, air conditioning and / or other non-propulsive demands, while the aircraft 1 is stationary (e.g. at a gate) or in motion; * taxiing of aircraft 1; and “the launch of the takeoff roll, possibly including the lifting of a front wheel, if necessary. The first fuel tank 52 (or one or more other fuel tanks containing a SAF or the high % SAF blend, in other examples) may therefore be used to provide some or all of the fuel used by the aircraft fueling system 4 at the parking position (e.g., at a gate), and during warm-up, taxiing, and takeoff. Advantageously, this can meet airport requirements for emissions and / or SAF use. If, after ground operations are completed, there is still fuel remaining in one or more of the first fuel tanks 52, it could possibly be retained for use at the destination airport for further ground operations upon landing, including landing taxi and arrival taxi. Any fuel remaining in the one or more first fuel tanks 52 may additionally or alternatively be used for the first part of the climb (at takeoff) and / or the final part of the approach (at landing) - i.e., for one or more parts of the flight that do not take place on the ground but take place close to the ground and are therefore relevant to local air quality and human health. The use of a SAF on the approach to a destination airport (as opposed to takeoff) may be particularly beneficial in terms of nyPM, since the power required is lower and thus the reduction in nvPM achievable by the use of a SAF is therefore higher. A selection can be made based on the amount of SAF available, the types of SAF mixtures available, and an order of priority for the use of SAF in different parts of aircraft operations. The fuel management device 214 may be arranged to control the supply of fuel to the propellant gas turbine engine(s) 10 in flight by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks 50, 52. The first fuel tank 52 of the plurality of fuel tanks 50, 52, arranged to contain only the highest % SAF fuel (which may be pure SAF) may be smaller than the one or more other fuel tanks 50, 53. For example, the first fuel tank 52 may represent 3% to 20%, and possibly 5% to 10%, of the total available tank volume of the aircraft 1. Optionally, this tank 52 may be arranged to be used exclusively for ground operations of the aircraft 1. Each fuel tank 50, 52, 53 on board the aircraft 1 may be arranged to contain a different type of fuel (e.g., a petroleum-based fuel or a SAF, or different varieties of SAF), and some tanks may contain fuels blended with a proportion of a sustainable aviation fuel mixed with a conventional jet fuel or another petroleum-based fuel. In some examples, two or more tanks 50, 52 may contain the same fuel, provided that at least two different fuels are available in the fuel tanks throughout the aircraft 1. In some examples, at least one tank 52 contains a SAF - i.e., purely a sustainable aviation fuel, and not a blend. In implementations such as that shown in [Fig. 14], in which a trim tank 52 is present on the aircraft 1 and is used as the first fuel tank 52, the approach and advantages of using SAF described above can also be achieved. The propulsion system 2 of the re-described examples comprises an adjustable fuel distribution system 220, allowing selection of which tank(s) 50, 52, and thus which fuel or fuel mixture to use. In such examples, fuel characteristics may vary during a journey (including flight and ground operations at the beginning and / or end of a journey) - a specific fuel or fuel mixture may be selected to improve operation at certain stages of flight or under certain external conditions. In examples where sensing is used for one or more fuel characteristics (either by direct sensing or by inference from sensed parameters), for example to discover or verify which tank contains the highest % SAF fuel, any of the sensing approaches described above may be implemented. In other examples, no sensing may be performed and provided fuel composition data may be used instead - such data may simply be SAF proportions, for example "100% SAF" versus "Other", or % SAF for each tank, or may include more detailed information on the fuel characteristics. fuel characteristics. In other examples, no fuel data may be provided - instead, each tank 50, 52, 53 may be identified as a "SAF only" tank or a "non-SAF only" tank, or as a "Highest SAF%" tank or "Other" tank, and the method may rely on the tanks 50, 52, 53 being properly filled accordingly. In some examples, the heating values ​​for each available fuel may be calculated or provided, and a fuel or mixture of fuels is provided based on a thrust demand as described above (optionally also taking into account flight altitude) - a portion of the fuel from the first tank 52 may be used alone and / or in one or more mixtures in such examples. In examples with a single first tank 52, control based on heating value may be performed in flight only. In examples with more than one first tank 52, a selection between the first tanks may be made based on heating value for different thrust requirements during ground operations, as well. A method 2061 of operating an aircraft 1 comprising a gas turbine engine 10, 44 and a plurality of fuel tanks 50, 52 arranged to store fuel for powering the gas turbine engine 10, 44 is shown in [Fig.15]. The method 2061 comprises arranging 2063 at least two fuel tanks 50, 52 of the plurality of fuel tanks to each store a different fuel, in particular, fuels having different SAF proportions. A first fuel tank 52 contains a fuel having a higher SAF proportion than a second fuel tank 50, 53. A first fuel tank 52 of the plurality of fuel tanks may be arranged to contain only a fuel that is a sustainable aviation fuel. This arranging step 2063 may include fluidically isolating one or more tanks from each other, if necessary, to allow different fuels to be stored in different tanks (e.g., by closing valves). This arranging step 2063 may include filling the tanks 50, 52, 53 appropriately. In some examples, one or more of the fuel tanks 50, 52 may be part of a separate set of interconnected fuel tanks. In other examples, each fuel tank 50, 52 may be a self-contained, single-tank fuel source. In some examples, the fuel in the first fuel tank 52 has a SAF% greater than 50%, e.g., greater than or equal to 55%, 60%, or 70%, and may optionally be 100% SAF. The method 2061 of some examples includes identifying 2067 the tank 52 that contains the fuel having the highest proportion of a sustainable aviation fuel. If multiple tanks 50, 52 each contain a fuel having the same highest SAF%, the multiple tanks may all be identified as first fuel tanks 52. Optionally, one or more other characteristics (e.g., heating value, nvPM emissions) may be used to select which first fuel tank 52 to use in such scenarios. The identification 2067 may be performed by sensing or determining from engine operating parameters, e.g., using any of the approaches described herein, or by using provided data (e.g., data transmitted or otherwise obtained by or accessed by the fuel management device 214). The method 2061 further comprises controlling 2065 the supply of fuel to the gas turbine engine 10 so as to use only a fuel from the tank 52 containing the fuel having the highest proportion of a sustainable aviation fuel (or, in some cases, any fuel with more than 50% SAF) when the aircraft 1 is conducting at least the majority of ground operations. This fuel may have a proportion of SAF greater than 50% SAF, and possibly at least 55% SAF.Here, the use of this fuel for "at least the majority" of operations may mean that at least 90% or 95% of the fuel used for ground operations is this fuel, and / or that at least 90% or 95% of the operating time required for ground operations is provided by this fuel, and / or that the fuel having the highest proportion of SAF is used for all ground operations except initial engine start (for which a dedicated fuel may be used as described below). The method 2061 further optionally comprises storing 2064 information about the fuel contained in each fuel tank 50, 52, optionally in a memory of an on-board fuel management device 214. The stored information may simply be a flag indicating whether or not a particular tank 50, 52 contains the highest SAF% fuel, or fuel with more than 50% SAF. Additional information may be stored in other examples. This stored information may be used for the control step 2065, and in particular may be used to identify the first fuel tank 52 (and / or correspondingly one or more other tanks, if multiple tanks contain SAF or the highest SAF% fuel), if not hard-coded / hard-wired into the propulsion system 2.A tank property lookup table can be used to identify the first fuel tank(s) 52 if the tanks are . always arranged to contain specific fuels. A fuel system 4 for an aircraft 1 may therefore comprise a fuel management device 214 arranged to store information about each fuel tank SO, 52 / about the fuel contained in each fuel tank 50, 52 and to control the supply of fuel to the operating gas turbine engine 10. The stored information may simply comprise a flag indicating whether or not each tank is the highest SAF% tank, or may comprise more detailed information, such as a SAF% content for each tank, and / or one or more other fuel characteristics of the fuel currently in each tank 50, 52 (the SAF% may be defined by volume and / or by mass, taking into account that densities may vary within accepted limits).In such examples, which tank 52 of the plurality of fuel tanks 50, 52, 53 is the first tank 52 may vary over the lifetime of the fuel system 4, for example depending on which tank is filled with which fuel. In other examples, the fuel delivery system 220 as shown in [Fig. 16] may be configured so that a specific tank 52 is always the tank with the highest SAF%, and it is not necessary to store this fuel information. The fuel management device 214 may also be arranged to identify which tank 52 contains the fuel having the highest proportion of a sustainable aviation fuel, and / or to identify all tanks containing fuels with more than 50% SAF. In implementations in which the SAF or the higher % SAF mixture is supplied to a main gas turbine engine 10 of the aircraft 1, which provides propulsive power to the aircraft 1, the fuel system 4 may be more specifically