VARIABLE OIL FLOW FOR GAS TURBINES

Abstract

A method 200 for operating a gas turbine engine for aircraft, comprising: an engine core including a turbine, a compressor, a combustion chamber configured to combust a fuel, and a core shaft connecting the turbine to the compressor; a fan disposed upstream of the engine core; and a gearbox receiving input from the core shaft and outputting drive to the fan; an oil circulation system arranged to supply oil to the gearbox; and a heat exchange system including an air-to-oil heat exchanger through which the oil flows; a fuel-to-oil heat exchanger through which the oil and fuel flow; and a valve configured to vary the proportion of oil passed over at least one of the heat exchangers, is disclosed, the method 200 comprising controlling 202 the valve such that, under idle conditions, an oil flow ratio of: Rate of oil flow into the air − oil heat exchanger Rate of oil flow into the fuel − oil heat exchanger in the range of 0.62 to 5.29.

Description

The present disclosure relates to aircraft propulsion systems and methods of operating aircraft that include the management of various fluids and the exchange of heat therebetween, and more particularly to the management of oil and / or air streams within an aircraft engine.In the aeronautical industry, development toward the use of fuels different from the currently widely used conventional kerosine-based jet fuels is expected. The fuels may have different fuel properties compared to petroleum-based hydrocarbon fuels. Thus, there is a need to account for fuel properties for these new fuels and adapt both the gas turbine engines themselves and the methods of operating gas turbine engines.According to a first aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; and at least one valve configured so that a part of the oil conducted via at least one of the heat exchangers can be varied.The method includes controlling the at least one valve such that an oil flow ratio of: is in the range of 0 to 0.59 under cruise conditions.The inventors have recognized that the use of fuels different from conventional kerosine-based jet fuels, such as sustainable aircraft fuels, may result in different fuel properties and that parameters under cruise conditions may be adjusted to take advantage of the different fuel properties. In particular, some fuels may be heated to higher temperatures than conventional fuels in one or more fuel-oil heat exchangers without significantly increasing the deposition of degradation products of the fuel, e.g., by coking or paint. This may enable a process that provides improved oil cooling (as the fuel may draw more heat) and may also improve overall thermal efficiency / thermodynamic efficiency of the engine as less heat is lost to the environment and possibly also more energy is recovered in the thermodynamic cycle. The ability to control oil flow in the engine plays a key role in controlling heat transfer. The inventors have recognized that these principles can be applied to engines having branched oil circulation paths with different heat exchangers on different branches (parallel arrangements) as well as to engines having a substantially linear, serial arrangement of heat exchangers in which one or more bypass pipes can be used as an alternative to a branched main path. The introduction of one or more controllable oil valves and / or the improved control of existing oil valves may therefore enable a method that provides better oil cooling (as the fuel to be used may absorb more heat than conventional fuels) and may also improve overall thermal efficiency of the engine. The one or more controllable oil valves may adjust the oil flow ratio for a particular fuel accordingly.Although the oil flow ratio is dimensionless, the units (cubic meters per second or kilograms per second) are given above as examples. In order to obtain a dimensionless ratio, the numerator and denominator must have the same units. The flow rates used can therefore be either volumetric flow rates or gravimetric flow rates (i.e., mass flow rates), provided that the numerator and denominator are consistent.The oil circulation system may branch such that a portion of the oil may flow along each branch. The air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of an oil circulation system. The at least one valve configured to allow for the change in the proportion of oil routed via at least one of the heat exchangers may be or may comprise a modulation valve configured to allow for the change in the proportion of oil routed via each branch.The oil circulation system may include at least one bypass pipe configured to allow a portion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger. The at least one valve configured such that the proportion of oil conducted via at least one of the heat exchangers can be varied can be or comprise a bypass valve configured such that a portion of the oil can bypass the at least one heat exchanger. The heat exchange system may include a plurality of bypass tubes, each configured to allow oil to bypass a heat exchanger. In such implementations, the step of controlling the at least one valve may be or may comprise controlling at least two bypass valves. In particular, the oil circulation system may comprise at least one bypass pipe configured such that a portion of the oil may bypass the air-oil heat exchanger.In implementations with one or more bypass pipes, the air-oil and fuel-oil heat exchangers in the oil circulation system may be arranged in series (such that a main oil flow path passes through both in succession) or in parallel (such that a main oil flow path splits and a branch passes through both in each case). In implementations where the air-oil and fuel-oil heat exchangers are arranged in parallel on different branches of the oil circulation system, the step of controlling the at least one valve may comprise controlling both one or more bypass valves and a modulation valve arranged such that the portion of oil routed via each branch may be varied.The method may comprise controlling the at least one valve such that the oil flow ratio is in the range of 0 to 0.50, optionally 0 to 0.40, and further optionally 0 to 0.30, 0 to 0.20, or optionally 0 to 0.10 under cruise conditions.The step of controlling the at least one valve to adjust the oil flow ratio may comprise reducing the amount of oil directed across the at least one air-oil heat exchanger when the oil flow ratio is too high. Additionally or alternatively, the amount of oil directed via the at least one fuel-oil heat exchanger may be increased if the oil flow ratio is too high.The method may include determining the fuel temperature downstream of the fuel-oil heat exchanger (and optionally at the inlet to the combustion chamber) and adjusting control of the oil flow ratio based on that fuel temperature if necessary. The method may comprise controlling the at least one valve under cruise conditions such that: (i) the oil flow ratio is in the range of 0 to 0.4, optionally 0 to 0.3, and more optionally 0 to 0.25, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 140°C; (ii) the oil flow ratio is in the range of 0 to 0.3, optionally 0 to 0.2, and more optionally 0 to 0.15, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 160°C; and / or (iii) the oil flow ratio is in the range of 0 to 0.2, optionally 0 to 0.1, and more optionally 0 to 0.075, provided that the fuel temperature downstream of the fuel-oil heat exchanger is at least 180°C.The method may include determining one or more fuel characteristics and adjusting control of the oil flow ratio based on the one or more determined fuel characteristics if necessary. For example, the method may comprise controlling the at least one valve such that the oil flow ratio is in the range of 0 to 0.3, optionally in the range of 0 to 0.2, and further optionally 0 to 0.15, provided that the fuel is at least 70% of sustainable flight fuel (SAF). The SAF fraction (X %) may be volumetric.The heat exchange system may include a refrigeration cycle device configured to provide thermal buoyancy by transferring additional heat from the oil to the fuel beyond the heat transferred from the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle device to adjust the amount of additional heat transferred to the fuel. In such implementations, the fuel temperature may be raised above the oil temperature.According to a second aspect, there is provided a gas turbine engine for an aircraft comprising:an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core; anda transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; and at least one valve configured so that a part of the oil conducted via at least one of the heat exchangers can be varied.The at least one valve is configured to be controllable to have an oil flow ratio of: in the range of 0 to 0.59 under cruise conditions.The gas turbine engine may further include a controller configured to control the valve. The gas turbine engine may further include one or more oil flow rate sensors. The controller may be configured to receive outputs from one or more oil flow rate sensors and make control decisions based on these outputs. The oil flow rate may be directly detected or inferred from one or more other measurements, e.g., by measuring the pressure drop across an orifice.The heat exchange system may further include a refrigeration cycle device configured to provide thermal lift by the transfer of further heat from the oil to the fuel, optionally such that the fuel temperature is raised above the oil temperature.The heat exchange system may further comprise branching fuel return paths and at least one valve that controls a split of fuel flow, the branching paths arranged to return fuel from the heat exchange system at at least two different locations along a main fuel path leading to the combustor from where fuel enters the gas turbine engine.The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core shaft.The engine of the second aspect may be configured to perform the method of the first aspect and may include any of the features described with respect to the first aspect.According to a third aspect, there is provided a method of operating a gas turbine engine for an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andat least one valve configured to vary a portion of the oil directed via at least one of the heat exchangers.The method includes controlling the at least one valve such that an oil flow ratio of: is in the range of 0.62 to 5.29 under idle conditions.As discussed in the first aspect, the inventors have recognized that the use of fuels different from conventional kerosine-based jet fuels, such as persistent flying fuels, may result in different fuel properties and that the parameters may be adjusted in operation to take advantage of the different fuel properties. In particular, some fuels may be heated to higher temperatures than conventional fuels in one or more fuel-oil heat exchangers without significantly increasing the deposition of degradation products of the fuel, e.g., by coking or paint. This may enable a method that provides improved oil cooling (as the fuel may draw more heat) and may also improve overall thermal efficiency of the engine as less heat is lost to the environment. The one or more controllable oil valves play a key role in controlling the oil flow ratio. Moreover, the inventors have recognized that while cruise conditions generally account for a much greater portion of the aircraft engine operating time, idle operation is also important. Since the mass flow rate of the fuel is much lower in idle than in cruising, even a relatively low heat load of the fuel may result in a large temperature rise. The use of non-conventional fuels can therefore have an even greater effect on optimum approaches for thermal management under idling conditions, for example in ground idling while the aircraft is starting, in operation during standstill times during boarding and rolling (to a runway or a hangar or between other ground-bound locations) or in flight idling, for example at the beginning of the descent. Since the operating conditions differ greatly between cruise and idle, in particular with respect to the desired thrust output of the engine, a different control of the oil flow is appropriate.As for the first and second aspects, although the oil flow ratio is dimensionless, the units (cubic meters per second or kilograms per second) are exemplified above to clarify that the numerator and denominator must have the same units. Idle operation while the aircraft is operating on the ground may be referred to as "ground idle.". Idle operation while the aircraft is in air may be referred to as "flight idle.". All options described below for this aspect may be considered relevant to ground idle conditions unless otherwise indicated. In flight idle, the thrust is generally somewhat higher than in ground idle. In some implementations, only the less restrictive areas may apply to the idle flight of a particular engine. In other implementations, all options described below for this aspect may also apply to the flight idle conditions.The oil circulation system may branch such that a portion of the oil may flow along each branch. The air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of an oil circulation system. The at least one valve configured to allow for the change in the proportion of oil routed via at least one of the heat exchangers may be or may comprise a modulation valve configured to allow for the change in the proportion of oil routed via each branch.The oil circulation system may include at least one bypass pipe configured to allow a portion of the oil to bypass at least one of the air-oil heat exchanger and the fuel-oil heat exchanger. The at least one valve configured such that the proportion of oil conducted via at least one of the heat exchangers can be varied can be or comprise a bypass valve configured such that a portion of the oil can bypass the at least one heat exchanger. The heat exchange system may include a plurality of bypass tubes, each configured to allow oil to bypass a heat exchanger. In such implementations, the step of controlling the at least one valve may be or may comprise controlling at least two bypass valves.In implementations with one or more bypass pipes, the air-oil and fuel-oil heat exchangers in the oil circulation system may be arranged in series (such that a main oil flow path passes through both in succession) or in parallel (such that a main oil flow path splits and a branch passes through both in each case). In implementations where the air-oil and fuel-oil heat exchangers are arranged in parallel on different branches of the oil circulation system, the step of controlling the at least one valve may comprise controlling both one or more bypass valves and a modulation valve arranged such that the portion of oil routed via each branch may be varied.The method may comprise controlling the at least one valve such that the oil flow ratio is below 5.50, optionally below 5.0, optionally below 4.5, and further optionally below 4.0 under idling conditions.The step of controlling the at least one valve to adjust the oil flow ratio may comprise reducing the amount of oil directed across the at least one air-oil heat exchanger when the oil flow ratio is too high.The method may comprise determining the fuel temperature downstream of the fuel-oil heat exchanger, and optionally at the entrance to the combustion chamber, and adjusting control of the oil flow ratio, if necessary, based on that fuel temperature. The method may comprise controlling the at least one valve under idle conditions such that: (i) the oil flow ratio is in the range of 0.62 to 4.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 140° C.; (ii) the oil flow ratio is in the range of 0.62 to 3.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 160° C.; and / or (iii) the oil flow ratio is in the range of 0.62 to 2.00 when the fuel temperature downstream of the fuel-oil heat exchanger is above 180° C.The method may include determining one or more fuel characteristics and adjusting control of the oil flow ratio based on the one or more determined fuel characteristics if necessary. For example, under idle conditions, the method may include controlling the at least one valve such that the oil flow ratio is in the range of 0.62 to 3.67 if the fuel is at least 70% persistent flight fuel (SAF).The heat exchange system may include a refrigeration cycle device configured to provide thermal buoyancy by transferring additional heat from the oil to the fuel beyond the heat transferred from the fuel-oil heat exchanger. The method may include controlling the refrigeration cycle device to adjust the amount of additional heat transferred to the fuel. In such implementations, the fuel temperature may be raised above the oil temperature.The methods of the first and third aspects may complement each other and may be performed together in various implementations. The method of the third aspect may be performed using the engine of the second aspect.According to a fourth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core; anda transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andat least one valve configured to vary a portion of the oil directed via at least one of the heat exchangers.The at least one valve is configured to be controllable to have an oil flow ratio of: in the range of 0.62 to 5.29 under idle conditions.The heat exchange system may further include a refrigeration cycle device configured to provide thermal lift by the transfer of further heat from the oil to the fuel, optionally such that the fuel temperature is raised above the oil temperature.The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core shaft.The heat exchange system may further comprise branching fuel return paths and at least one valve that controls a split of fuel flow, the branching paths arranged to return fuel from the heat exchange system at at least two different locations along a main fuel path leading to the combustor from where fuel enters the gas turbine engine.The engine of the fourth aspect may be configured to perform the method of the first and / or third aspect and may include any of the features described with respect to the first, second or third aspect.According to a fifth aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andan air valve configured to control the flow rate of the air through the air-oil heat exchanger.The method comprises:determining at least one fuel property of the fuel to be burned by the combustion chamber; andcontrolling the air valve based on the at least one fuel property to adjust the flow rate of the air through the air-oil heat exchanger.The inventors have recognized that the use of fuels different from conventional kerosine-based jet fuels, such as sustainable aircraft fuels, may result in different fuel properties and that operating parameters may be adjusted to take advantage of the different fuel properties. In particular, some fuels may be heated to higher temperatures than conventional fuels in one or more fuel-oil heat exchangers without significantly increasing the deposition of degradation products of the fuel, e.g., by coking or paint, which may allow the fuel to absorb more heat and thus require less air cooling. This may improve the combustion efficiency of the fuel and also increase the overall efficiency of the engine, with less heat being lost to the environment (via the air-oil heat exchanger). Higher fuel temperatures when entering the combustion chamber may therefore allow a method that provides improved oil cooling, provided the fuel properties allow for this. The thermal stability of the fuel (a fuel property) has an effect, for example, on how much heat the fuel can absorb or to which temperature the fuel can be brought without deposits forming in lines, burners and / or a hydromechanical unit or another engine component. Taking into account the thermal stability of the fuel and reducing the air flow rate through the air-oil heat exchanger (optionally to zero) as the thermal stability of the fuel is higher may result in a more thermally efficient engine while avoiding coking or excessive fouling and thus improving the performance of the aircraft. Thus, utilizing the fuel to draw more heat from the oil rather than rely on heat transfer from the oil to the ambient air (in the air-to-oil heat exchanger) can both provide a thermodynamically more efficient engine and improve cooling of the oil before it is recirculated to the remainder of the turbine engine.In a transient period, where the available flying fuels change over time during the life of a gas turbine engine and depend on the geographical location of a refueling point (among other variables), it is important that a relevant fuel property be determined and the control of the air flow rate be based on the fuel used. Thus, the operation of gas turbines may be adjusted to retrieve the best from a variety of fuels. A key role in adapting engine performance is the controllable airflow valve which adjusts the air flow rate through the air-oil heat exchanger.The method may include, depending on the suitability of the at least one determined fuel property, controlling the air valve under idle conditions such that: (i) the air flow rate into the air-to-oil heat exchanger is reduced to less than 60% of the flow rate that would prevail at fully open valve and under engine idle conditions; and / or (ii) the air flow rate into the air-to-oil heat exchanger is reduced to less than 40% of the flow rate that would prevail at fully open valve and under engine idle conditions.The method may include, depending on the suitability of the at least one determined fuel property, controlling the air valve under cruise conditions such that: (i) the air flow rate into the air-to-oil heat exchanger is reduced to less than 20%, and optionally to less than 17%, of the flow rate that would prevail when the valve is fully open and under cruise conditions of the engine; and / or (ii) the air flow rate into the air-to-oil heat exchanger is reduced to at least substantially zero when the engine is running under cruise conditions.The suitability of the at least one determined fuel property may be determined by comparing one or more fuel properties with a threshold value established for the respective fuel property(s). For example, a fuel having a SAF content (% SAF) of over 60%, 65%. For a reduction of the air flow rate compared to conventional flying fuels, 70%, 75% or 80% may be considered suitable. In some implementations, multiple fuel characteristics may be checked together - for example, by using a lower threshold for one fuel characteristic when another fuel characteristic is within a certain range. The fuel properties may be considered independent / isolated in other implementations.The methods of the first, third, and fifth aspects may complement each other, and two or more of them may be performed together in various implementations. For example, both oil flow and air flow rates may be controlled in cruise and / or idle operation. The method of the fifth aspect may be performed using the engine of the second or fourth aspect.The at least one ascertained fuel property may be or comprise thermal stability. In such implementations, the air flow rate into the air-oil heat exchanger during cruise operation may be reduced to less than 15% of the flow rate that would prevail when the valve is fully open, provided that the fuel is stable during operation at temperatures above 160° C. Alternatively or additionally, the air flow rate into the air-oil heat exchanger during cruising may be reduced to less than 5% of the flow rate that would prevail when the valve is fully open, provided that the fuel is stable during operation at temperatures above 180° C. In some implementations, the air flow rate may be reduced with increasing thermal stability, optionally linearly.The at least one ascertained fuel property can be the content of aromatic hydrocarbons in the fuel or can comprise this. In such implementations, the airflow rate into the air-oil heat exchanger during cruising may be reduced to less than 5% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a molar proportion of aromatic hydrocarbons of less than 12%, and optionally less than 10% or less than 5%.The at least one determined fuel property may be or include the percentage of sustainable flying fuel - SAF - in the fuel (% SAF). In such implementations, the air flow rate into the air-oil heat exchanger during cruise operation may be reduced to less than 5% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a SAF content of over 60%. Alternatively or additionally, the air flow rate into the air-oil heat exchanger during cruising may be reduced to less than 2% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a SAF content of over 80%. In some implementations, the air flow rate may be reduced as the % SAF increases, optionally linearly, once the fuel exceeds 60%, 65%, 70%, 75%, or 80% SAF.The at least one ascertained fuel property may be the heating value of the fuel or may comprise the latter. In such implementations, the air flow rate into the air-oil heat exchanger during cruise operation may be reduced to less than 4% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a heating value of at least 43.5 MJ / kg. The at least one ascertained fuel property may be or comprise the thermal stability of the fuel.Possibilities for the fuel properties and for their determination are listed below. These possibilities are provided by way of example only and are not intended to be limiting.According to a sixth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged to supply the transmission with oil; anda heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andan air valve configured to control the flow rate of the air through the air-oil heat exchanger; anda fuel composition determination module configured to determine at least one fuel property of the fuel to be burned by the combustion chamber.The air valve is configured to be controllable based on the at least one fuel characteristic to adjust the flow rate of air through the air-oil heat exchanger.A control device may be provided for controlling the valve.The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core shaft.The heat exchange system may further comprise branching fuel return paths and at least one valve that controls a split of fuel flow, the branching paths arranged to return fuel from the heat exchange system at at least two different locations along a main fuel path leading to the combustor from where fuel enters the gas turbine engine.The engine of the sixth aspect may be configured to perform the method of the first, third and / or fifth aspect and may include any of the features described with respect to any of the preceding aspects.According to a seventh aspect, there is provided a method of operating a gas turbine engine of an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged for supplying the transmission with oil;a heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andat least one valve configured to allow variation of at least one of the oil flow rate and the air flow rate through at least one of the fuel-oil heat exchangers and the air-oil heat exchangers; anda temperature sensor configured to indicate fuel temperature downstream of the fuel-oil heat exchanger (optionally at the entrance to the combustion chamber),the method comprising:determining whether the fuel temperature has increased above a set threshold during cruise conditions based on an output of the temperature sensor; andresponsive to determining that the fuel temperature has increased above the set threshold during cruise conditions, controlling the at least one valve to change the at least one flow rate through the at least one heat exchanger.For example, the at least one valve may be or may comprise a valve configured to vary a portion of oil directed via the fuel-oil heat exchanger, and the method may comprise, responsive to determining that the fuel temperature has increased above a predetermined threshold during cruise conditions, controlling the at least one valve to direct less oil through the fuel-oil heat exchanger.The inventors have recognized that it is important that the oil temperature remains within a desired range and the fuel temperature does not exceed a threshold. Therefore, in some implementations, the focus in flow rate adjustment may be on the air-oil heat exchanger. The at least one valve may therefore be or comprise a valve configured to vary a portion of the oil directed via the air-to-oil heat exchanger, and the method may comprise, responsive to determining that the fuel temperature has increased above a predetermined threshold during cruise conditions, controlling the at least one valve to direct more oil through the air-to-oil heat exchanger.The lack of an oil bypass at the air-oil heat exchanger could result in overcooling of the oil (e.g., solidification, or in less extreme cases overcooling may result in less efficient operation of the transmission, e.g., with higher heat losses, and generally efficiency degradation occurs throughout the engine cycle). Therefore, a bypass pipe can be provided for both heat exchangers. In some implementations, at least a portion of the oil may bypass both heat exchangers.The at least one valve may be or may comprise a valve configured to control the air flow rate through the air-oil heat exchanger. The method may include directing more air through the air-to-oil heat exchanger in response to determining that the fuel temperature has increased above the set threshold during cruise conditions.The method may include controlling multiple valves, for example: in some implementations.• separately controlling the flow of oil through each of the fuel-oil heat exchanger and the air-oil heat exchanger; and / or• Control of the air and oil flow.As described for the above aspects, the inventors have recognized that the use of fuels different from the conventional kerosine-based jet fuels, such as persistent flying fuels, may result in different fuel properties and that the parameters may be adjusted to take advantage of the different fuel properties. In particular, some fuels may be heated to higher temperatures than conventional fuels in one or more fuel-oil heat exchangers without significantly increasing the deposition of degradation products of the fuel, e.g., by coking or paint, which may draw more heat and thus require less air cooling of the oil and improve engine efficiency. However, when improper fuels are permitted to reach these higher temperatures, this may impair engine performance and in some cases even result in blockages of the fuel injectors (or other components such as small screen filters, apertures, and any small passages generally present in the hydromechanical unit and / or fuel heat exchangers). Therefore, checks and compensations are required to ensure that engine performance is optimized for a particular fuel. The method of this seventh aspect comprises monitoring the fuel temperature to check whether there is an excessive fuel temperature and taking measures to reduce heating of the fuel when appropriate. Fuel temperatures downstream of the fuel-oil heat exchanger (e.g., upon entering the combustion chamber) under cruise conditions may be defined as an average over at least 1, 2, 3, 4, or 5 minutes, and optionally over ten, twenty, or thirty minutes under stable cruise conditions. These average temperatures do not include transient temperature spikes, which may be defined as fluctuations in the temperature of the fuel during operation, often increasing the temperature. A transient rise to a higher temperature - for example a rise of only a few seconds or minutes - may therefore not be sufficient to trigger a change in the oil flow.In a transient period, where the available flying fuels change over time during the life of a gas turbine engine and depend on the geographical location of a refueling point (among other variables), it is important that a level of fuel warming be decided based on the specific fuel used. As less oil is directed through the fuel-oil heat exchanger, less heat is transferred to the fuel, which in turn results in the fuel having a lower temperature upon reaching the combustion chamber. Thus, the operation of gas turbines may be adjusted to retrieve the best from a variety of fuels. A key role in adapting engine performance is the controllable oil flow valve which adjusts the oil flow rate through the fuel-oil heat exchanger.The inventors have recognized that these principles can be applied to engines having branched oil circulation paths with different heat exchangers on different branches (parallel arrangements) as well as to engines having a substantially linear, serial arrangement of heat exchangers in which one or more bypass pipes can be used as an alternative to a branched main path.The oil circuit system may branch such that a portion of the oil may flow along each branch, and the air-oil and fuel-oil heat exchangers may be arranged in a parallel configuration on different branches of the oil circuit system. In such implementations, the at least one valve configured to allow for the change in the fraction of oil routed via the fuel-oil heat exchanger may be or may comprise a modulation valve configured to allow for the change in the fraction of oil routed via each branch.The oil circulation system may include at least one bypass pipe configured to allow a portion of the oil to bypass at least one of the fuel-oil heat exchanger and the air-oil heat exchanger. In such implementations, the at least one valve may be or comprise at least one bypass valve configured to allow a portion of the oil to bypass the respective heat exchanger and to control the portion of the oil directed via the bypass pipe, respectively. In such implementations, the air-oil and fuel-oil heat exchangers in the oil circulation system may be arranged in series or in parallel. In implementations where the air-oil and fuel-oil heat exchangers are arranged in parallel on different branches of the oil circulation system and where there is additionally at least one bypass pipe, the method may comprise controlling both the bypass valve and a