referred to as the propulsion system 2. The more general term "fuel system" 4 mentioned above is used herein to ensure that implementations in which the fuel is supplied in addition or alternatively to an APU 44 are included, as propulsive power may not be provided by such fuel systems 4. The fuel management device 214 may be provided as part of a fuel delivery system 220 arranged to allow control and adjustment of the fuel supplied to the gas turbine engine 10, 44, and may be as described above for the preceding examples. The fuel management device 214 is generally as described above. In examples with APUs 44, the fuel management device 214 may further be arranged to control the fuel or fuel mixture supplied to the APU 44. In some of the examples described, the fuel management device 214 is arranged to draw fuel exclusively from the first fuel tank 52, i.e., the fuel tank 52 containing the fuel having the highest proportion of SAF, for ground operations of the fuel system 4 As described above for other examples, the fuel management device 214 may further receive other data (in addition to information indicating which tank(s) contain(s) SAF, providing a % SAF for each tank, and / or providing other fuel characteristic data), and use this other data and the fuel characteristic data to determine a desired fuel or fuel mixture for the gas turbine engine 10, 44 in flight. The fuel management device 214 may be arranged to control the supply of fuel to the gas turbine engine 10, 44 so as to draw fuel primarily, or solely, from a first fuel tank 52 when the aircraft 1 is performing ground operations. In some examples, an aircraft 1 may be modified to perform the method 2061 described above, possibly by installing an adjustable fuel delivery system 220. A method 2071 of modifying an aircraft 1 in this manner is shown in [Fig. 17]. The original aircraft 1 comprises a gas turbine engine 10, the gas turbine engine 10 of this example comprising an engine core 11 comprising a turbine 19, a compressor 14, and a main shaft 26 connecting the turbine to the compressor. The aircraft | also comprises a fan 23 located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The original aircraft 1 may further comprise an APU 44, the APU itself being or comprising a gas turbine engine 44. The method 2071 includes arranging 2073 at least two separate fuel tanks 50, 52 to store a different fuel, such that one fuel has a higher proportion of SAF than the other. One or more fuel tanks may be arranged to contain only a fuel that is a sustainable aviation fuel in some implementations (in one described example, the first fuel tank 52 of the plurality of fuel tanks 50, 52 is the only tank containing pure SAF). In some cases, the aircraft 1 may already include a plurality of fuel tanks 50, 52 arranged to store fuel to power the gas turbine engine(s) 10, 44; in such examples, the step 2073 of arranging the tanks of fuel may simply comprise selectively filling the tanks with different fuels. In cases where the aircraft 1 previously had only a single fuel tank 50, a new fuel tank 52 may be added to provide a plurality of fuel tanks. In cases where the aircraft 1 previously had only a single fuel source, albeit composed of multiple tanks, a new fuel tank 52 may be added and / or fuel lines may be adjusted so that the original tanks 50, 53 are no longer all fluidly interconnected, thereby providing at least two separate fuel sources. The arrangement step 2073 may therefore vary depending on the initial configuration of the aircraft. The method 2071 further comprises providing 2075 a fuel management device 214 arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to use only, or primarily, a SAF when the aircraft 1 is performing ground operations. The fuel management device 214 may further be arranged to identify which tank 52 contains the fuel having the highest proportion of sustainable aviation fuel - this tank 52 may be identified as a first fuel tank 52. In cases where two or more tanks contain fuel having the same highest % SAF, a plurality of tanks may be identified as first fuel tanks. A selection between these may be made using one or more of the approaches described above. The fuel management device 214 may use fuel from the first fuel tank 52 (e.g., SAF in a specific example described, or more generally the highest available SAF% fuel, or possibly any fuel above 50% SAF) for some, but not all, ground operations. For example, the fuel management device 214 may use a different fuel for startup before switching to use of the first tank 52, may switch to a different fuel if the fuel in the first fuel tank 52 is exhausted before ground operations are completed, and / or the fuel management device 214 may provide the fuel from the first fuel tank 52 to the APU 44 but a different fuel to the other gas turbine engine(s) 10. In some examples, such as arrangements in which the tank(s) 52 used to store the fuel at the highest SAF% may vary over the lifetime of the propulsion system 2, the fuel management device 214 may further be arranged to store information about the fuel contained in each fuel tank 50, 52 to enable identification of the first tank 52, or equivalent. Storage and control functions can be performed by entities separate or by the same entity; it will be understood that the fuel management device 214 may therefore be a distributed system or a single unit or module. The step 2075 of providing the fuel management device 214 may include installing or consist of installing software into existing memory, to be executed using existing systems, in some examples. In other examples, a new physical unit or module may be mounted on the propulsion system 2, possibly including one or more flow regulators 216 and / or replacement fuel line sections, if applicable, to achieve the desired fuel flow and mixture control. In some examples, the fuel management device 214 may further be arranged to perform other functions, for example, to control the supply of fuel to the gas turbine engine 10 by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks 50, 52 based on a thrust demand of the gas turbine engine 10 such that a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa. It will be understood that the thrust demand may be determined using one or more approaches known in the art, for example as mentioned above. The inventors also appreciated that the fuel differences may allow for redesign of the aircraft fuel system to provide technical and environmental benefits for aircraft ground operations. In the present examples, described with respect to Figures 14 and 19, the use of the first fuel tank 52 is adjusted accordingly. [Fig. 19] shows an aircraft | with a propulsion system 2 generally identical to that shown in [Fig. 14], but with three separate fuel sources 50, 52, 53 instead of two. The first, second and third fuel sources are therefore not fluidically coupled to each other so as to separate the different fuels (at least under normal operating conditions). In other examples, such as that shown in [Fig. 14], only two separate fuel sources may be provided. In this example, the first fuel source is the first fuel tank 52. In other examples, the first fuel source may include multiple interconnected tanks. The first fuel tank 52 is arranged to be used at least primarily (and in some cases, solely) for ground operations. The fuel in the first fuel tank 52 is arranged to be used for at least the majority of ground operations, similar to the use of a SAF or the high SAF% blend described in relation to the previous examples. In the example shown in [Fig. 19], the second fuel source comprises a central fuel tank 50, located primarily in the fuselage of the aircraft, and the third fuel source comprises a plurality of wing fuel tanks 53a, 53b, where at least one wing fuel tank is located in the left wing and at least one wing fuel tank is located in the right wing for balancing. The tanks 53a and 53b are fluidically interconnected in the example shown, thus forming a single third fuel source. A fluidic interconnection between the wing fuel tanks 53 of the third fuel source may be provided for balancing the aircraft 1, as previously described. Each of the center fuel tank 50 and the wing fuel tanks 53 may include a plurality of fluidically interconnected fuel tanks. In another example, the wing fuel tanks 53a, 53b may be in fluid communication with the center tank 50, thereby forming a single second fuel source. For balancing purposes, one or more fuel tanks in the left wing may be in fluid communication with one or more fuel tanks in the right wing. This may be accomplished via a center fuel tank 50 (if that tank is not part of a separate fuel source), or by bypassing the center fuel tank(s), or both (for maximum flexibility and safety). In some examples, the distribution of fuel tanks 50, 53 available on the aircraft 1 may be constrained so that each fuel source is substantially symmetrical about the aircraft axis. In cases where an asymmetric distribution of fuel tanks is permitted, a suitable means of transferring fuel may be provided between the fuel tanks of the first fuel source and / or between the fuel tanks of the second fuel source so that the position of the center of mass of the aircraft can be maintained within acceptable lateral limits throughout the flight. However, in examples where the first fuel tank 52 is much smaller than the other fuel tanks 50, 53, its change in mass as fuel from that tank is used may be less significant and thus symmetry may not be an issue. In the examples shown in Figures 14 and 19, the first fuel tank 52 is even smaller than the second fuel tank 50. In the example of [Fig. 14], it is located further towards the rear of the fuselage. The first fuel tank 52 of this implementation can therefore be used more easily as a trim tank 52 in flight; the approach described above with regard to trim can therefore also be used in conjunction with dedicating initial initially a tank 52 for use on the ground. If the first tank 52 is to be used as an in-flight trim tank, the fuel in this tank 52 is either depleted in the initial stages of ground operations and possibly at takeoff, or the remainder is blended with other fuel pumped into this tank for in-flight trim. In contrast, in implementations in which this first tank 52 is not used as a trim tank, rather