modulation valve arranged to allow the variation of the proportion of oil routed via each branch.In implementations where the method is configured to adjust the flow rate of the oil through the fuel-oil heat exchanger, at least a portion of the oil diverted from the fuel-oil heat exchanger may be directed to the air-oil heat exchanger. In some of these implementations, the air flow through the air-to-oil heat exchanger may be increased as more oil is directed through the air-to-oil heat exchanger.In implementations where the method is configured to adjust the flow rate of the oil through the fuel-oil heat exchanger or the air-oil heat exchanger, at least a portion of the oil diverted from the respective heat exchanger may be diverted around the respective heat exchanger via a bypass pipe.The method may further comprise determining the determined threshold based on at least one fuel property of the fuel. In such implementations, the at least one fuel property of the fuel may be or include at least one of: thermal stability of the fuel, nitrogen content of the fuel, sulfur content of the fuel, and sustainable flight fuel (SAF) content of the fuel.The step of determining the predetermined threshold may comprise increasing the predetermined threshold, optionally linearly, with increasing thermal stability of the fuel.The step of determining the fixed threshold may comprise increasing the fixed threshold, optionally linearly, as the SAF content of the fuel increases for fuels having a SAF content of over 70%.The method may further comprise determining the at least one fuel property of the fuel, optionally by any of the methods described below.The heat exchange system may include an air valve configured to control the rate of air flow into the air-to-oil heat exchanger. The method may further comprise, responsive to determining that the fuel temperature has increased above a set threshold during cruise conditions, controlling the air valve to direct more air through the air-to-oil heat exchanger.The heat exchange system may include a refrigeration cycle device configured to provide thermal buoyancy by transferring additional heat from the oil to the fuel beyond the heat transferred from the fuel-oil heat exchanger. The method may further include controlling the refrigeration cycle device to reduce the amount of additional heat transferred to the fuel in response to determining that the fuel temperature has increased above a set threshold during cruise conditions. For example, the refrigeration cycle device may be turned off / deactivated.The heat exchange system may comprise at least one bypass pipe and the at least one valve may be or comprise a bypass valve arranged to control the flow rate through that bypass pipe. The heat exchange system may include a plurality of bypass tubes, each configured such that the oil may bypass a heat exchanger (e.g., the fuel-oil heat exchanger or air-oil heat exchanger described above, or a secondary fuel-oil heat exchanger, an oil-oil heat exchanger configured to transfer heat between two separate oil circuits of the heat exchange system, or any other suitable heat exchanger). The method may include controlling at least two bypass valves. For example, a bypass valve for the air-oil heat exchanger may be adjusted to direct more oil to the air-oil heat exchanger when the bypass valve for the fuel-oil heat exchanger is adjusted to direct less oil to the fuel-oil heat exchanger. In some implementations, the same valve (e.g., a three-way valve) may adjust the flow of oil to both heat exchangers.The predetermined threshold may be in the range of 140° C. to 300° C., and optionally 250° C. to 300° C. The set threshold may be 140° C., 180° C., 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C., 270° C., 275° C., 280° C., 290° C., or 300° C.The determining step may be performed at regular intervals during cruise operation of the aircraft.The methods of the first, third, fifth, and seventh aspects may complement each other, and two or more of them may be performed together in various implementations. The method of the seventh aspect may be performed using the engine of the second, fourth or sixth aspect.According to an eighth aspect, there is provided a gas turbine engine for an aircraft, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;a transmission receiving input from the core shaft and outputting output for driving the fan to drive the fan at a lower speed than the core shaft;an oil circuit system arranged for supplying the transmission with oil;a heat exchange system comprising:an air-oil heat exchanger through which the oil in the oil circulation system flows;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; andat least one valve configured to allow variation of at least one of the oil flow rate and the air flow rate through at least one of the fuel-oil heat exchangers and the air-oil heat exchangers;a temperature sensor configured to indicate fuel temperature downstream of the fuel-oil heat exchanger (optionally at the inlet to the combustion chamber); anda controller configured to receive an output from the temperature sensor during cruise conditions, determine whether the fuel temperature has increased above a predetermined threshold based on this output, and control the at least one valve to change the at least one flow rate through the at least one heat exchanger in response to determining that the fuel temperature has increased above a predetermined threshold during cruise conditions.For example, the at least one valve may be configured such that a portion of the oil directed via the fuel-oil heat exchanger may be varied; and the controller may be configured to:receiving an output from the temperature sensor during cruise conditions;determining, based on the output, whether the fuel temperature has increased above a predetermined threshold; andresponsive to determining that the fuel temperature has increased above a predetermined threshold during cruise conditions, controlling the at least one valve to pass less oil through the fuel-oil heat exchanger.The controller or other processing module may be configured to determine the set threshold based on one or more fuel characteristics of the fuel.The heat exchange system may include a refrigeration cycle device configured to provide thermal buoyancy by transferring further heat from the oil to the fuel beyond the heat transferred from the fuel-oil heat exchanger. Optionally, the chiller may allow for an increase in fuel temperature above oil temperature. The controller may be configured to deactivate the refrigeration cycle device in response to determining that the fuel temperature has increased above a set threshold during cruise conditions.The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core shaft.The heat exchange system may further comprise branching fuel return paths and at least one valve that controls a split of fuel flow, the branching paths arranged to return fuel from the heat exchange system at at least two different locations along a main fuel path leading to the combustor from where fuel enters the gas turbine engine.The engine of the eighth aspect may be configured to perform the method of the first, third, fifth and / or seventh aspect, and may include any of the features described with respect to any of the preceding aspects.According to a ninth aspect, there is provided a method of operating a gas turbine engine, the gas turbine engine comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;an oil circuit system configured to supply oil for cooling at least one engine component;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred from the oil to the fuel; anda refrigeration cycle device through which the oil in the oil cycle system and the fuel flow, the refrigeration cycle device being configured to transfer further heat from the oil to the fuel.The method includes controlling the refrigeration cycle device such that the fuel temperature entering the combustion chamber is higher than a maximum oil temperature in the oil cycle system.The inventors have recognized that the use of fuels different from conventional kerosine-based jet fuels, such as sustainable aircraft fuels, may result in different fuel properties and that parameters under cruise conditions may be adjusted to take advantage of the different fuel properties. In particular, some fuels may be heated to higher temperatures than conventional fuels without significantly increasing deposition of degradation products of the fuel, e.g., by coking or paint, and in some cases, the thermal buoyancy force may be used to bring the fuel above the temperature of the oil and thus optimize the performance of the fuel. This may additionally enable a method that provides improved oil cooling (as the use of a refrigeration cycle device enables heat to be pumped out of the oil even when the oil is cooled to the fuel temperature) and may also improve overall engine heat efficiency as hotter fuel is burned. By controlling the refrigeration cycle device, a thermal lift force may be provided and optionally the level of the thermal lift force provided may also be adjusted according to fuel and engine operating conditions.As used herein, the maximum oil temperature in the oil circulation system refers to the highest oil temperature at any location in the oil circulation system at any particular time--i.e., at the time when the comparison is made with the fuel temperature. It will be appreciated that the oil temperature generally increases as it flows through the one or more engine components to be cooled by the oil and then decreases as it flows through the at least one fuel-oil heat exchanger (and through any other heat exchanger configured to remove heat from the oil - e.g., an air-oil heat exchanger). A temperature sensor providing an indication of the maximum oil temperature in the oil circulation system may therefore be arranged downstream of the engine component(s) to be cooled and upstream of the fuel-oil heat exchanger(s) (and any / any other heat exchanger(s) arranged to remove heat from the oil). Fuel and oil temperatures may both be evaluated as described above; particularly if transient spikes are ignored. An average may be determined over a period of time of, for example, one minute or a few minutes.The engine component(s) to be cooled may include one or more of the following: a main drive transmission, i.e., a transmission that receives input from the core shaft and outputs drive to the fan, an auxiliary transmission, one or more bearings (e.g., shaft bearings, for example, for the core shaft), one or more generators, electrical lines, and / or one or more pumps. It will be appreciated that any structure within the engine to be cooled may be classified as such an engine component.The engine may include a transmission that receives input from a core shaft and outputs a drive to the fan to drive the fan at a lower speed than the core shaft, and the oil circulation system may supply oil to the transmission. The refrigeration cycle device can therefore be used for additionally cooling the main transmission in a transmission engine.The refrigeration cycle device may include a refrigerant liquid, the refrigerant liquid serving to transfer heat from the oil to the fuel. The refrigerant liquid may be 1,1,1,2-tetrafluoroethane.The refrigeration cycle device may include an evaporator, a refrigerant compressor, and a condenser. The method may include:transferring heat from the oil to the refrigerant liquid using the evaporator to evaporate liquid in the refrigerant liquid and form a saturated vapor;compressing the saturated steam to form a super heated steam using the refrigerant compressor and sending the super heated steam to the condenser; andtransferring the heat from the superheated steam to the fuel using the condenser, wherein the superheated steam is at least partially converted into a liquid in the process.The refrigerant compressor may also function as a pump and circulate the refrigerant liquid around the refrigeration cycle device. In some implementations, a separate refrigerant pump may be provided.The method may comprise converting the superheated steam into a saturated liquid in the condenser (a saturated liquid in the context of thermodynamics / refrigeration cycles is a liquid containing as much thermal energy as possible without boiling). The capacitor may form a near saturated liquid in some implementations.The refrigeration cycle device may further include an expansion valve located between the condenser and the evaporator. The method may include using the expansion valve to convert the liquid from the condenser (which may be a saturated liquid) to a liquid and vapor mixture of reduced temperature (i.e., a mixture of liquid and gas having a lower temperature than the liquid discharged from the condenser). Subsequently, the refrigerant may be returned to the evaporator and the cycle may begin again.The engine may further include a temperature sensor configured to sense the temperature of the fuel downstream of the refrigeration cycle device. The fuel temperature sensor may be located at or near an inlet to the combustion chamber. The engine may further comprise a temperature sensor configured to provide an indication of the maximum oil temperature in the oil circulation system. The oil temperature sensor may be located at or near an inlet to the fuel-oil heat exchanger or at or near an outlet from the one or more engine components to be cooled. The method may further include comparing the oil and fuel temperatures and adjusting control of the refrigeration cycle device based on the comparison.The method may include controlling one or more additional components of a heat exchange system of the engine, and controlling the refrigeration cycle device - the multiple components may cooperate to increase the fuel temperature to the desired level.For example, the method may include controlling the flow of liquid through the fuel-oil heat exchanger to increase heat transfer to the fuel by: (i) increasing the flow rate of oil through the fuel-oil heat exchanger, e.g., by controlling a bypass valve modulation valve or a recirculation valve as described elsewhere herein or by controlling the pump speed of an oil pump of the oil circulation system; and / or (ii) recirculating the fuel through the fuel-oil heat exchanger, e.g., by using a recirculation valve as described elsewhere herein.In some implementations, the engine further includes an air-to-oil heat exchanger configured upstream of the fuel-to-oil heat exchanger with respect to the oil flow. In such implementations, the method may include increasing heat transfer from the oil to the fuel in the fuel-oil heat exchanger by decreasing air cooling of the oil before the oil enters the fuel-oil heat exchanger.The method may be performed during cruise operation. The engine may be controlled to run at a fuel temperature entering the combustion chamber that is above the maximum oil temperature for at least 10%, 20%, or 30% of the cruise flight time.The method may include controlling the refrigeration cycle device such that the fuel temperature entering the combustion chamber is higher than the maximum oil temperature in the oil cycle system by at least 2° C., 5° C., 10° C., 15° C., 20° C., or 25° C. The method may include controlling the refrigeration cycle device such that the fuel temperature entering the combustion chamber is higher than the maximum oil temperature in the oil cycle system by between 2° C. and 50° C.The method may include controlling the refrigeration cycle device such that the fuel temperature at entering the combustion chamber is higher than the maximum oil temperature in the oil cycle system by a margin determined based on at least one fuel property of the fuel.The methods of the first, third, fifth, seventh, and ninth aspects may complement each other, and two or more of them may be performed together in various implementations. The method of the ninth aspect may be performed using the engine of the second, fourth, sixth, or eighth aspect.According to a tenth aspect, there is provided a gas turbine engine for an aircraft, comprising:an engine core comprising a turbine, a compressor, a combustor arranged to combust a fuel, and a core shaft connecting the turbine to the compressor;a fan disposed upstream of the engine core;an oil circuit system configured to supply oil for cooling at least one engine component;a fuel-oil heat exchanger through which the oil in the oil circulation system and the fuel flow so that heat is transferred from the oil to the fuel; anda refrigeration cycle device through which the oil in the oil cycle system and the fuel flow, the refrigeration cycle device being configured to transfer further heat from the oil to the fuel.The fuel-oil heat exchanger and the refrigeration cycle device are configured to transfer heat to the fuel so that the fuel temperature when entering the combustion chamber is higher than the maximum oil temperature in the oil cycle system.The gas turbine engine may further include a gearbox that receives input from a core shaft and outputs output to the fan to drive the fan at a lower speed than the core shaft. The gas turbine engine may therefore be a gas turbine engine. The oil circuit system may be configured to supply the transmission with oil.The turbine may be a first turbine, the compressor may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core shaft.The gas turbine engine may further include branching fuel return paths and at least one valve that controls a division of fuel flow along the branching fuel return paths. The branching paths may be configured to recirculate fuel from the fuel-oil heat exchanger at at least two different locations along a main fuel path leading to the combustor from where the fuel enters the gas turbine engine.The engine of the tenth aspect may be configured to perform the method of the first, third, fifth, seventh and / or ninth aspect, and may include any of the features described with respect to any of the preceding aspects.In each of the above aspects:In implementations where one or more fuel properties of the fuel are used, the at least one fuel property of the fuel may comprise at least one of: i. percent of residual flying fuel in the fuel (% SAF); ii. concentration of the heteroatomic species of the fuel; iii. content of aromatic hydrocarbons in the fuel; iv. content of multiaromatic hydrocarbons in the fuel; v. percent of nitrogenic species in the fuel; vi. presence or percent of a tracer species or trace element in the fuel; vii. hydrogen-carbon ratio of the fuel; viii. hydrocarbon distribution of the fuel; ix. level of non-volatile particle emissions during combustion; x. naphthalene content of the fuel; xi. sulfur content of the fuel; xii. cycloparaffinic content of the fuel; xiii. oxygen content of the fuel; xiv. thermal stability of the fuel (e.g., thermal decomposition temperature; thermal stability may be assigned a numerical value at each temperature by forming the reciprocal of the deposition rate of fuel degradation products at the given temperature); xv. degree of coking of the fuel or general degree of deposition of fuel degradation products; xvi. an indication that the fuel is a fossil fuel; xvii. at least one of density, viscosity, heating value and heat capacity.The method may further include chemically or physically sensing one or more parameters relevant to the fuel in the fuel tank after refueling. The detected parameters may be fuel characteristics or may be used to calculate or derive fuel characteristics. For example, the detected parameters may be the shaft speed and the mass flow rate of the fuel from which the heating value (a fuel property) may be determined, or the detected parameters may be the fuel density and / or the presence of a tracer, both of which are themselves fuel properties. Determining at least one fuel property may include retrieving stored data about fuel properties. Chemically and / or physically determining one or more parameters of the fuel in the fuel tank may be performed by sampling the fuel from the fuel tank for off-wing testing.Determining the at least one fuel property of the fuel may include obtaining the at least one fuel property of a fuel already present in the fuel tank before refueling; determining at least one fuel property of a fuel supplied to the fuel tank during refueling; and calculating (based on this information) at least one fuel property of the resulting fuel in the fuel tank after refueling.The determining of the at least one fuel property may be performed based on the detection of at least one fuel feature. The fuel characteristic may be the fuel property or may be used to calculate or otherwise determine (e.g., by retrieving from a look-up table) the fuel property. The detection can be performed on the wing.The determination of the at least one fuel property may be performed based on the received data regarding the composition of the fuel. The fuel composition data may be provided to the aircraft during refueling. The fuel composition data may be manually input.From the performance of the gas turbine engine during coasting, takeoff, and climb of the aircraft, at least one fuel property may be inferred.It is understood that a feature described with respect to one aspect may be used, mutatismutantly, in combination with any other aspect.As noted elsewhere herein, the present disclosure may be applicable to a relevant configuration of a gas turbine engine. Such a gas turbine engine may be, for example, a turbofan gas turbine engine, an open rotor gas turbine engine (in which the propeller is not surrounded by a nacelle), a turboprop engine, or a turbojet engine. Each such engine may or may not be equipped with an afterburner. Such a gas turbine engine may be configured for, for example, land or sea power generation applications.A gas turbine engine according to the present disclosure may include an engine core including a turbine, a combustor, a compressor, and a core shaft connecting the turbine to the compressor. Such a gas turbine engine may comprise a fan (with fan blades). Such a fan may be located upstream of the engine core. Alternatively, in some examples, the gas turbine engine may include a fan disposed downstream of the engine core, for example, when the gas turbine engine has an open rotor or is a turboprop engine (in which case the fan may be referred to as a propeller).If the gas turbine engine has an open rotor or is a turboprop engine, the gas turbine engine may comprise two contrarotating propeller stages which are connected via a shaft to a free power turbine and driven by it. The propellers may rotate in the opposite direction so that one rotates clockwise and the other rotates counter-clockwise about the axis of rotation of the engine. Alternatively, the gas turbine engine may include a propeller stage and a vane stage configured downstream of the propeller stage. The guide vane stage may have a variable pitch. Accordingly, high pressure, intermediate pressure and free turbines may drive high and intermediate pressure compressors and propellers, respectively, through suitable connecting shafts. The propellers can thus provide the majority of the drive thrust.When the gas turbine engine has an open rotor or is a turboprop engine, one or more of the propeller stages may be driven by a gearbox. The transmission may be of the type described herein.An engine according to the present disclosure may be a turbo fan engine. Such an engine may be a direct drive turbo fan engine in which the fan is directly connected to the fan drive turbine via a core shaft, for example without a gearbox. In such a direct drive turbo fan engine, the fan may be arranged to rotate at the same speed as the fan drive turbine. By way of example only, the fan power turbine may be a first turbine, the core shaft may be a first core shaft, and the gas turbine engine may further include a second turbine and a second core shaft connecting the second turbine to the compressor. The second turbine, compressor, and second core shaft may be arranged to rotate at a higher speed than the first core shaft. In such an arrangement, the second turbine may be positioned axially upstream of the first turbine.An engine according to the present disclosure may be a gear fan engine. In such an arrangement, the engine includes a fan driven via a gearbox. Accordingly, such a gas turbine engine may include a gearbox that receives input from the core shaft and outputs output to the fan to drive the fan at a lower speed than the core shaft. The input to the transmission can be effected directly from the core shaft or indirectly from the core shaft, for example via a spur gear shaft and / or a gearwheel. The core shaft may rigidly connect the turbine and the compressor such 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 include any number of shafts connecting turbines and compressors, for example, one, two, or three shafts. For example only, the turbine connected to the core shaft may be a first turbine, the compressor connected to the core shaft may be a first compressor, and the core shaft may be a first core shaft. The engine core may further include a second turbine, a second compressor, and a second core shaft connecting the second turbine to the second compressor. The second turbine, the second compressor, and the second core shaft may be arranged to rotate at a higher speed than the first core 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) a flow from the first compressor, e.g., via a generally annular channel).The transmission may be arranged to be driven by the core shaft configured to rotate (e.g., in use) at the lowest speed (e.g., by the first core shaft in the above example). For example, the transmission may be arranged to be driven only by the core shaft configured to rotate (e.g., in use) at the lowest speed (e.g., only by the first core shaft and not by the second core shaft in the above example). Alternatively, the transmission may be arranged to be driven by any one or more shafts, for example the first and / or second shaft in the above example.The gearbox may be a reduction gearbox (in that the output to the fan has a lower speed than the input from the core shaft). Any type of transmission may be used. For example, the transmission may be a "planetary gear" or a "star gear" as described in more detail elsewhere herein. Such a transmission can be a single-stage transmission. Alternatively, such a transmission may be a compound transmission, for example a compound planetary transmission (which may have the input to the sun gear and the output to the ring gear and thus may be referred to as a "compound star" transmission) having, for example, two reduction stages.The transmission may have any desired reduction ratio (defined as the speed of the input shaft divided by the speed of the output shaft), for example greater than 2.5, for example in the range of 3 to 4.2 or 3.2 to 3.8, for example on 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 transmission ratio may be between any two of the values mentioned in the previous sentence, for example. By way of example only, the transmission may be a "star" transmission having a gear ratio in the range of 3.1 or 3.2 to 3.8. Further, by way of example only, the transmission may be a "star" transmission having a gear ratio in the range of 3.0 to 3.1. Further, by way of example only, the transmission may be a "planetary gear" having a gear ratio in the range of 3.6 to 4.2. In some arrangements, the gear ratio may be outside of these ranges.In any gas turbine engine described and / or claimed herein, fuel of a given composition or mixture is provided to a combustor, which may be provided downstream of the fan and compressor(s) with respect to the flowpath (e.g., axially downstream). For example, the combustor may be located directly downstream of (e.g., at the exit of) the second compressor where a second compressor is provided. As another example, the flow at the exit to the combustor may be provided to the inlet of the second turbine 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 include a row of rotor blades and a row of stator blades, which may be variable stator blades (as their angle of incidence may be variable). The row of rotor blades and the row of stator blades may be axially offset from each other. For example, the gas turbine engine may be a direct drive turbo fan engine that includes 13 or 14 compressor stages (in addition to the fan). Such an engine may, for example, comprise 3 stages in the first (or "low pressure") compressor and either 10 or 11 stages in the second (or "high pressure") compressor. As another example, the gas turbine engine may be a "gearbox" gas turbine engine (in which the fan is driven by a first core shaft via a reduction gearbox) that includes 11, 12 or 13 compressor stages (in addition to the fan). Such an engine may comprise 3 or 4 stages in the first (or "low pressure") compressor and 8 or 9 stages in the second (or "high pressure") compressor. As another example, the gas turbine engine may be a "gearbox" gas turbine engine having 4 stages in the first (or "low pressure") compressor and 10 stages in the second (or "high pressure") compressor.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 blades or vice versa, as required. The respective rows of rotor blades and stator blades may be axially offset from each other. The second (or "high pressure") turbine may include 2 stages in any arrangement (e.g., whether it be a transmission or direct drive engine). The gas turbine engine may be a direct drive turbo fan engine that includes a first (or "low pressure") 5, 6 or 7 stage turbine. Alternatively, the gas turbine engine may be a "gearbox" gas turbine engine having a first (or "low pressure") 3 or 4 stage turbine.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 radius of the fan blade at the hub to the radius of the fan blade at the tip may be less than (or on the order of) 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 radius of the fan blade at the hub to the radius of the fan blade at the tip may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower limits), for example, in the range of 0.28 to 0.32, or 0.29 to 0.30 These ratios may be referred to generally as the hub-to-tip ratio. The radius at the hub and the radius at the tip may both be measured at the leading edge (or the axially foremost part) 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 outward of any platform.The radius of the fan may be measured between the centerline of the engine and the tip of a fan blade at its leading edge. The fan diameter (which may be simply twice the fan radius) may be greater than (or on the order of) 140 cm, 170 cm, 180 cm, 190 cm, 200 cm, 210 cm, 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 (about 160 inches), or 420 cm (about 165 inches). The fan diameter may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower bounds), for example, in the range of 210 cm to 240 cm or 250 cm to 280 cm or 320 cm to 380 cm. By way of non-limiting example only, the fan diameter may be in the range of 170 cm to 180 cm, 190 cm to 200 cm, 200 cm to 210 cm, 210 cm to 230 cm, 290 cm to 300 cm, or 340 cm to 360 cm.The speed of the fan may vary in use. Generally, the speed is lower for larger diameter fans. Purely as a non-limiting example, the speed of the fan may be less than 3500 U / min, for example less than 2600 U / min, or less than 2500 U / min, or less than 2300 U / min, under cruise conditions. By way of further non-limiting example, the speed of the fan under cruise conditions for a "gearbox" gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 2750 to 2900 U / min. By way of further non-limiting example, the speed of the fan may be in the range of 2500 to 2800 U / min under cruise conditions for a "gearbox" gas turbine engine having a fan diameter in the range of 210 cm to 230 cm. By way of further non-limiting example, the speed of the fan may be in the range of 1500 to 1800 U / min under cruise conditions for a "gearbox" gas turbine engine having a fan diameter in the range of 340 cm to 360 cm. By way of further non-limiting example, the speed of the fan may be in the range of 3600 to 3900 U / min under cruise conditions for a direct drive engine having a fan diameter in the range of 190 cm to 200 cm. By way of further non-limiting example, the speed of the fan may be in the range of 2000 to 2800 U / min under cruise conditions for a direct drive engine having a fan diameter in the range of 300 cm to 340 cm.In use of the gas turbine engine, the fan (with associated fan blades) rotates about an axis of rotation. This rotation causes the tip of the fan blade to move Spitze at a speed U. The work performed by the fan blades in the flow results in an enthalpy increase dH of the flow. A fan tip load may be defined as dH / U Spitze2 where dH is the enthalpy increase (e.g., the average 1-D enthalpy increase) across the fan and U Spitze is the (translatory) speed of the fan tip, e.g., at the leading edge of the tip (which may be defined as the fan tip radius at the leading edge multiplied by the angular speed). The fan peak load under cruise conditions may be greater than (or on the order of) 0.28, 0.29, 0.30, 0.31, 0.32, 0.33, 0.34, 0.35, 0.36, 0.37, 0.38, 0.39, or 0.4 (all values being dimensionless). The fan tip load may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower limits), for example, in the range of 0.28 to 0.31 or 0.29 to 0.3 (for example, for a transmission gas turbine engine).Gas turbine engines according to the present disclosure may have a desired bypass ratio (BPR), 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. In some arrangements, the bypass ratio under cruise conditions may be greater than (or on the order of) one of: 9.5, 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, or 20. By way of further non-limiting example, the bypass ratio of a gas turbine engine according to the present disclosure may be in the range of 12:1 to 15:1 under cruise conditions. The bypass channel may be substantially annular. The bypass duct may be located radially outside of the core engine. The radially outer surface of the bypass duct may be defined by a nacelle and / or a fan casing.The total pressure ratio (OPR) of a gas turbine engine as