than being depleted in-flight or diluted, the SAF remaining in the first tank 52 could be retained until the aircraft 1 has landed and then used to fuel final ground operations (e.g., landing and / or taxiing on arrival) to achieve further air quality benefits at the destination airport. In various examples, an APU 44 as described above may be used to provide some or all of the power for ground operations, and the first fuel tank 52 may be used to provide fuel to the APU 44. In alternative implementations, the first fuel tank 52 may be a dedicated APU fuel tank, and may not be fluidly interconnected to the main gas turbine engines 10 as is the case in [Fig. 14], nor to any other fuel tank. In some of these examples, a fuel management device as described elsewhere herein may not be present or used. An aircraft 1 may be refueled by connecting a fuel storage container 60, such as that supplied by an airport tanker, or a pipeline, to a fuel line connection port 62 of the aircraft, via a fuel line 61, as described above. In particular, in the examples described, the first tank 52 may be refueled directly from a fuel supply rather than having to be filled by transfer from another fuel tank 50, 53 of the aircraft 1. In [Fig. 14], internal fuel lines from the port(s) 62 to the tanks are not shown, for clarity. In [Fig.19], an internal fuel line is shown (to the larger tank 50); it will be appreciated that at least a second fuel line to the first fuel tank 52 could also be provided - rather than filling the first tank 52 via a different tank - but this is not shown for clarity. In examples of the present invention, the aircraft 1 has a plurality of fuel tanks 50, 52, 53, and in particular, a first fuel tank 52 arranged to be used to power some or all of the ground operations of the aircraft 1, and one or more secondary fuel tanks 50, 53, each arranged to contain fuel to be used to power the gas turbine engine 10 in flight. The ground operation of the aircraft 1 may involve one or more gas turbines 10, 44 - the gas turbine engine 44 of the APU 44 may be used to certain ground operations, and one or more of the main gas turbine engines 10 may be used for other ground operations. Each fuel tank 50, 52, 53 is arranged to store a fuel to be used to power one or more gas turbine engines 10, 44 of the aircraft 1. One of the fuel tanks - called the first fuel tank 52 - is arranged to be used for some or all of the ground operations of the aircraft 1. In the example shown in [Fig. 21], the first fuel tank 52 is dedicated to ground operations exclusively and the fuel in this tank 52 is not used in flight. The fuel management device 214 of this example (which may generally be as described above) is arranged to control the supply of fuel to the gas turbine engines 10, 44 so as to draw fuel only from the first fuel tank 52 when the aircraft 1 is performing ground operations, and to draw fuel only from the one or more secondary fuel tanks 50, 53 for other operations. The flow regulator 216c may prevent the drawing of fuel from the first fuel tank 52 in flight. In other examples, a portion of the fuel in the first tank 52 may also be used in flight, and / or a portion of the fuel from other tanks may be used for ground operations. In some examples, the first fuel tank 52 is arranged to contain a fuel that is a sustainable aviation fuel (SAF), at least during ground operations—i.e., the tank 52 may contain 100% pure SAF. The fuel in the first tank 52 may be selected so that the propulsion system 2 may operate only with that fuel for ground operations, whether or not that fuel is certified for flight use. Alternatively, the fuel in the first tank 52 may not be suitable for use in the main gas turbine engines 10, and may be used only for the APU 44. In the examples currently described, the first fuel tank 52 is therefore arranged to be used to fuel the aircraft 1 when the aircraft is performing ground operations, whether or not this fuel tank 52 contains SAF. In some implementations, such as that shown in [Fig. 19], this first fuel tank 52 is arranged to be used only for ground operations (possibly, with a small amount remaining after the aircraft 1 has completed the pitch-up during climb as previously described), and may still be isolated from the other fuel source(s) 50, 53. In other implementations, such as that shown in [Fig. 14], this first fuel tank 52 is arranged to be used initially for ground operations, and to be fluidically isolated from the other fuel source(s) 50, 53 when used for ground operations, but may then be fluidically connected to one or more other fuel sources. fuel 50, 53 in flight, and may be used as a trim tank 52, and / or to fuel an engine 10, 44, in flight. The first fuel tank 52 may have a different fuel from a different fuel source 50, 53 pumped into it in flight, than the fuel with which it was originally filled to fuel ground operations. In some examples, the first fuel tank 52 of some implementations is configured to be used to supply fuel to one or more gas turbine engines 10, 44 only during ground operations, i.e., this tank is arranged to be used exclusively for ground operations of the aircraft 1. In some implementations, fuel suitable for use in an APU 44 only and not suitable for combustion in a main gas turbine engine 10 may be provided in the first tank 52. However, the fuel management device 214 of some examples may be arranged to allow fuel from the first fuel tank 52 to be supplied to the main gas turbine engine 10, for example, alone in emergency situations or as part of normal in-flight mixing, or the first fuel tank 52 may serve a different role—such as as a trim tank—in flight.In other examples, a block may be provided (e.g., by a flow regulator 216c) to prevent any introduction of fuel from the first tank 52 to the gas turbine engine 10 in flight. In some of the described examples, all fuel used for ground operations is drawn from the first fuel tank 52. The fuel management device 214 may be arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to draw fuel only from the first fuel tank 52 when the aircraft 1 is performing all ground operations. In other examples, fuel drawn from the first fuel tank 52 may be used for a subset of ground operations, with fuel from other tanks 50, 53 being used as appropriate. For example, the first fuel tank 52 may supply fuel to the APU 44 for its ground operations (e.g., lighting, air conditioning, main engine start), but a different fuel tank 50, 53 may supply fuel to the main gas turbine engine(s) 10 for ground operations (e.g., taxiing) thereof, or a different fuel may be used for initial engine warm-up / start, and power may then be switched to the first fuel tank 52 (e.g., when a threshold fuel temperature has been reached). As used herein, "ground operations" is as defined above. The first fuel tank 52 can therefore be used to supply part or all fuel used by the aircraft fuel system 4 at the parking position (e.g. at a gate), and during warm-up, taxiing and take-off. Advantageously, this may allow the storage and use of fuel suitable for relatively low power requirements of ground operations, and / or may facilitate compliance with airport emissions and / or SAF usage requirements, and / or may improve the operation of the engine 10 at low thrusts. The first fuel tank 52 of the plurality of fuel tanks may be arranged to contain only a fuel, which is a sustainable aviation fuel, or to contain a high SAF% blend; some or all of the nvPM and air quality benefits described above may therefore be achieved. For the same reasons, 100% SAF or a high SAF% / fuel blend from the first fuel tank 52 may be selected for use in an auxiliary power unit (APU) 44 of the aircraft 1 at an airport gate. The first fuel tank 52 of the plurality of fuel tanks may be smaller than the one or more other fuel tanks. For example, the first fuel tank 52 may represent 3% to 20%, and possibly 5% to 10%, of the total available tank volume of the aircraft 1. In implementations such as that shown in [Fig. 14] in which the first fuel tank 52 may function as a trim tank 52, the approach described above may be used whether or not the fuel in the first fuel tank 52 is a SAF or a high % SAF blend. Each fuel tank 50, 52, 53 on board the aircraft may be arranged to contain a fuel having a different type or proportion of a sustainable aviation fuel. Although it is appreciated that a synthetic fuel may be manufactured to exactly mimic a traditional kerosene fuel, one or more fuel characteristics of a SAF stored on board the aircraft 1, either as neat SAF fuel or as part of a blend, may differ from the fuel characteristics of the one or more other fuels (SAF blends or otherwise) stored on board the aircraft 1, in other tanks. The fuel characteristics may include one or more of those listed above, and may be determined using any of the techniques listed above, alone or in combination as appropriate, including the various examples of detection methods mentioned. In other examples, no detection can be performed and the data provided on fuel composition may be used instead - this data may simply be "ground use fuel" versus "flight use fuel", or may include more detailed information on fuel characteristics. In other examples, no fuel data may be provided - instead, each tank 50, 52, 53 may be identified as a ground use tank 52 or a standard use tank 50, 53, and these examples may rely on the tanks 50, 52, 53 being properly filled accordingly. The fuel system 4 includes an adjustable fuel delivery system 220, allowing selection of which tank(s) 50, 52, 53, and thus which fuel or fuel mixture to use. A fuel management device 214 as described above may control this system 220. In examples where the fuel mixture or fuel may be changed in flight (rather than having an in-flight setting and a ground setting), the fuel characteristics may vary during a flight - a specific fuel or fuel mixture may be selected to improve operation at certain stages of flight or under certain external conditions. In some examples, the calorific values ​​for each available fuel may be calculated or provided, and the in-flight fuel supply may be controlled accordingly. In implementations in which fuel from the first fuel tank 52 is supplied to a main gas turbine engine 10 of the aircraft 1, which provides propulsive power to the aircraft 1, the fuel system 4 may be more specifically referred to as the propulsion system 2. The more general term "fuel system" 4 is used to ensure that implementations in which fuel is additionally or alternatively supplied to an APU 44 are included, as propulsive power may not be provided by such fuel systems 4, as discussed above. A method 2080 of operating an aircraft 1 comprising one or more gas turbine engines 10, 44 and a plurality of fuel tanks 50, 52, 53 arranged to store fuel for powering the gas turbine engine(s) 10, 44 is shown in [Fig.20]. The method 2080 comprises providing 2082 at least two mutually separate fuel tanks 50, 52, 53 - i.e., at least two fuel sources. The method 2080 further comprises controlling 2086 the supply of fuel to the gas turbine engine(s) 10, 44 so as to use only fuel from the first fuel tank 52 when the aircraft 1 is performing at least the majority of ground operations. Here, the use of fuel from the first tank 52 for “at least the majority" of operations may mean that at least 90% or 95% of the fuel used for ground operations is fuel from the first tank 52, and / or that at least 90% or 95% of the operating time required for ground operations is provided by fuel from the first tank 52, and / or that this fuel from the first tank 52 is used for all ground operations except the initial engine start. The fuel management device 214 may therefore use fuel from the first fuel tank 52 for some, but not all, ground operations.For example, the fuel management device 214 may switch to a different fuel if the fuel in the first fuel tank 52 is depleted before ground operations are completed, and / or the fuel management device 214 may provide the fuel in the first fuel tank 52 to the APU 44 but a different fuel to the other gas turbine engine(s) 10. Optionally, only the fuel in this tank 52 may be used for all ground operations. Optionally, the method 2080 may include drawing fuel only from the one or more secondary fuel tanks 50, 53 for other operations. In other examples, the fuel from the first fuel tank 52 may be provided in flight as part of a mixture, but possibly not alone (except in emergency situations). In some examples, one or more of the fuel tanks 50, 52 may be part of a separate set of interconnected fuel tanks. In other examples, each fuel tank 50, 52 may be a self-contained, single-tank fuel source. The method 2080 of some examples further includes storing 2084 information about the fuel contained in each fuel tank 50, 52, 53 and / or the identification of each fuel tank 50, 52, 53, possibly in a memory of an onboard fuel management device 214. The stored information may simply be a flag indicating whether or not a particular tank 50, 52 is the first tank 52 / is intended to be used for ground operations. Additional information may be stored in other examples. The control step 2086 may be executed based on this stored information. A propulsion system 2, or other power system 4, for an aircraft 1 may therefore comprise a fuel management device 214 arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to draw fuel from the first fuel tank 52 when the aircraft 1 is performing ground operations, and draw fuel from the one or more secondary fuel tanks 50, 53 for other operations. In some implementations In practice, the fuel management device 214 may be arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to draw fuel only from the first fuel tank 52 when the aircraft 1 is performing ground operations, and draw fuel only from the one or more secondary fuel tanks 50, 53 for other operations. In some examples, the fuel management device 214 may be arranged to store information about each tank / about the fuel contained in each fuel tank 50, 52, 53 and to control the supply of fuel to the operating gas turbine engine(s) 10, 44 accordingly. The information about the fuel contained in each fuel tank 50, 52, 53 may simply comprise a flag indicating whether each tank is the ground use tank or not. In such examples, which of the plurality of fuel tanks 50, 52, 53 is the first tank 52 may vary over the lifetime of the propulsion system 2, for example depending on which tank is filled with which fuel.In other examples, the fuel dispensing system 220 may be configured so that a specific tank 52 is always the ground use tank, and no such information need be stored. In examples in which fuel information is stored, the information about the fuel in each fuel tank 50, 52, 53 may further include more information, such as a % SAF content for each tank, and / or one or more other fuel characteristics of the fuel currently in each tank 50, 52, 53. The fuel management device 214 may be provided as part of a fuel delivery system 220 arranged to allow control and adjustment of the fuel supplied to the gas turbine engine 10, 44, and may be as described above. In the example shown in [Fig. 21], the fuel management system 220 is arranged to allow the fuels of the second and third fuel sources 50, 53 to be mixed, so as to form a mixture for supplying the engine 10, but does not allow the fuel of the first ground use tank 52 to be mixed with the other fuels. Different approaches may be used in other examples. The fuel management device 214 of the presently described examples is arranged to draw fuel exclusively from the first fuel tank 52 for ground operations of the fuel system 4. It will be appreciated that, although the aircraft 1 still technically has one or more wheels on the ground for the majority of the takeoff, takeoff is a high power activity and is generally classified as part of the takeoff envelope. flight, not as a ground operation. Optionally, the fuel management device 214 may further receive other data (in addition to information indicating which tank 50, 52, 53 is the first tank 52, and other optional fuel characteristic data), and use this other data and the fuel characteristic data to determine a desired fuel composition for the gas turbine engine 10, 44 in flight. As mentioned above, the fuel management device 214 may be provided as a separate fuel management unit 214 integrated into the propulsion system 2, and / or as software and / or hardware incorporated into pre-existing aircraft control systems. In some examples, the fuel composition data and / or the tank identification data may be stored separately from the circuitry performing the fuel supply management and retrieved as needed - regardless of where the data is stored, this storage may be considered part of the fuel management device 214, whether or not it is an integral part thereof, or physically connected thereto in any way. The fuel management device 214 of the presently described examples is arranged to control the supply of fuel to the gas turbine engine 10, 44 so as to draw fuel from the first fuel tank 52 to power at least the majority of ground operations. In some examples, the fuel management device 214 may be arranged to be able to mix the fuels of the secondary fuel tanks 50, 53 arranged to supply the gas turbine engine 10 in flight, but not to be able to mix the fuel of the first fuel tank 52 with the fuel of the secondary fuel tanks 50, 53. In some examples, the fuel management device 214 may be arranged to select a specific fuel or combination of fuels from one or more of the plurality of fuel tanks based on a thrust demand of the gas turbine engine in flight. In particular, a fuel having a lower heating value may be supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa. A fuel having a higher heating value may therefore be used in high power stages of the flight envelope, such as during takeoff. The fuel management device 214 may be arranged such that a fuel having a lower heating value is supplied to the gas turbine engine 10 during cruise than during climb. Optionally, a fuel having an even lower heating value may be supplied to the gas turbine engine 10 at ground idle (low idle). The fuel management device 214 may make changes to the fuel supply directly, or may provide notification or a suggestion to the pilot regarding the change, for approval, as described in more detail above. In some examples, the same fuel management device 214 may automatically make some changes and request others, depending on the nature of the change. In some examples, an aircraft 1 may be modified to perform the method 2080 described above, possibly by installing an adjustable fuel delivery system 220. A method 2081 of modifying an aircraft 1 in this manner is shown in [Fig. 22]. The original aircraft 1 comprises a gas turbine engine 10, and in this example, the gas turbine engine 10 comprises an engine core 11 comprising a turbine 19, a compressor 14, and a main shaft 26 connecting the turbine to the compressor. The aircraft 1 also comprises a fan 23 located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The original aircraft 1 may further comprise an APU 44, the APU itself being or comprising a gas turbine engine 44. The method 2081 comprises providing 2083 at least two separate fuel tanks 50, 52, 53 (not fluidically connected, or at least capable of being fluidly isolated from each other) arranged to be able to supply fuel to the engine 10. A first fuel tank 52 of the plurality of fuel tanks 50, 52, 53 is arranged to be used to supply fuel for at least the majority of ground operations. One or more other fuel tanks 50, 53 are arranged to be used to supply fuel for all operations not covered by the first fuel tank 52. In some cases, the aircraft 1 may already include a plurality of fuel tanks 50, 52, 53 arranged to store fuel to power one or more gas turbine engines 10, 44; in such examples, the step 2083 of arranging the fuel tanks may simply consist of ensuring that one of the tanks 52 is configured to be used for ground operations and that at least one different tank 50, 53 is configured to be used in flight. In cases where the aircraft 1 previously had only one fuel tank 50, a new fuel tank 52 may be added to provide a plurality of fuel tanks. In cases where the aircraft 1 previously had only one fuel source, albeit comprised of multiple tanks, a new fuel tank 52 may be added and / or fuel lines may be adjusted so that the original tanks 50, 53 are no longer all in- fluidly interconnected, thus providing at least two separate fuel sources. The step 2083 of arranging fuel tanks / providing a first separate fuel tank 52 may therefore vary depending on the