described and / or claimed herein may be defined as the ratio of the ram pressure at the exit of the super compressor (prior to entry into the combustor) to the ram pressure upstream of the fan. By way of non-limiting example, the total pressure ratio of a gas turbine engine as described and / or claimed herein may be greater than (or on the order of) one of the following under cruise conditions): 35, 40, 45, 50, 55, 60, 65, 70, 75. The total pressure ratio may be in an inclusive range bounded by any two of the values in the preceding set (i.e., the values may form upper or lower limits), for example, in the range of 50 to 70. Purely by way of non-limiting example, the total pressure ratio under cruise conditions of a transmission gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 40 to 45. By way of non-limiting example only, the total pressure ratio may be in the range of 45-55 under cruise conditions of a gas turbine engine having a fan diameter in the range of 210 cm to 230 cm. By way of non-limiting example only, the total pressure ratio may be in the range of 50 to 60 under cruise conditions of a gas turbine engine having a fan diameter in the range of 340 cm to 360 cm. By way of non-limiting example only, the total pressure ratio may be in the range of 50 to 60 under cruise conditions of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm.The specific thrust of an engine may be defined as the net thrust of the engine divided by the total mass flow through the engine. In some examples, the specific thrust for a given state of thrust may depend on the specific composition of fuel provided to the combustion chamber. Under cruise conditions, the specific thrust of an engine described and / or claimed herein may be less than (or on the order of) 110 Nkg -1 s, 105 Nkg -1 s, 100 Nkg -1 s, 95 Nkg -1 s, 90 Nkg -1 s, 85 Nkg -1 s, or 80 Nkg -1 s. The specific thrust may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower bounds), for example, in the range of 80 Nkg -1 s to 100 Nkg -1 s or 85 Nkg -1 s to 95 Nkg -1 s. Such engines may be particularly efficient compared to conventional gas turbine engines. By way of non-limiting example only, the specific thrust of a gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 90 Nkg -1 s to 95 Nkg -1 s. By way of non-limiting example only, the specific thrust of a gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 80 Nkg -1 s to 90 Nkg -1 s. By way of non-limiting example only, the specific thrust of a gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 70 Nkg -1 s to 90 Nkg -1 s. By way of non-limiting example only, the specific thrust of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 90 Nkg -1 s to 120 Nkg -1 s.A gas turbine engine as described and / or claimed herein may have any desired maximum thrust. By way of non-limiting example only, a gas turbine as described and / or claimed herein may be capable of generating a maximum thrust of at least one of (or on the order of) 100 kN, 110 kN, 120 kN, 130 kN, 135 kN, 140 kN, 145 kN, 150 kN, 155 kN, 160 kN, 170 kN, 180 kN, 190 kN, 200 kN, 250 kN, 300 kN, 350 kN, 400 kN, 450 kN, 500 kN, or 550 kN. The maximum thrust may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower bounds). By way of non-limiting example only, a gas turbine as described and / or claimed herein may be capable of generating a maximum thrust in the range of 155 kN to 170 kN, 330 kN to 420 kN, or 350 kN to 400 kN. By way of non-limiting example only, the maximum thrust of a gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 140 kN to 160 kN. By way of non-limiting example only, the maximum thrust of a gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 150 kN to 200 kN. By way of non-limiting example only, the maximum thrust of a gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 370 kN to 500 kN. By way of non-limiting example only, the maximum thrust of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 370 kN to 500 kN. The above thrust may be the maximum net thrust at normal sea level atmospheric conditions at +15 degrees C. (ambient pressure 101.3 kPa, temperature 30 degrees C.) for a static engine.In use, the temperature of the stream at the inlet to the high pressure turbine may be particularly high. This temperature, which may be referred to as TET, may be measured at the exit to the combustor, for example immediately upstream of the first turbine vane, which may itself be referred to as a nozzle vane. In some examples, for a given state of thrust, the TET may depend on the specific composition of fuel provided to the combustion chamber. Under cruise conditions, the TET may be at least one of (or on the order of): 1400 K, 1450 K, 1500 K, 1520 K, 1530 K, 1540 K, 1550 K, 1600 K, or 1650 K. Thus, purely by way of non-limiting example, under cruise conditions of a gas turbine engine having a fan diameter in the range of 200 cm to 210 cm in the range of 1540 K to 1600 K, the TET may be. By way of non-limiting example only, the TET may be in the range of 1590 K to 1650 K under cruise conditions of a gas turbine engine having a fan diameter in the range of 210 cm to 230 cm. By way of non-limiting example only, the TET may be in the range of 1600 K to 1660 K under cruise conditions of a gas turbine engine having a fan diameter in the range of 340 cm to 360 cm. By way of non-limiting example only, the TET of a direct drive gas turbine engine having a fan diameter under cruise conditions in the range of 300 cm to 340 cm may be in the range of 1590 K to 1650 K. By way of non-limiting example only, the TET may be in the range of 1570 K to 1630 K under cruise conditions of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm.The TET under cruise conditions may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower limits), for example 1530 K to 1600 K. The maximum TET in use of the engine may be, for example, at least one of (or on the order of): 1700 K, 1750 K, 1800 K, 1850 K, 1900 K, 1950 K, 2000 K, 2050 K, or 2100 K. Thus, purely by way of non-limiting example, the maximum TET of a gas turbine engine having a fan diameter in the range of 200 cm to 210 cm may be in the range of 1890 K to 1960 K. By way of non-limiting example only, the maximum TET of a gas turbine engine having a fan diameter in the range of 210 cm to 230 cm may be in the range of 1890 K to 1960 K. By way of non-limiting example only, the maximum TET of a gas turbine engine having a fan diameter in the range of 340 cm to 360 cm may be in the range of 1890 K to 1960 K. By way of non-limiting example only, the maximum TET of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 1935 K to 1995 K. By way of non-limiting example only, the maximum TET of a direct drive gas turbine engine having a fan diameter in the range of 300 cm to 340 cm may be in the range of 1890 K to 1950 K. The maximum TET may be in an inclusive range bounded by any two of the values in the previous set (i.e., the values may form upper or lower limits), for example, in the range of 1800 K to 1950 K or 1900 K to 2000 K. The maximum TET may occur, for example, at a high thrust condition, for example, at a maximum lift-off (MTO) condition.A fan blade and / or airfoil portion of a fan blade as described and / or claimed herein may be made of any suitable material or combination of materials. For example, at least a portion of the fan blade and / or the air baffle may be at least partially made of a composite material, for example a metal matrix composite material and / or an organic matrix composite material, such as a carbon fiber composite material. As another example, at least a portion of the fan blade and / or air baffle may be made at least partially 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 that may be made using a material that is better able to resist impact (e.g., birds, ice, or other material) than the remainder of the blade. Such a leading edge may be made using titanium or a titanium-based alloy, for example. Thus, for example only, the fan blade may include a carbon fiber or aluminum-based body (such as an aluminum-lithium alloy) having a leading edge of titanium.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 include a fastener that may engage a corresponding slot in the hub (or disc). Such a fastening may be in the form of a dovetail, purely by way of example, which may be inserted and / or snap-fitted into a corresponding slot in the hub / disk in order to fasten the fan blade to the hub / disk. As another example, the fan blades may be integrally formed with a central portion. Such an arrangement may be referred to as a blade disk or blade ring. Any suitable method may be used to produce such a blade disc or ring. For example, at least a portion of the fan blades may be made from a block and / or at least a portion of the fan blades may be attached to the hub / disk 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 for varying the exit area of the bypass passage in use. The general principles of the present disclosure may be applied to engines with or without VAN.The fan of a gas turbine as described and / or claimed herein may include any number of fan blades, for example 14, 16, 18, 20, 22, 24 or 26 fan blades. If the fan blades comprise a carbon fibre composite body, 16 or 18 fan blades may be present. If the fan blades comprise a metallic body (for example an aluminium-lithium or titanium alloy), 18, 20 or 22 fan blades may be present.As used herein, the terms coasting, rolling, takeoff, climbing, cruise, descent, approach, and landing (or one or more portions thereof) have the usual meaning and are readily understood by those skilled in the art. Thus, for a particular gas turbine engine for an aircraft, those skilled in the art would readily appreciate that each term refers to all or part of an operating phase of the engine within a given deployment of an aircraft for which the gas turbine engine was designed for mounting thereto.In this regard, ground idle may refer to an operating phase of the engine in which the aircraft is stationary and is in contact with the ground, but there is a requirement that the engine be running. During idle, the engine may generate between 3% and 9% of the available thrust of the engine. In further non-limiting examples, the engine may generate between 5% and 8% of available thrust. In further non-limiting examples, the engine may generate between 6% and 7% of available thrust. Coasting may refer to an operating phase of the engine in which the aircraft is propelled along the ground by the thrust generated by the engine. During coasting, the engine may generate between 5% and 15% of the available thrust. In further non-limiting examples, the engine may generate between 6% and 12% of the available thrust. In further non-limiting examples, the engine may generate between 7% and 10% of the available thrust. Takeoff may refer to an operating phase of the engine in which the aircraft is propelled by the thrust generated by the engine. At an initial stage within the launch phase, the aircraft may be propelled while the aircraft is in contact with the ground. At a later stage within the launch phase, the aircraft may be propelled while the aircraft is not in contact with the ground. During launch, the engine may generate between 90% and 100% of the available thrust. In further non-limiting examples, the engine may generate between 95% and 100% of the available thrust. In further non-limiting examples, the engine may generate 100% of the available thrust.Climbing flight may refer to an operating phase of the engine in which the aircraft is driven by the thrust generated by the engine. During climbing flight, the engine may generate between 75% and 100% of the available thrust. In further non-limiting examples, the engine may generate between 80% and 95% of the available thrust. In further non-limiting examples, the engine may generate between 85% and 90% of the available thrust. In this context, the ascent may refer to an operating phase within the flight cycle of an aircraft between the start and the attainment of the cruise conditions, wherein the attainment of the cruise conditions defines the beginning of the cruise phase or a portion thereof of the aircraft flight. Additionally or alternatively, the term climbing flight may refer to a nominal point or time periods during the flight cycle of an aircraft between takeoff and landing where a relative elevation is required that may require additional propulsion demands of the engine.As used herein, cruise conditions that may define the cruise phase (or a portion thereof) of aircraft flight are of conventional significance and will be readily understood by those skilled in the art. In some examples, the cruise conditions for a particular gas turbine engine for an aircraft may refer to the operating point of the engine at cruise of a given mission (which may be referred to in the industry as "economic mission") of an aircraft for which the gas turbine engine has been designed for attachment thereto. In this context, cruise may be considered to be the point in the flight cycle of an aircraft where 50% of the total fuel burned between the highest point of climb and the beginning of descent has burned (which may be approximated by the midpoint - in terms of time and / or distance - between the highest point of climb and the beginning of descent). The cruise conditions may thus define an operating point, phase, or portion thereof of the flight that provides thrust that provides stable operation (i.e., maintaining a constant altitude and / or Mach number) or at least substantially stable operation (i.e., maintaining an at least substantially constant altitude and / or Mach number) of an aircraft for which it was designed for attachment thereto, taking into account the number of engines provided for that aircraft. For example, if an engine is configured to be mounted on an aircraft having two engines of the same type, the engine may provide, under cruise conditions, half of the total thrust that would be required for stable operation or at least substantially stable operation of the aircraft during cruise.In other words, the cruise conditions for a given gas turbine engine for an aircraft may be defined as the operating point of the engine that provides a particular thrust (required to provide, in combination with any other engines on the aircraft, stable operation or at least substantially stable operation of the aircraft for which it has been designed to be mounted thereto at a given Mach number during cruise) under cruise atmospheric conditions (defined by the international standard atmosphere according to ISO 2533 at altitude during cruise). For any given gas turbine engine for an aircraft, the thrust at cruise, atmospheric conditions and Mach number are known and thus the operating point of the engine under cruise conditions can be clearly defined.Purely by way of example, the forward speed under cruise conditions may be any point in the range of 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 on the order of Mach 0.8, on the order of Mach 0.85, or in the range of 0.8 to 0.85. Any single speed within these ranges may be part of the cruise conditions. In some aircraft, cruise conditions may be outside these ranges, for example below Mach 0.7 or above Mach 0.9.By way of example only, the cruise conditions may correspond to standard atmospheric conditions (according to the International Standard Atmosphere, ISA) at a height that is in the range of 10,000 m to 15,000 m, for example in the range of 10,000 m to 12,000 m, for example in the range of 10,400 m to 11,600 m (about 38,000 feet), for example in the range of 10,500 m to 11,500 m, for example in the range of 10,600 m to 11,400 m, for example in the range of 10,700 m (about 35,000 feet) to 11,300 m, for example in the range of 10,800 m to 11,200 m, for example in the range of 10,900 m to 11,100 m, for example in the order of 11,000 m. The cruise conditions may correspond to standard atmospheric conditions at any given altitude in these ranges.By way of example only, the cruise conditions may correspond to a forward Mach number of 0.8 and standard atmospheric conditions (according to the International Standard Atmosphere) at a level of 35,000 feet (10,668 m). Under such cruise conditions, the engine may provide a known net thrust level required. The known net thrust level required is, of course, dependent on the engine and its intended use and may be, for example, a value in the range of 20 kN to 40 kN.By way of example only, cruise conditions may correspond to a forward Mach number of 0.85 and standard atmospheric conditions (according to the International Standard Atmosphere) at a altitude of 38,000 feet (11,582 m). Under such cruise conditions, the engine may provide a known net thrust level required. The known net thrust level required is, of course, dependent on the engine and its intended use and may be, for example, a value in the range of 35 kN to 65 kN.In use, a gas turbine engine as 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., the center of flight conditions) of an aircraft to which at least one (e.g., 2 or 4) gas turbine engine may be mounted to provide propulsion