initial configuration of the aircraft. The method 2081 further comprises providing 2085 a fuel management device 214 arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 so as to use only the fuel from the first fuel tank 52 to power at least the majority of the ground operations of the aircraft. The fuel management device 214 may also be arranged to store information about the fuel contained in each fuel tank 50, 52, 53, and / or an identifier for each tank to indicate its intended use. The storage and control functions may be performed by separate entities or by the same entity; it will be understood that the fuel management device 214 may therefore be a distributed system or a single unit or module, as described above. The step 2085 of providing the fuel management device 214 may include installing or consist of installing software into existing memory, to be executed using existing systems, in some examples. In other examples, a new physical unit or module may be mounted on the propulsion system 2 (or other fuel system 4), possibly including one or more flow regulators 216 and / or replacement fuel line sections, as appropriate to achieve the desired fuel flow and mixture control. In some examples, the fuel management device 214 may further be arranged to provide other functions, for example, the fuel management device 214 may further be arranged to control the supply of fuel to the gas turbine engine 10 in flight by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks 50, 53 based on a thrust demand of the gas turbine engine 10 such that a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa.It will be understood that the thrust demand may be determined using one or more approaches known in the art, for example based on fuel flow and / or cockpit throttle angle, or one or more other pilot settings, and possibly taking into account the outside air density, or an indirect indicator thereof such as altitude, ambient temperature and / or pressure. The examples currently described may therefore be used in conjunction with the examples described above. The inventors also appreciated that, although the use of SAF could offer advantages across many parts of the flight envelope, initial engine start-up (i.e., from the point the engine is 'cold' / not running while an aircraft is parked) may be affected by certain properties of some SAF types, such as the potentially increased viscosity and / or lower lubricity of SAF compared to more traditional fossil-based fuels. Aircraft fuel system control changes may therefore allow the technical and environmental benefits of SAF to be realized without compromising the start-up operation.In particular, using a short "burst" of a fossil hydrocarbon fuel, such as Jet A, Jet A-1, or another fuel optimized for use under cold start / start conditions (SAF or otherwise), for start-up - before switching to a SAF fuel (or to a different SAF fuel) can ensure a smooth start while still allowing the benefits of the SAF to be obtained thereafter. More generally, regardless of the fuel to be used later in operation, a fuel optimized for start-up can be used initially - such a fuel may have a lower freezing temperature and / or a lower viscosity, at a given temperature, than that(s) of other fuels on board the aircraft 1. . Although it is appreciated that a synthetic fuel may be manufactured to exactly mimic a traditional kerosene fuel, one or more fuel characteristics of a SAF stored on board the aircraft 1 may differ from the fuel characteristics of the one or more other fossil-based fuels stored on board the aircraft in other tanks. In particular, the viscosity of SAFs may be higher, at a given temperature, than that of a traditional fossil-based fuel, and decrease with increasing temperature. Thus, once the engine has warmed up, such a fuel will have an improved viscosity for use due to the increased temperature. A fuel optimized for start-up—for example, one with higher lubricity at a given temperature and / or lower heat capacity—may therefore be selected, which may be particularly advantageous for cold start conditions, regardless of whether any or all fuels carried are or include SAF. The fuel optimized for start-up may or may not be certified for flight use—it may be dedicated for start-up use. A first fuel source and a second fuel source may therefore be used, providing different first and second fuels. The second fuel may be selected to have improved characteristics with respect to starting operation. In some cases, the first fuel may be a sustainable aviation fuel and the second fuel may be of fossil origin; however, this option is not intended to be limiting. In the present examples, described with respect to Figures 14 and 18, the first fuel source is the first fuel tank 52. In other examples, the first fuel source may comprise multiple interconnected tanks. In some examples, the first fuel tank 52 is arranged to contain only fuel that is pure sustainable aviation fuel (SAF), i.e., 100% sustainably sourced and not derived from kerosene / fossil fuel.In other examples, multiple fuel tanks of a plurality of fuel tanks may all contain SAF - any of the subset of fuel tanks containing SAF may thus be used to provide SAF, or the first fuel tank 52 may contain a blend of SAF or a fossil-based fuel; it will be understood that the example of a single fuel tank 52 containing SAF is described herein by way of non-limiting example only. In the present examples, described with respect to Figures 14 and 18, the second fuel source is or comprises the second fuel tank 50. The second fuel tank 50 is arranged to contain only a fuel that is selected for improved starting properties, and which may be of fossil / petroleum-based origin.Again, it will be understood that these arrangements are described by way of example only, and are not intended to be limiting. In some implementations, the first fuel tank 52 and the second fuel tank 50 may be used to supply fuel to both the main gas turbine engine(s) 10 and the APU 44. In other implementations, one or both of the tanks 50, 52 may be used to supply fuel to the main (propulsor) gas turbine engine(s) 10 or the APU 44, but not both - other fuel tanks may be provided to supply fuel to the other gas turbine engine(s) 10, 44 in such implementations. In some of the described examples, all fuel used for ground operations, except that used for initial startup, is a sustainable aviation fuel or a high SAF% blend, and any other fuel used for ground operations is therefore taken from the first fuel tank 52 containing the sustainable aviation fuel or the high SAF% blend (in examples with multiple tanks containing SAF, one or more of these tanks may be used, as appropriate). The fuel used for startup may be a SAF or a SAF blend in some cases. A fuel management device 214 as described above may be arranged to control the supply of fuel to the gas turbine engine(s) 10, 44 to draw fuel from the second fuel tank 50 at startup, and then switch to another fuel tank 52. A SAF or a high % SAF blend may therefore be used when the aircraft is conducting at least the majority of ground operations (as defined above), possibly achieving one or more of the benefits described above (e.g., nvPM reduction), whether or not the fuel used for initial engine start is a SAF. In some implementations, SAFs or SAF blends may be used for the majority, or all, of aircraft operations, both on the ground and in flight. Although Figures 14 and 18 show the second fuel tank 50 as being relatively large, in some implementations, the second fuel tank 50 may be smaller than the one or more other fuel tanks 52, 53. For example, the first fuel tank 50 may represent 1% or 2% to 15%, and possibly 3% to 5%, of the total available tank volume of the aircraft 1. Optionally, this tank 50 may be arranged to be used exclusively for starting the engine. A fuel optimized for use at start-up, even at the expense of performance in normal use, may therefore be selected as the second fuel. Each fuel tank 50, 52 on board the aircraft 1 may be arranged to contain a different type of fuel (e.g., petroleum-based fuel or SAF, or different varieties of SAF), and some tanks may contain fuels blended with a proportion of a sustainable aviation fuel blended with a conventional jet fuel or another petroleum-based fuel. At least one tank 52 may contain SAF—i.e., purely a sustainable aviation fuel, and not a blend, in some examples. At least one tank 50 contains a start-optimized fuel, which may or may not be a fossil fuel. The propulsion system 2 of the examples described again comprises an adjustable fuel distribution system 220, allowing selection of which tank(s) 50, 52, 53, and thus which fuel or fuel mixture to use. In such examples, the fuel characteristics may vary during a journey - a specific fuel or fuel mixture may be selected to improve operation at certain stages of flight or under certain external conditions, for example as described above with respect to other aspects. In examples where sensing is used for one or more fuel characteristics (either by direct sensing or by inference from sensed parameters), for example to discover or verify which tank contains the fuel to be used at startup, any of the sensing approaches described above may be implemented. In other examples, no sensing may be performed and the provided fuel composition data may be used instead - this data may simply be, for example, "Fuel for Start" or "Other", or may include more detailed information about the fuel characteristics. In other examples, no fuel data may be provided - instead, each tank 50, 52, 53 may be identified, for example, as a "start" tank or a "normal use" tank, and the example may rely on the tanks 50, 52, 53 being correctly filled accordingly. In some examples, the calorific values ​​for each available fuel may be calculated or provided, and a fuel or mixture of fuels is provided based on a thrust demand as described above (optionally also taking into account altitude) - fuel from the first tank 52 (e.g., a SAF) and / or fuel from the second tank 50 (e.g., a fossil fuel) may be used alone and / or in one or more mixtures in such examples. A method 2021 of operating an aircraft 1 comprising a gas turbine engine 10, 44 and a plurality of fuel tanks 50, 52 arranged to store fuel for powering the gas turbine engine 10, 44 is shown in [Fig.23]. The method 2021 comprises arranging 2023 two fuel tanks 50, 52 of the plurality of fuel tanks to each store a different fuel. In particular, a first fuel tank 52 of the plurality of fuel tanks is arranged to contain only a fuel that is a sustainable aviation fuel in this example, and a second fuel tank 50 of the plurality of fuel tanks is arranged to contain a fuel selected for improved starting properties, which may be or comprise a fossil hydrocarbon-based fuel. This arranging step 2023 may comprise fluidically isolating one or more tanks from each other to allow different fuels to be stored in different tanks (e.g., by closing valves). This arranging step 2023 may comprise filling the tanks appropriately. In some examples, one or more of the fuel tanks 50, 52 may be part of a separate set of interconnected fuel tanks. In other examples, each fuel tank 50, 52 may be a self-contained, single-tank fuel source. The method 2021 further includes controlling 2025 the supply of fuel to the gas turbine engine 10 so as to draw fuel from the second tank 50 for engine start, before switching to the first fuel tank 52. The first fuel tank 52 may contain SAF, and may be the sole source of fuel used for ground operations after the start-up, so that the SAF is used when aircraft 1 is performing at least the majority of ground operations. The fuel control 2025, possibly managed by a fuel management device 214, may include switching from drawing fuel from the second tank 50 to drawing fuel from the first fuel tank 52 when a selected parameter, such as fuel temperature, turbine gas temperature, oil temperature, shaft speed, or time since engine start, reaches a certain threshold. For example, the fuel switch may be performed when: (i) the fuel reaches a temperature of at least 60°C, and optionally 80°C, 85°C, 90°C, 95°C or 100°C, at the inlet to the combustion chamber 16; or (ii) after the gas turbine engine 10, 44 has been running for a period of at least thirty seconds, at least one minute, at least three minutes, or at least five minutes. For some engines 10, a period of at least 10 minutes or 15 minutes may be selected. It will be understood that an appropriate period may depend on environmental conditions (e.g., a lower air temperature and corresponding lower initial fuel temperature may result in a longer start-up period) and the properties of the aircraft 1 and fuel system, and may be adjusted as appropriate for a given aircraft | and environment. In some implementations, the switching from drawing fuel from the second tank 50 to drawing fuel from the first fuel tank 52 may be actuated when the engine 10 reaches idle conditions, and possibly a short period (e.g., thirty seconds) after idle conditions have been reached to ensure that idle operation has stabilized. In some cases, the engine 10 may be allowed to idle for at least two minutes, or at least five minutes. In some implementations, a period may be set based on how long the engine 10 has been shut down since its last use—for example, setting the period to two minutes if the engine 10 has been shut down for less than 90 minutes, and five minutes if the engine has been shut down for more than 90 minutes. The instant at which idle operation is reached may be identified based on a temperature or shaft speed, for example, which may be specific to the engine 10 and / or aircraft 1 in question. In some implementations, switching from fuel sampling in the second tank 50 to fuel sampling in the first fuel tank 52 may be actuated when the engine 10 reaches a limit defined in the engine operating instructions that prevent takeoff until certain criteria are met. It will be understood that specific parameters and values ​​may be consulted in the engine operating instructions for a given engine 10. The engine 10 reaching a state in which it would be ready for takeoff indicates that a start phase has been completed (although the fuel change may be performed earlier in some implementations). The method 2021 further optionally includes storing 2027 information about the fuel contained in each fuel tank 50, 52, optionally in a memory of an on-board fuel management device 214. The stored information may simply be a flag indicating whether a particular tank 50, 52 contains a fuel selected for its starting properties (which may be a fossil fuel). A flag indicating that one or more tanks contain 100% SAF or a high SAF% blend may also be provided. Additional information may be stored in other examples.This stored information may be used for control step 2025, and in particular may be used to identify the first fuel tank 52 (and / or correspondingly one or more tanks containing a fuel for general use / for purposes other than starting, if there are multiple such tanks) and the second fuel tank 50 (and / or correspondingly one or more other tanks, if multiple tanks contain fuels suitable for starting), if not hard-coded / wired into the propulsion system 2. A fuel system 4 for an aircraft 1 may therefore comprise a fuel management device 214 arranged to store information about each fuel tank 50, 52, 53 / about the fuel contained in each fuel tank 50, 52, 53 and to control the supply of fuel to the gas turbine engine 10, 44 in operation. The stored information may simply comprise a flag indicating whether or not each tank is the starting tank, or may comprise more detailed information, such as a % SAF content for each tank, and / or one or more other fuel characteristics of the fuel currently in each tank 50, 52.In such examples, which tank 50, 52 of the plurality of fuel tanks 50, 52, 53 is the first tank 52 and which tank is the second fuel tank 50 may vary over the lifetime of the fuel system 4, for example depending on which tank is filled with which fuel. In other examples, the fuel distribution system 220 as shown in [Fig. 16] may be configured so that a specific tank 50 is always the starting tank, and no such information needs to be stored. Optionally, a specific tank . 52 can still be a / the SAF tank or a / the high % SAF mix tank. In implementations in which the starting fuel and the other first fuel are supplied to a main gas turbine engine 10 of the aircraft 1, which provides propulsive power to the aircraft 1, the power system 4 may be more specifically referred to as the propulsion system 2. The more general term "power system" 4 mentioned above is used herein to ensure that implementations in which the fuels are supplied in addition or alternatively to an APU 44 are included, as propulsive power may not be provided by such power systems 4. As described above for other examples, the fuel management device 214 may be provided as part of a fuel delivery system 220 arranged to allow control and adjustment of the fuel supplied to the gas turbine engine 10, 44; all of the features described above may be suitably applied to the examples currently described. In examples with APUs 44, the fuel management device 214 may be arranged to control the fuel or fuel mixture supplied to the APU 44 as well as to the main propulsion engine(s) 10. The APU 44 may be required in flight under certain circumstances, and its availability in such circumstances may be time-critical—for example, to restart the main engines 10 after a main engine flameout. When starting the APU 44 in flight, it may be cold (unused for several hours); the fuel management device 214 may therefore be (alternatively or additionally) arranged to provide the second fuel to effect the start of the APU 44 in flight quickly and reliably.To avoid wasting time by inadvertently trying to start the APU 44 with fuel having poor starting properties, the APU 44 could automatically switch to drawing fuel from the second tank (e.g. with a low viscosity fuel at a given temperature) whenever the aircraft 1 is in flight (e.g. with reference to the "weight on wheels" indicator), but use any fuel when on the ground (when start-up time is less likely to be crucial). In some implementations, the first tank 52 may be empty for at least part of the in-flight portion of the journey, or even refilled with a different fuel if the tank 52 is used as a trim tank. The fuel management device 214 may therefore be arranged to take appropriate action based on the current contents and / or usage of the first fuel tank 52 when considering whether or not to draw fuel. fuel in the first fuel tank for starting engine 10, 44 in flight. As described above for other examples, the fuel management device 214 may further receive other data (in addition to information indicating which tank(s) contain(s) a fuel selected for startup, and other optional fuel characteristic data, such as SAF content), and use this other data and the fuel characteristic data to determine a desired fuel composition for the gas turbine engine 10, 44 in flight. In some examples, an aircraft 1 may be modified to perform the method 2021 described above, possibly by installing an adjustable fuel delivery system 220. A method 2031 of modifying an aircraft 1 in this manner is shown in [Fig. 24]. The original aircraft 1 comprises a gas turbine engine 10, the gas turbine engine 10 optionally comprising an engine core 11 comprising a turbine 19, a compressor 14, and a main shaft 26 connecting the turbine to the compressor. The aircraft 1 also comprises a fan 23 located upstream of the engine core, the fan comprising a plurality of fan blades and being arranged to be driven by an output of the main shaft. The original aircraft 1 may further comprise an APU 44, the APU itself being or comprising a gas turbine engine 44. The method 2031 comprises arranging 2033 two fuel tanks 50, 52 of the plurality of fuel tanks to each store a different fuel. In particular, a first fuel tank 52 of the plurality of fuel tanks is arranged to contain a first fuel, which may be a sustainable aviation fuel, and a second fuel tank 50 of the plurality of fuel tanks is arranged to contain a fuel selected for its improved starting properties compared to the first fuel, and which may be a fossil hydrocarbon-based fuel. In some cases, the aircraft 1 may already include a plurality of fuel tanks 50, 52 arranged to store fuel to power the one or more gas turbine engines 10, 44; in such examples, the step 2033 of arranging the fuel tanks may simply include selectively filling the tanks with different fuels. In cases where the aircraft 1 previously had only