thrust.Furthermore, the skilled person would immediately recognize that descent and approach, or both, relate to an operating phase within a flight cycle between cruise and landing of the aircraft, wherein in particular the approach forms part of the landing and take-off phase (LTO phase). During descent or landing approach, or both, the engine may generate between 0% and 50% of the available thrust. In further non-limiting examples, the engine may generate between 25% and 40% of the available thrust. In further non-limiting examples, the engine may generate between 30% and 35% of the 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 propulsion demand of the engine.According to one aspect, there is provided an aircraft comprising a gas turbine engine as described and / or claimed herein. The aircraft of this aspect is the aircraft for which the gas turbine engine has been designed for mounting thereto. Accordingly, in this aspect, cruise conditions may correspond to an operating point, phase, or portion thereof of aircraft flight, as defined elsewhere herein.According to one aspect, there is provided a method of operating a gas turbine engine as described and / or claimed herein. Operation may be under suitable conditions as defined elsewhere herein (e.g., in terms of thrust, atmospheric conditions, and Mach number).According to one aspect, there is provided a method of operating an aircraft comprising a gas turbine engine as described and / or claimed herein. Operation in accordance with this aspect may include (or be) operation under suitable conditions, for example, at the center of flight of the aircraft as defined elsewhere herein.The skilled person will understand that, except in the case of mutual exclusion, a feature or parameter described with respect to one of the above-mentioned aspects can be applied to any other aspect. Moreover, except in the case of mutual exclusion, any feature or parameter included or described herein may be applied to any aspect and / or combined with any other feature or parameter included or described herein.Unless mutually exclusive, any parameter or value included or described herein may be applied to and / or combined with one or more other parameters and / or values included or described herein. For example, a first parameter or value (e.g., parameter A) included or described herein may be applied to and / or combined with one or more other parameters and / or values included or described herein (e.g., one or more parameters B; parameter C; and parameter D, etc.) to express a product of their relationship. For example, those skilled in the art would understand that in cases where parameter A is disclosed separately from parameter B, a product of their relationship may be expressed as, for example, A / B, B / A, B*A, or any other such application, combination, or function of parameter A with respect to parameter B, as desired.Embodiments will now be described, by way of example only, with reference to the figures, in which: FIG. 1 is a cross-sectional side view of a gas turbine engine; FIG. 2 is a cross-sectional side view of an upstream portion of a close-up gas turbine engine; FIG. 3 is a partially cut-away view of a transmission for a gas turbine engine; FIG. 4 is an illustration of an aircraft having a propulsion system including two gas turbine engines; FIG. 5 is an illustration of an example fuel system; FIG. 6 is an illustration of an alternative exemplary fuel system; FIG. 7 is a diagram of a portion of an example oil circulation system (a primary oil circulation system); FIG. 8 is a diagram of another part of an exemplary oil circulation system (a secondary oil circulation system); FIG. 9 is a representation of a portion of an alternative example of an oil circulation system also showing features of the air-to-oil heat exchanger; FIG. 10 is an illustration of a portion of the example fuel system of FIG. 5 and the example oil circulation system of FIGS. 7 and 8 ; FIG. 11 is a diagram of another alternative exemplary oil circulation system illustrating all major components of a heat exchange system; FIG. 12 illustrates an example method of operating a gas turbine engine; FIG. 13 illustrates another example method of operating a gas turbine engine; FIG. 14 illustrates another example method of operating a gas turbine engine; FIG. 15 illustrates another example method of operating a gas turbine engine; FIG. 16 illustrates another example method of a recirculating oil system for a gas turbine engine; FIG. 17 illustrates another example method of operating a gas turbine engine; and FIG. 18 illustrates a refrigeration cycle device integrated into a gas turbine engine.FIG. 1 illustrates a gas turbine engine 10 having a main axis of rotation 9. the engine 10 includes an air inlet 12 and a propulsion fan 23 that produces two airflows: a core airflow A and a bypass airflow B. The gas turbine engine 10 includes a core 11 that receives the core airflow A. The engine core 11 comprises, in axial flow sequence, 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 core 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 B flows through the bypass duct 22, the fan 23 is mounted on and driven by the low pressure turbine 19 via a shaft 26 and an epicyclic gearing 30.In use, the core air stream A is accelerated and compressed by the low pressure compressor 14 and directed into the high pressure compressor 15 where it is further compressed. The compressed air discharged from the high pressure compressor 15 is introduced into the combustion equipment 16 where it is mixed with fuel F and the mixture is burned. Combustion equipment 16 may be referred to as combustor 16, with the terms "combustion equipment 16" and "combustor 16" being used interchangeably herein. The resulting hot combustion products then expand through and thereby drive the high pressure and low pressure turbines 17, 19 before being discharged through the nozzle 20 to provide some propulsion thrust. The high-pressure turbine 17 drives the high-pressure compressor 15 via a suitable connecting shaft 27. The fan 23 generally acts to pressurize the bypass airflow B flowing through the bypass duct 22 such that the bypass airflow B is expelled through the bypass outlet nozzle 18 to generally provide the majority of the motive thrust. The planetary gear 30 is a reduction gear.An exemplary arrangement for a gear 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 central pinion or sun gear 28 of the epicyclic gear assembly 30. Radially outwardly of and engaging sun gear 28 is a plurality of planet gears 32 which are coupled together by a planet carrier 34. The planet carrier 34 restricts the planet gears 32 to orbit synchronously about the sun gear 28 while enabling each planet gear 32 to rotate about its own axis. Planet carrier 34 is coupled via linkages 36 to fan 23 to drive rotation thereof about engine axis 9. Radially outwardly of and engaging the planet gears 32 is a ring gear 38 which is coupled via linkages 40 to a stationary support structure 24.It should be noted that the terms "low pressure turbine" and "low pressure compressor" as used herein mean the lowest pressure turbine stages and the lowest pressure compressor stages, respectively (i.e., not including the fan 23) and / or the turbine and compressor stages connected by the lowest speed in the engine connecting shaft 26 (i.e., not including the transmission output shaft that drives the fan 23). In some references, the "low pressure turbine" and "low pressure compressor" referred to herein may alternatively be known as the "intermediate pressure turbine" and "intermediate pressure compressor.". Where such alternative nomenclature is used, the fan 23 may be referred to as a first or lowest compression stage.The planetary gear 30 is shown in more detail in FIG. 3 by way of example. Each of the sun gear 28, the planet gears 32, and the ring gear 38 includes teeth on its periphery to engage the other gears. However, for clarity, only exemplary portions of the teeth are illustrated in FIG. 3. Four planet gears 32 are illustrated, although it will be apparent to those skilled in the art that more or fewer planet gears 32 may be provided within the scope of the claimed invention. Practical applications of planetary epicyclic gearing 30 generally include at least three planetary gears 32.The planetary gear 30 illustrated by way of example in FIGS. 2 and 3 is of the planetary type, since the planetary carrier 34 is coupled to an output shaft via linkages 36, with the ring gear 38 being fixed. However, any other suitable type of epicyclic gear train 30 may be used. As another example, the epicyclic gear 30 may be a star arrangement in which the planet carrier 34 is held fixed and the ring gear (or ring gear) 38 may rotate. In such an arrangement, the fan 23 is driven by the ring gear 38. As another alternative example, the transmission 30 may be a differential transmission in which both the ring gear 38 and the planet carrier 34 may rotate.It should be understood that the arrangement shown in FIGS. 2 and 3 is exemplary only and that various alternatives are within the scope of the present disclosure. By way of example only, any suitable arrangement may be used for housing the transmission 30 in the engine 10 and / or for connecting the transmission 30 to the engine 10. As another example, the connections (such as linkage 36, 40 in the example of FIG. 2 ) between transmission 30 and other parts of engine 10 (such as driveshaft 26, output shaft, and fixed structure 24) may have any desired degree of stiffness or flexibility. As another example, any suitable arrangement of bearings between rotating and stationary parts of the engine (e.g., between the input and output shafts of the transmission and the stationary structures such as the transmission case) may be used, and the disclosure is not limited to the example arrangement of FIG. 2. For example, if the transmission 30 has a star configuration (described above), one skilled in the art would readily understand that the arrangement of output and support linkages and bearing positions would typically vary from the configuration shown in FIG. 2 by way of example.Accordingly, the present disclosure extends to a gas turbine engine having any arrangement of transmission types (e.g., star or planetary gear), support structures, input and output shaft assemblies, and bearing positions.Optionally, the transmission may drive additional and / or alternative components (e.g., the intermediate pressure compressor and / or a booster compressor).Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. For example, such engines may include an alternative number of compressors and / or turbines and / or an alternative number of connection shafts. As another example, the gas turbine engine shown in FIG. 1 has a split flow nozzle 18, 20, meaning that the flow through the bypass duct 22 has its own nozzle 18 separate from and located radially outside the core 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 passage 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 mixed or split flow) may have a fixed or variable area.As another example, other gas turbine engines to which the present disclosure may be applied may not include a gearbox for the mainshaft(s) and instead may be direct drive engines.Other gas turbine engines to which the present disclosure may be applied may have alternative configurations. As an example, such engines may include an alternative number of connection shafts (e.g., two) and / or an alternative number of compressors and / or turbines. Further, the engine may include a gearbox provided in the powertrain from a turbine to a compressor and / or a fan.While the example described relates to a turbo fan engine, the disclosure may be applied to any type of gas turbine engine, such as an open rotor (where the fan stage is not surrounded by a nacelle) or a turboprop engine, for example. In some arrangements, the gas turbine engine 10 may not include a gearbox 30.The geometry of the gas turbine engine 10 and components thereof is defined by a conventional axis system that includes an axial direction (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 side in the view of FIG. 1 ). The axial, radial and circumferential directions are perpendicular to each other.The fuel F provided to the combustion equipment 16 may include a fossil-based hydrocarbon fuel such as kerosine. Thus, fuel F may comprise molecules from one or more of the chemical families of n-alkanes, isoalkanes, cycloalkanes, and aromatics. Additionally or alternatively, the fuel F, when mixed with or replaced by an alternative fuel, may comprise renewable hydrocarbons made from biological or non-biological resources, also known as sustainable aviation fuel (SAF). In any of the examples provided, the fuel F may include one or more trace elements including, for example, sulfur, nitrogen, oxygen, inorganics, and metals.For example, SAF is understood by those skilled in the art to mean a bio-fuel, a renewable flying fuel, a renewable nozzle fuel, an alternative fuel or a biojet fuel made from biological or non-biological resources. SAF is typically synthesized from carbonaceous gases taken from the atmosphere and / or separated in industrial processes; or from a wide range of sustainable raw materials such as waste oil and fats; solid boiling wastes; cellulosic wastes (such as corn stalks); crop crops such as flax yolk, carinate and light herb; non-biogenic alternative fuels; jatropha; halophytes and algae instead of fossil hydrocarbons. SAF is to be understood as not comprising fossil fuels.The functional performance of a particular fuel composition or mixture F for use in a particular mission may be defined, at least in part, by the fuel's ability to operate the Brayton cycle of the gas turbine engine 10. Parameters defining performance may include, for example, specific energy; energy density; thermal stability; and emissions including gaseous and / or particulate matter. In this context, the particulate emissions may include soot particles that arise during combustion of the fuel F and are also referred to as non-volatile particulates (nvPM). All references herein to soot or smoke apply equally to other types of particulate emissions known in the art. Gaseous emissions may include one or more of the following: oxides of nitrogen (NOx), carbon monoxide (CO), carbon dioxide (CO 2), unburned hydrocarbons (UHC), sulfur oxides (SO) including, for example, sulfur dioxide (SO 2) and / or sulfur trioxide (SO 3), and volatile organic compounds (VOC) produced during combustion of the fuel F. All references herein to gaseous emissions apply equally to other types of gaseous emissions known in the art.A relatively higher specific energy (i.e., energy per unit mass) expressed as MJ / kg may at least partially reduce the starting weight, thereby potentially providing a relative improvement in fuel efficiency. A relatively higher energy density (i.e., energy per unit volume) expressed as MJ / L may at least partially reduce the starting fuel volume, which may be particularly important for volume limited or military uses with refueling. Relatively higher thermal stability (i.e., inhibiting degradation or coking of the fuel under thermal load) may allow the fuel to maintain elevated temperatures in the engine and fuel injectors, thereby potentially providing relative improvements in combustion efficiency. Reduced emissions, including particulate matter, may allow for reduced condensate streaking while reducing the environmental impact of a given mission.Other properties of the fuel may also be central to the performance. For example, a relatively lower freezing point (°C) may allow remote deployments to optimize flight profiles; minimum aromatic concentrations (%) may ensure sufficient swelling of certain materials used in the construction of O-rings and gaskets previously exposed to fuels with high aromatic ingredients; and maximum surface tension (mN / m) may ensure sufficient spray distribution and atomization of the fuel.The ratio of the number of hydrogen atoms to the number of carbon atoms in a molecule can affect the specific energy of a given composition or mixture of fuel. Fuels having higher hydrogen to carbon ratios may have higher specific energies in the absence of bonding stress. For example, fossil-based hydrocarbon fuels may comprise molecules of about 7 to 18 carbons, with a significant portion of a given composition being derived from molecules of 9 to 15 carbons with an average of 12 carbons.A number of sustainable aircraft fuel mixtures have been approved for use. For example, some permitted mixtures comprise mixing ratios of up to 10% sustainable aviation fuel, while other permitted mixtures comprise mixing ratios of between 10% and 50% sustainable aviation fuel (the balance comprising one or more fossil hydrocarbon fuels such as kerosine), with the approval being pending further compositions. However, it is expected in the aeronautical industry that sustainable aircraft fuel mixtures comprising up to (and including) 100% sustainable aircraft fuel (SAF) will eventually be permitted for use.Sustainable aircraft