one fuel tank 50, one or more new fuel tanks 52, 53 may be added so as to provide a plurality of fuel tanks. In cases where the aircraft 1 previously had only one fuel source, albeit comprised of multiple tanks, a new fuel tank fuel 52 may be added and / or fuel lines may be adjusted so that the original tanks 50, 53 are no longer all fluidically interconnected, thereby providing at least two separate fuel sources. The arrangement step 2033 may therefore vary depending on the initial configuration of the aircraft. The method 2031 further comprises providing 2035 a fuel management device 214 arranged to control the fuel supply so as to draw fuel from the second tank 50 for engine start, before switching to the first fuel tank 52. The fuel management device 214 may use fuel from the first fuel tank 52 (which is an SAF in a specific example described, but may be a blend of SAF or a pure fossil fuel in other examples) for all ground operations after start. In some examples, such as arrangements in which the tank(s) used to store the fuel selected for start-up may vary over the lifetime of the propulsion system 2, the fuel management device 214 may further be arranged to store information about the fuel contained in each fuel tank 50, 52 to enable identification of the first tank 52 and the second tank 50, or equivalent. The storage and control functions may be performed by separate entities or by the same entity; it will be understood that the fuel management device 214 may therefore be a distributed system or a single unit or module. The step of providing 2035 the fuel management device 214 may include installing or consist of installing software into existing memory, to be executed using existing systems, in some examples. In other examples, a new physical unit or module may be mounted on the propulsion system 2, possibly including one or more flow regulators 216 and / or replacement fuel line sections, if applicable, to achieve the desired fuel flow and mixture control. In some examples, the fuel management device 214 may further be arranged to perform other functions, for example, to control the supply of fuel to the gas turbine engine 10 by selecting a specific fuel or combination of fuels from one or more of the plurality of fuel tanks 50, 52, 53 based on a thrust demand of the gas turbine engine 10 such that a fuel having a lower heating value is supplied to the gas turbine engine 10 at a lower thrust demand, and vice versa. It will be understood that the thrust demand may be determined using one or more approaches known in the art, for example as mentioned above. The inventors also appreciated that, since different fuels may have different properties, while still remaining compliant with standards, knowledge of the flight profile may allow selection of which of the fuels available to an aircraft 1 is used for which part(s) of the flight profile (when multiple fuels are available) - this may improve the performance of the aircraft. For example, a fuel with improved emissions results may be selected for operations at or near an airport, and a fuel with a higher heating value may be used for operations with higher thrusts. A refueling schedule defining which fuel, or mixture of fuels, to use for each part of the flight profile may therefore be determined based on knowledge of the flight profile and the available fuels. Such a method 3020 is illustrated in [Fig. 25]. It will be understood that an aircraft 1 for which the method 3020 is implemented must include at least two fuel sources 50, 53, such that at least two different fuels (i.e., fuels having at least one difference in fuel characteristics) are available for use. It will be understood that the use of fuel blends with different mixing ratios can allow for many more than two refueling options even with only two different fuels stored on board. The method 3020 may be performed on-site, for example by a refueling schedule determination module 250 of the aircraft 1 as shown in [Fig. 21]. Such a refueling schedule determination module 250 may be part of an electronic engine governor (EEC) 42 of the aircraft 1, possibly provided as software installed on an existing EEC 42, or added as a module thereto, or may be provided by a separate module. Alternatively, the method 3020 may be performed in a workshop, and the refueling schedule is provided to the aircraft 1 for implementation. It will be understood that any suitable processing means may be used to act as the refueling schedule determination module 250, and that computer-readable instructions for causing the processing means to implement the described method 3020 may be provided. The method 3020 may therefore be executed by processing circuits of the aircraft 1, or by separate processing circuits, in the workshop. The method 3020 may be executed by processing circuits of a portable computing device, for example the personal computing device of a pilot. A fuel management device 214 as described above may be used to implement the refueling schedule. The refueling schedule determination module 250, or other processing circuitry, may provide the refueling schedule to the fuel management device 214 for implementation. The method 3020 includes obtaining a flight profile for a flight of the aircraft 1. The flight profile may be provided to a refueling schedule determination module 250 of the aircraft 1, or to a shop refueling schedule determination module. The flight profile may be obtained in any suitable manner, for example by electronic transmission, manual entry via a user interface, or retrieval from memory. The method 3020 further includes determining 3024 a refueling schedule for the flight of the aircraft 1 based on the flight profile and fuel characteristics of the available fuels. When determining 3024 the refueling schedule, an amount of each fuel available on board the aircraft 1 is also considered. The altitude and route of the aircraft 1 for the intended flight are defined in the flight profile. The predicted thrust demands may be included in the flight profile, or determined or inferred based on the flight profile, and may be used to guide fuel selection. In addition, data relating to the predicted weather conditions along the intended route of the aircraft 1 as defined in the flight profile may be provided with the flight profile, or requested based on the flight profile.Weather data can be used to influence the determination 3024 of fuel scheduling. . The determined refueling schedule specifies a desired variation over time in the amount of fuel drawn from each tank 50, 53; that is, it lists which fuel or fuel mixture is to be used for each stage of flight as defined in the flight profile, and thus determines when a refueling change is to be made and the nature of the change. The fuel characteristics considered include one or more of the fuel characteristics as defined elsewhere herein. For example, the amount of sustainable aviation fuel - SAF - available for aircraft 1, possibly both as neat SAF and in blends, can be determined. As described above, the use of SAF or high SAF% blends for ground operations can reduce nyPM emissions and thus improve air quality at airports, and the refueling program can prioritize the use of high SAF / SAF% fuel for aircraft 1's ground operations. Similarly, and as described above, a heating value of each fuel on board the aircraft 1 may be a fuel characteristic of interest, and the fueling program may prioritize the use of higher heating value fuels for high thrust operations of the aircraft 1, and lower heating value fuels for low thrust operations of the aircraft 1. The refueling program may be determined on board the aircraft |; for example in an electronic engine governor 42 or other processing circuits of the aircraft 1, or in a device belonging to the pilot. Alternatively, the refueling program may be determined in the workshop, for example in a server or other computer system on the ground. The determined refueling program may therefore be provided 3025 to the aircraft 1 before or at the start of the flight to which the refueling program applies. Different steps of the method 3020 may therefore be carried out by entirely separate entities in some cases, or all within or by the aircraft 1 in other cases. In some implementations, the method 3020 further includes controlling 3026 the supply of fuel to the operating gas turbine engine 10 in accordance with the refueling schedule. In particular, a fuel management device 214 as described above may receive the refueling schedule and control the supply of fuel accordingly, for example, by opening or closing one or more valves, or by activating or deactivating one or more pumps, as appropriate to provide the desired fuel or fuel mixture at each stage of the flight. A propulsion system 2 for an aircraft 1 may be arranged to implement the method 3020 described above. The propulsion system 2 comprises a gas turbine engine comprising at least two separate fuel sources 50, 53, such that at least two different fuels (i.e. fuels having different fuel characteristics) are stored on board the aircraft 1. The propulsion system 2 of such examples comprises a refueling schedule determination module 250, which is arranged to obtain 3022 a flight profile for a flight of the aircraft 1; and determine 3024 a refueling schedule for the flight based on the flight profile and the fuel characteristics. The refueling schedule determination module 250 may itself implement the control 3026 of the fuel supply to the gas turbine engine 10, or may transmit the refueling schedule to a fuel management device 214 for implementation. The propulsion system 2 of some examples further comprises a receiver 251 arranged to receive forecast weather conditions for the intended route of the aircraft 1, which is defined in the flight profile. The received forecast weather conditions may be used to influence the refueling schedule, as mentioned above. It will be understood that the invention is not limited to the embodiments described above and that various modifications and improvements may be made without departing from the concepts described herein. Unless mutually exclusive, any of the features may be used separately or in combination with any other feature and the disclosure extends to and includes all combinations and subcombinations of one or more features described herein.