fuels may include one or more of n-alkanes, isoalkanes, cycloalkanes, and aromatics, and may be made, for example, from one or more of syngas (syngas); lipids (e.g., fats, oils, and greases); sugars; and alcohols. Thus, sustainable aircraft fuels may comprise one or both of a lower aromatic and sulfur content compared to fossil-based hydrocarbon fuels. Additionally or alternatively, sustainable aircraft fuels may comprise one or both of a higher isoalkane and cycloalkane content relative to fossil-based hydrocarbon fuels. Thus, in some examples, persistent flying fuels may include a density between 90% and 98% of the density of kerosine and a heating value between 101 % and 105% of the density of kerosine, or both.In some examples, the sustainable fuel(s) or mixture(s) provided to the combustion equipment 16 may have a relatively lower content of aromatic and / or other non-paraffinic constituents than kerosine. The sustainable flying fuel may comprise an aromatic content of, for example, 30%, 20%, 15%, 10%, 8%, 5%, or less than 5%; e.g., 4%, 3%, 2%, 1%, or less than 1%; e.g., 0.75%, 0.5%, 0.25%, or less than 0.25%; e.g., 0.2%, 0.1%, or less than 0.1%; e.g., 0.01%, 0.001%, or 0%. The aromatic content of the sustainable flying fuel may be in an inclusive value or range that is limited by or within any of two of the values mentioned in the previous sentence (i.e. the values may form upper or lower limits), e.g. 13.5%, 8.5%, 2.5%, 0.35%, 0.15%, 0.05%, 0.005 % or 0%; or 0% to 0.75%, 0% to 0.5% or 0.1% to 0.25%; or 0.15% to 0.65%, 0.35% to 0.55% or 0.035% to 0.055%; depending on one or more of preference, fuel supply or supplier and variations of the composition thereof.Due, at least in part, to the molecular structure of sustainable aircraft fuels, sustainable aircraft fuels may provide advantages including, for example, one or more of a higher specific energy (despite, in some examples, a lower energy density); a higher specific heat capacity; a higher thermal stability; a higher lubricity; a lower viscosity; a lower surface tension; a lower freezing point; lower soot emissions; less NOx; and lower CO 2- emissions compared to fossil-based hydrocarbon fuels (e.g., when burned in the combustion equipment 16). Accordingly, persistent aircraft fuels may result in one or both of a relative decrease in specific fuel consumption and a relative decrease in maintenance costs, as compared to fossil-based hydrocarbon fuels, such as kerosine.As shown in FIG. 4, an aircraft 1 may include a plurality of fuel tanks 50, 53; for example, a larger primary fuel tank 50 located in the aircraft fuselage and a smaller fuel tank 53 a, 53 b located in each wing. In other examples, an aircraft 1 may have only a single fuel tank 50, and / or the airfoil fuel tanks 53 may be larger than the central fuel tank 50, or no central fuel tank may be provided (with all fuel instead stored in the airfoils of the aircraft) - it should be understood that many different tank arrangements are provided and that the depicted examples are provided for ease of description and are not intended to be limiting.FIG. 4 shows an aircraft 1 with a drive system 2 comprising two gas turbine engines 10. The gas turbine engines 10 are supplied with fuel from a fuel supply system onboard the aircraft 1. The fuel delivery system 1000 of the illustrated example includes a single fuel source. For purposes of the present application, the term "fuel source" means either 1) a single fuel tank or 2) a plurality of fuel tanks that are fluidly connected together. Each fuel source is arranged to provide a separate fuel source, i.e., a first fuel source may include a first fuel having a different property or properties than a second fuel included in a second fuel source. Thus, the first and second fuel sources are not fluidly connected to each other to separate the different fuels (at least under normal operating conditions). The use of several fuel sources allows an aircraft 1 to carry several different fuels and to change the fuel used during operation, optionally even in cruise flight or when changing between different operating stages in flight.In the present example, the first (and only in these examples) fuel source comprises a central fuel tank 50 located primarily in the fuselage of the aircraft 1, and a plurality of wing fuel tanks 53 a, 53 b, wherein for balancing at least one wing fuel tank is located in the port wing and at least one wing fuel tank is located in the starboard wing. All tanks 50, 53 are fluidly connected to each other in the example shown, thereby forming a single fuel source. Each of the middle fuel tank 50 and the wing fuel tank 53 may include a plurality of fuel tanks fluidly connected to each other.In another example, the wing fuel tanks 53 a, 53 bmay not be fluidly connected to the middle tank 50, thus forming a separate, second fuel source. For purposes of balancing, one or more fuel tanks in the port wing may be fluidly connected to one or more fuel tanks in the starboard wing. This may be done either via a middle fuel tank (unless this tank is part of the other fuel source), or bypassing the middle fuel tank / tanks, or both (for maximum flexibility and safety). In another example, the first fuel source includes wing fuel tanks 53 and a middle fuel tank 50, while a second fuel source includes another separate middle fuel tank.For balancing the aircraft 1, a liquid connection may be provided between the wing fuel tanks and the central fuel tank of the first fuel source. Thus, in multi-fuel source aircraft 1, two or more fuel sources may contain fuels that are different from each other so that aircraft 1 may change fuel during flight. Being able to determine which fuel is provided to the combustion chamber 16 may therefore be more complex than simply detecting a single identity of a fuel on board the aircraft 1 or checking once at start.In some examples, the allocation of the fuel tanks 50, 53 available in aircraft 1 may be limited such that the first fuel source and the second fuel source are each substantially symmetrical with respect to the centerline of the aircraft. In cases where asymmetric fuel tank allocation is permitted, a suitable means of fuel transfer is typically 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 gravity of the aircraft can be maintained within acceptable lateral limits throughout flight.Aircraft typically stop at several different airports, for example at the beginning and at the end of a long-distance flight. As mentioned above, while there are standards to which all of the aviation fuels must meet, different aviation fuels have different compositions, for example, depending on the source (e.g., different petroleum sources, bio-fuels, or other synthetic aviation fuels (often referred to as sustainable aviation fuels - SAFs - ) and / or mixtures of petroleum-based fuels and other fuels) and the additives included (e.g., antioxidants and metal deactivators, biocides, anti-static agents, anti-icing agents, anti-corrosion agents) and impurities. The composition of the available aviation fuel may vary not only from airport to airport and from fuel supplier to fuel supplier, but even from batch to batch of the same airport or fuel supplier. Moreover, the fuel tanks 50, 53 of the aircraft 1 are usually not emptied before being refilled for a further flight, which results in mixtures of different fuels being present in the tanks - and a fuel having a different composition being effectively formed from the mixture.The inventors have realized that by allowing different fuels to have different characteristics without conflicting with the standards, knowing the fuel / fuels available / available to an aircraft 1 may enable more efficient, individual control of the aircraft 1, and in particular the propulsion system 2 of the aircraft (i.e., the one or more gas turbine engines 10 of the aircraft 1 and the associated controllers and components). Knowledge of the fuel may therefore be used as an aid to improving aircraft performance, thus determining or monitoring the composition of the fuel may provide advantages. In particular, determining one or more fuel properties of the fuel to be supplied to the combustion chamber 16, whether it be a fuel from a single fuel source or a mixture of one or more fuels from different fuel sources, is therefore important for determining the operation of the engine. An essential feature of the operation of the engine is thermal management-thermal management of the engine is primarily via heat transfer to and from oil and fuel in the engine, such that control of a heat exchange system 3000-for example, by control of the oil flow, control of the fuel flow, control of the air flow, and / or control of one or more heat exchangers 1004, 1006, 2020, 2030 or other heat exchange components in some other way-may allow the performance of the engine 10 to be optimized for a fuel having particular fuel properties.As used herein, the term "fuel properties" refers to intrinsic or inherent fuel properties such as the composition of the fuel, and not to variable properties such as volume or temperature. One or more fuel characteristics may be determined and this data may be used to adjust control of the engine 10, and more particularly the thermal management system 3000. Examples of fuel properties include one or more of the following: i. the percentage of sustainable flying fuel (SAF) in the fuel or the indication that the fuel is a fossil fuel, e.g. fossil kerosine, or that the fuel is a pure SAF fuel; ii. parameters of a hydrocarbon distribution of the fuel, such as:• the content of aromatic hydrocarbons of the fuel and optionally also / alternatively the content of multiaromatic hydrocarbons of the fuel;• the hydrogen-carbon ratio (H / C) of the fuel;• Information on the % composition of some or all hydrocarbons present;iii. the presence or percentage of a particular element or species, such as:• Nitrogen content of the fuel / the percentage of nitrogen-containing species in the fuel;• the presence or percentage of a tracer species or trace element in the fuel;• the naphthalene content of the fuel;• Sulfur content of fuel / percent sulfur-containing species in fuel;• the cycloparaffin content of the fuel;• the oxygen content of the fuel;iv. one or more properties of the fuel used in a gas turbine engine 10, such as:• Level of non-volatile particulate (nvPM) or CO 2- emissions during combustion;• Degree of coking or paint of fuel (or other measure of deposition of degradation products of fuel);v. one or more properties of the fuel itself, regardless of use in an engine 10 or combustion, such as:• Thermal stability of the fuel (e.g. thermal decomposition temperature); and• one or more physical properties such as density, viscosity, heating value, freezing temperature and / or heat capacity.The fuel property(s) to be determined may be selected based on those fuel properties most relevant to possible changes to the thermal management system 3000. The determination of the fuel properties may include obtaining the fuel properties of the fuel that was already in the fuel tank 50, 53 before refueling and an indication of the amount of fuel remaining, and then combining this information with the information about the new fuel that is filled into the fuel tank 50, 53 during refueling.Obtaining the fuel properties of a fuel already in the fuel tank 50, 53 prior to refueling and / or obtaining the fuel properties of a fuel provided during refueling may comprise one or more of: (i) physically and / or chemically detecting one or more features or parameters of the composition of the fuel (this may allow direct detection of the fuel properties and / or allow determination of the fuel properties based on the detection results) and / or detecting one or more tracer elements or compounds added to the fuel to facilitate its identification (e.g., a colorant); (ii) retrieving information about fuel properties from an integrated memory / data store; and / or (iii) receiving data, for example from an input provided at a user interface or from data transmitted to the aircraft 1.In some examples, one or more fuel characteristics may be determined during operation of the gas turbine engine 10, for example, by deriving the fuel characteristics of the fuel provided to the combustor 16 during operation from the performance metrics of the engine or by making a detection on the airfoil.In some examples, multiple different methods may be performed to obtain the fuel properties - for example, different methods may be used for different properties and / or different methods may be used for testing for the same property. For example, stored or otherwise provided fuel property data may be compared to the results of chemically or physically detecting one or more parameters of the fuel. If the stored fuel property and the corresponding detected parameter do not match, a warning message may be provided.The fuel properties may be determined by physically and / or chemically detecting one or more characteristics of the composition of the fuel (e.g., in a test unit external to the wing or during transport of the fuel to a fuel tank on the wing or even during use in the gas turbine engine 10) such that the fuel properties may be directly detected or data may be provided from which they may be determined as mentioned above, and / or by detecting one or more tracer elements or compounds added to the fuel to facilitate its identification (e.g., a colorant); or by receiving data, for example via input provided at a user interface, or by data transmitted to the aircraft, e.g., by scanning a barcode associated with the fuel delivery.In the physical and / or chemical determination, the fuel properties may be determined in various ways, both directly (e.g., based on sensor data corresponding to the fuel property in question) and indirectly (e.g., by derivation or calculation from other properties or measurements, or by reference to data for a particular tracer detected in the fuel). The properties may be determined as relative values compared to another fuel or as absolute values. For example, one or more of the following detection methods may be used:• The aromatic or cycloparaffin content of the fuel can be determined from measurements of swelling of a sensor component made of a sealing material, such as a nitrile sealing material.• Trace materials or species either naturally present in the fuel or added as tracer may be used to determine fuel properties, for example the percentage of sustainable flying fuel in the fuel or whether the fuel is kerosine.• Measurements of the vibration mode of a piezoelectric crystal exposed to the fuel can be used as a basis for determining various fuel properties, including the aromatic content of the fuel, the oxygen content of the fuel and the thermal stability or the coking / paint level of the fuel - for example, by measuring the formation of surface deposits on the piezoelectric crystal that lead to a change in the vibration mode.• Various fuel properties may be determined by collecting performance parameters of the gas turbine engine 10 during a first operating period (e.g., during takeoff), and optionally also during a second operating period (e.g., during cruise flight), and comparing these collected parameters to expected values using fuel with known properties.• Various fuel properties, including the content of aromatic hydrocarbons in the fuel, may be determined based on sensor measurements regarding the presence, absence or degree of formation of a condensate strip by the gas turbine 10 during its operation.• The fuel properties, including the content of aromatic hydrocarbons, can be determined by means of a UV-Vis spectroscopy measurement performed on the fuel.• Various fuel properties, including sulfur content, naphthalene content, aromatic hydrogen content, and hydrogen-carbon ratio, can be determined by measuring substances contained in the exhaust gases exhausted from the gas turbine engine 10 during operation.• The heating value of the fuel can be determined during the operation of the aircraft 1 on the basis of measurements carried out during the combustion of the fuel - for example on the basis of the fuel flow rate and the shaft speed or the temperature change in the combustion chamber 16.• Various fuel characteristics may be determined by making an operational change that affects operation of the gas turbine engine 10, detecting a response to the operational change; and determining the one or more fuel characteristics of the fuel based on the response to the operational change.