Claims

Demands

1. Power supply system (4) for an aircraft (1) comprising: at least one gas turbine engine (10, 44) arranged to burn a fuel to provide power to the aircraft (1); a plurality of fuel tanks (50, 52, 53), each arranged to contain a fuel to be used to provide power to the aircraft (1), in which at least two tanks of the plurality of re- Fuel tanks (50, 52, 53) contain different fuels, the different fuels each having a different proportion of a sustainable aviation fuel; and a fuel management device (214) arranged to: store information about the fuel contained in each tank of fuel (50, 52, 53); identify which tank (52) contains the fuel having the proportion the higher than sustainable aviation fuel; and control the fuel supply in such a way as to draw from fuel only in the tank (52) containing the fuel having the highest proportion of sustainable aviation fuel for to supply at least the majority of ground operations.

2. Power supply system (4) of claim 1, wherein a motor at least one gas turbine engine (10, 44) is a gas turbine engine (44) of an Auxiliary Power Unit - APU.

3. Power supply system (4) of claim 2, wherein a first fuel tank (52) of the plurality of tanks of fuel (50, 52, 53) is arranged to contain aviation fuel sustainable and is exclusively dedicated to the APU (44) so ​​that the fuel from the first fuel tank (52) is not arranged to be supplied to any other gas turbine engine (10) of the aircraft (1).

4. Power supply system (4) of claim 1, wherein the fuel with the highest proportion of aviation fuel sustainable - SAF - contains more than 50% SAF.

5. Power supply system (4) of claim 1, wherein a first fuel tank (52) of the plurality of tanks of fuel (50, 52, 53) is arranged to contain the fuel having the the highest proportion of a sustainable aviation fuel, is more smaller than at least one other fuel tank (50, 53), and is designed for use exclusively for ground operations the aircraft (1).

6. Power supply system (4) of claim 1, wherein a a plurality of fuel tanks (50, 52, 53) contain a fuel having the same highest proportion of aviation fuel durable, and in which the tank to be used is selected on the basis from the comparison of at least one among: (i) levels of non-volatile particulate matter emissions during of combustion; and (ii) hydrogen to carbon ratios of fuels.

7. Power supply system (4) of claim 1, wherein the fuel with the highest proportion of aviation fuel durable for use in ground operations has a calorific value lower than that of any fuel stored in another tank of fuel (50, 53) from the plurality of fuel tanks (50, 52, 53).

8. Fuel supply system (4) of claim 1, wherein the fuel with the highest proportion of aviation fuel durable is used to power all ground operations of the aircraft (1).

9. Power supply system (4) of claim 1, wherein a first fuel tank (52) of the plurality of tanks of fuel (50, 52, 53) is arranged to contain the fuel having the the highest proportion of sustainable aviation fuel and is arranged to supply fuel to the gas turbine engine (10, 44) during the execution of ground operations, and to serve as a reservoir of com- pensation (52) in flight.

10. Power supply system (4) for an aircraft (1) comprising: a gas turbine engine (10, 44) arranged to burn a fuel in order to to provide power to the aircraft (1); a plurality of fuel tanks (50, 52, 53), each arranged to contain a fuel to be used to provide power to the aerone[ (1), in which at least two reservoirs of the plurality of re- Fuel tanks (50, 52, 53) contain different fuels, a first tank (52) containing a fuel that is more than 50% sustainable aviation fuel and a second tank (50, 53) containing a fuel that is less than 50% aviation fuel sustainable; and a fuel management device (214) arranged to: store information about the fuel contained in each tank of fuel (50, 52, 53); and control the fuel supply in such a way as to draw from fuel only in the tank (52) containing the fuel which is more than 50% sustainable aviation fuel to power the less the majority of ground operations.

11. Method (2061) of operating an aircraft (1) comprising a gas turbine engine (10, 44) and a plurality of tanks of fuel (50, 52, 53) arranged to store fuel to power the gas turbine engine (10), the process comprising: the arrangement (2063) of two fuel tanks (50, 52, 53) or plus the plurality of fuel tanks to store different fuels, the different fuels each having a proportion different from sustainable aviation fuel: the identification (2067) of the tank (52) which contains the fuel having the highest proportion of sustainable aviation fuel; and the fuel supply control (2065) so as to draw fuel only from the tank (52) containing the fuel with the highest proportion of aviation fuel durable when the aircraft (1) performs at least the majority of the ground operations.

12. Method (2061) of claim 11, further comprising storage (2064) information on the fuel contained in each tank of fuel (50, 52, 53), and in which the order (2065) is carried out based on the stored information.

13. Method (2071) of modifying an aircraft (1) comprising an engine a gas turbine (10, 44) and a plurality of fuel tanks (50, 52, 53) arranged to store fuel to power the engine at gas turbine (10, 44), the process comprising: the arrangement (2073) of two fuel tanks (50, 52, 53) or plus the plurality of fuel tanks to store different fuels, the different fuels each having a proportion different from sustainable aviation fuel; and the supply (2075) of a fuel management device (214) arranged For: identify which tank (52) contains the fuel having the proportion the higher than sustainable aviation fuel; and control the fuel supply in such a way as to draw from fuel only in the tank (52) containing the fuel having the highest proportion of sustainable aviation fuel when the aircraft (1) performs at least the majority of ground operations.

14. Method (2071) of claim 13, wherein the device of fuel management (214) is further arranged to store in- training on the fuel contained in each fuel tank (50, 52, 53), and in which the fuel management device (214) is arranged to perform tank identification and ordering fuel supply based on stored information.

15. Fuel system (4) for an aircraft (1) comprising: an Auxiliary Power Unit (APU) comprising an engine gas turbine (44) arranged to burn a fuel in order to provide a power to the aircraft (1); and a first fuel tank (52) arranged to contain only a fuel that is a sustainable aviation fuel; and in which the first fuel tank (52) is dedicated to the APU, so that all the fuel used by the APU is drawn from the first fuel tank (52).