• Different fuel characteristics may be determined with respect to the 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 the ratio between T 30 and one of the values T 40 and T 41 (the ratio indicating the temperature rise in the combustion chamber 16). The properties may be determined as relative values to the first fuel or as absolute values, e.g., by reference to known values for the first fuel.As used herein, T30, T40, and T41, as well as any other numbered pressure readings and temperatures, are defined based on the station numbering set forth in the SAE AS755 standard, particularly:• T30 = total temperature at the outlet of the high pressure compressor (HPC);• T40 = total combustion outlet temperature;• T41 = Total temperature at the rotor inlet of a high pressure turbine (HPT).Any suitable approach known in the art may be used and the determination of fuel properties will not be discussed further herein.The aircraft 1 may therefore comprise a fuel composition determination module 57 configured to determine at least one fuel property of the fuel. The determination may be made by retrieving a value from a data store (e.g., from data provided to the aircraft 1 during refueling) or by performing one or more calculations based on data provided by one or more sensors or other engine components using any of the methods described above. In the example shown in FIG. 4, a fuel composition determination module 57 is provided as part of each engine 10 where fuel enters the engine 10. In other implementations, module 57 may be located elsewhere in engine 10 or elsewhere on aircraft 1, e.g., in, on, or adjacent to a fuel tank 50, 53. In implementations with a fuel composition determination module 57, module 57 may be configured to provide output to an electronic engine control unit (EEC), or module 57 may be provided as part of an EEC. The one or more fuel characteristics determined by the fuel composition determination module 57 may therefore be used to affect control of the engine 10. A special controller 58 may be provided to process the output of a fuel composition determination module 57 and provide control instructions to controllable components of the engine based on the fuel property information. In other implementations, the control functionality 58 may be provided by the EEC and no separate unit or module may be provided.An example fuel system 1000 for a transmission gas turbine engine 10 is shown in FIG. 5, where the fuel system 1000 includes a fuel flow path from the fuel tank 50 to the combustor 16 of the gas turbine engine 10 of the aircraft 1. The fuel system 1000 includes both the fuel delivery system 50, 1002 (which supplies fuel to the engine 10) and the fuel management system 1500 (which operates within the engine 10) of the aircraft 1. the fuel management system 1500 is operable to control fuel temperature as well as fuel flow by directing fuel via one or more heat exchangers 1004, 1006 of the engine's heat exchange system 3000. Heat exchange system 3000 includes portions of fuel management system 1500 as well as oil circulation system 2000, 2000' (described in more detail below). Heat exchange system 3000 is the general term for the systems and components used to transfer heat between fluids (particularly oil and fuel) within engine 10, and includes heat exchangers, valves, connection pipes, and associated components, e.g., pumps, refrigerators, etc.Returning to fuel system 1000, fuel is pumped from fuel tank 50 to gas turbine engine 10 by low pressure fuel supply pump 1002. The fuel then flows through a secondary fuel-oil heat exchanger 1004 and a primary fuel-oil heat exchanger 1006. The primary fuel-oil heat exchanger 1006 may be described as a main fuel-oil heat exchanger because the oil flowing through it may be used to cool and lubricate the main transmission 30 of the engine 10. The gas turbine engine 10 of the described implementations may further include a generator (particularly an integrated propulsion generator) and a secondary oil circulation system configured to provide oil to the generator.The secondary fuel-oil heat exchanger 1004 may be described as an integrated propulsion generator fuel-oil heat exchanger because the oil flowing through it may be used to cool and / or lubricate one or more integrated propulsion generator (IDG) components of the engine 10. In other implementations, a different type of generator may be used in place of an IDG - e.g., a variable frequency generator (VFG) or a variable frequency starter generator (VFSG). The fuel system 1000 of such implementations may be otherwise equivalent.The engine 10 of the described example therefore comprises two fuel-oil heat exchangers 1004, 1006. In other implementations, more or fewer fuel-oil heat exchangers may be provided. Depicted fuel management system 1500 is configured such that fuel reaches secondary fuel-oil heat exchanger 1004 prior to primary fuel-oil heat exchanger 1006. After exiting the fuel-oil heat exchanger 1006, the fuel passes through an engine fuel pump 1003 and then further into the combustion chamber 16. In other implementations, the engine fuel pump 1003 may be located upstream of one or more of the heat exchangers 1004, 1006.In the example shown in FIG. 5, fuel system 1000 further includes a temperature sensor 1009 configured to sense the temperature of fuel approaching or reaching combustion chamber 16. The temperature sensor 1009 is shown adjacent to an inlet of the combustion chamber 16 in the depicted example, however, in various examples, the sensor 1009 may be located anywhere downstream of the primary fuel-oil heat exchanger 1006 (or the fuel-oil heat exchanger that is most downstream in other implementations) on the fuel side to provide a measurement of fuel temperature. In gas turbine engines 10 such as those described herein, the fuel passes through fuel spray nozzles before entering the actual combustor - these nozzles may be very susceptible to thermal degradation of the fuel, which results in blockages in the relatively narrow passages (e.g., metering slots); therefore, in some implementations, it is recommended to place the sensor 1009 as close as possible to the inlet of the nozzles. In some implementations, multiple temperature sensors 1009 may be used. The one or more temperature sensors 1009 are configured to provide an output (temperature data) to a controller 58.The primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are configured to carry an oil stream through each in addition to the fuel stream therethrough. The primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are configured such that heat may be transferred between the oil and the fuel flowing through them. During normal operation of the engine 10, for example, cruise conditions, the average temperature of the oil stream entering the primary fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel entering the primary fuel-oil heat exchanger 1006, and the average temperature of the oil stream entering the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the secondary fuel-oil heat exchanger 1004. In this manner, the primary fuel-oil heat exchanger 1006 and the secondary fuel-oil heat exchanger 1004 are each configured to transfer thermal energy from an oil stream to a fuel stream flowing therethrough during operation.The two oil streams (oil stream through the primary heat exchanger and oil stream through the secondary heat exchanger) can be separated-physically separated and optionally also chemically different oils and / or have a different flow rate. Therefore, oil may flow through the primary fuel-oil heat exchanger 1006 other than through the secondary fuel-oil heat exchanger 1004. Each heat exchanger 1004, 1006 may be located in a separate closed loop oil circulation system 2000, 2000'.Generally, at least most of the fuel passing through the secondary fuel-oil heat exchanger 1004 also passes through the main fuel-oil heat exchanger 1006. The two heat exchangers 1004, 1006 may therefore be described as being in series with each other and along a main fuel flow path from the tank 50 to the combustion chamber 16 with respect to fuel flow. However, one or each of the heat exchangers 1004, 1006 may be provided with a bypass so that a portion of the fuel does not flow through the respective heat exchanger, for example in the form of a bypass pipe 1005, as shown in FIG. 5. A valve (not shown) may determine which portion of the fuel flows through the heat exchanger 1004 and which portion flows through the bypass tube 1005. In various implementations, a bypass tube may be provided for each heat exchanger 1004, 1006 so that a portion of the fuel may bypass one or both heat exchangers. Additionally or alternatively, one or more bypass pipes 2005, 2005' for the oil may be provided for one or both fuel-oil heat exchangers 1004, 1006, such that a portion of the oil may bypass one or more heat exchangers. A valve arranged to control the flow of a liquid through a bypass pipe 1005 may be referred to as a bypass valve.The secondary (IDG) fuel-oil heat exchanger 1004 and the main fuel-oil heat exchanger 1006 are therefore configured to also pass an oil stream through them in addition to the fuel stream - wherein in the described implementation the oil flowing through the one heat exchanger is different from the oil flowing through the other heat exchanger, although it is understood that in other implementations the same oil may flow through one fuel-oil heat exchanger and then through the other fuel-oil heat exchanger.The two heat exchangers 1004, 1006 are therefore located in separate closed-loop systems 2000, 2000' (FIGS. 7, 8 ) in the described implementation with respect to the oil flow, i.e. the oils, the oils flowing through the primary and secondary fuel-oil heat exchangers are fluidically separated and may be chemically different from one another. The two oil circulation systems 2000, 2000' serve to circulate oil through their respective fuel-oil heat exchangers 1006, 1004, and optionally also through one or more further heat exchangers, e.g. air-oil or oil-oil heat exchangers, as described below. The two oil circulation systems 2000, 2000' may be provided together as an oil circulation system for the engine 10.FIG. 6 shows an alternative example fuel system 6000 including a fuel delivery system and a fuel management system 6500 including a fuel flow path from the fuel tank 50 to the combustor 16 of the gas turbine engine 10 of the aircraft 1. As with fuel system 1000, fuel is pumped from fuel tank 50 by low pressure fuel supply pump 1002. The fuel then flows through a secondary fuel-oil heat exchanger 1004 and then a primary fuel-oil heat exchanger 1006 before reaching an engine fuel pump 1003 that pumps the fuel along its flowpath to the combustor 16. Fuel system 6000 differs from fuel system 1000 in that fuel system 6000 includes a recirculation valve 6010 located downstream of primary fuel-oil heat exchanger 1006 and configured to recirculate at least a portion of the fuel that has left the primary fuel-oil heat exchanger back to inlet 1006 aof primary fuel-oil heat exchanger 1006, thereby allowing further heat transfer between the primary circulation system oil and the fuel. The fuel temperature sensor 1009 is located downstream of the drop point for the recirculation valve 6010, and ideally adjacent to the combustion chamber 16, to provide a more accurate indication of the temperature at the inlet of the combustion chamber 16 (in some implementations, another temperature sensor may be provided between the heat exchanger outlet and the drop point for the recirculation valve 6010). Recirculation valve 6010 may determine which portion of fuel is recirculated via recirculation pipe 6011 and which portion is directed more directly to combustion chamber 16. In the illustrated example, the recirculation valve 6010 is downstream of the primary fuel-oil heat exchanger 1006. In the illustrated example, recirculation valve 6010 is positioned upstream of engine fuel pump 1003. In some implementations where the recirculation valve 6010 is positioned upstream of the engine fuel pump 1003, an additional recirculation pump (not shown) may be provided to provide a positive pressure gradient. A pump for returning fuel to the tank (FRTT) may be present and in some implementations may also aid in the recirculation. The recirculation valve 6010 is arranged to allow a controlled amount of fuel to be recirculated to the inlet 1006 aof the primary heat exchanger 1006 and thus flow through the primary heat exchanger 1006 a plurality of times before reaching the pump 1003 and the combustion chamber 16. It is contemplated that the recirculation valve could be positioned downstream of the engine fuel pump 1003, for example as shown in FIG. 10, in alternative implementations. In such implementations, the recirculation valve 6010 would be arranged such that a controlled amount of fuel can be recirculated to the inlet 1006 aof the primary heat exchanger 1006 and thus multiple times flow through the primary heat exchanger 1006 and the pump 1003 before reaching the combustion chamber 16. This feedback provides a mechanism for controlling fuel flow within fuel management system 6500, and also within thermal management system 3000, without altering fuel flow from tank 50 to engine 10.Thus, a tube 6011 may be provided, which may be referred to as a return tube, as it carries fuel from a point on the main flow path through the engine 10 to an earlier point on that flow path, so that the fuel must pass through the intermediate portion of the flow path again, and which leads from the return valve to a point on the flow path upstream of the inlet 1006a to the primary heat exchanger 1006. In some implementations, a recirculation pipe 6011 and a bypass pipe 1005 may be provided for any given heat exchanger 1004, 1006. In some implementations, the same pipe 1005, 6011may serve as both a return pipe and a bypass pipe. One or more valves may be used to control the direction of liquid flow through the tube.In the example shown in FIG. 10, unlike FIG. 6, the recirculation valve 6010 is downstream of the pump 1003, so that the recirculated fuel has already passed through both the primary heat exchanger 1006 and the pump 1003, and not just the primary heat exchanger 1006. The recirculation pipe 6011 recirculates the recirculated fuel to a point on the flow path that is upstream of both the main pump 1003 and the primary heat exchanger 1006, so that the recirculated fuel passes through both of these components once more. Recirculation of fuel through the pump may allow more adjustable control of fuel flow rate to the combustor 16 for a particular shaft speed of the engine 10, it being appreciated that pump speed (or a limited set of pump speed options) is often determined by the shaft speed.Controlling fuel flow in fuel system 6000, by recirculating through and / or bypassing one or more heat exchangers, may assist in thermal management (e.g., affecting the temperature of the fuel as it enters combustion chamber 16 or pump 1003, or the amount of heat transferred to the fuel). Thus, a portion of the control of the heat exchange system 3000 may be the control of one or more valves that control the flow of oil and / or fuel through one or more return and / or bypass pipes 6011, 1005.The described gas turbine engine 10 of the aircraft 1 includes an oil circulation system configured to provide oil for lubricating and heat removing a plurality of components. In the described implementation, the oil circulation system comprises a primary oil circulation system 2000 and a secondary oil circulation system 2000', each of which is a closed oil circulation system. An example of a primary closed loop oil system 2000 is schematically shown in FIG. 7 --as with the heat exchangers, this oil loop system is also referred to as "primary" because it is responsible for lubricating and cooling the main transmission 30 and generally for the main / primary cooling load of the engine 10. The primary closed loop oil system 2000 includes an oil tank 2002 that is adapted to receive a volume of oil. In some implementations, gases are removed from oil within the oil tank 2002 by a breather. A feed pump 2004 is configured to pump oil from the oil tank 2000 into the main fuel-oil heat exchanger 1006. The average temperature of the oil entering the main fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel entering the main fuel-oil heat exchanger 1006 during cruise conditions. In the main fuel-oil heat exchanger 1006, thermal energy is transferred from the oil stream to the fuel stream. In this way, the average temperature of the oil stream exiting the main fuel-oil heat exchanger 1006 is lower than the average temperature of the oil stream entering the main fuel-oil heat exchanger 1006 so that it is cooled prior to reuse as lubricant and / or coolant, thereby allowing the cooled oil to remove more heat from the system to be lubricated and / or cooled. Also in this manner, the average temperature of the fuel exiting the main fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel entering the main fuel-oil heat exchanger 1006.In a standard heat exchanger 1006, the temperature of the coolant (in this case the fuel) is the basic limiting factor for the cooling power provided - the temperature of the oil as it exits the heat exchanger 1006 cannot be lowered below the temperature of the fuel entering the heat exchanger 1006 (and also the temperature of the fuel cannot be raised higher than that of the oil entering the heat exchanger), even if a substantially infinite flow rate of fuel or oil were used. Temperature compensation is the threshold as defined in the second principle set of thermodynamics. Therefore, in some implementations, as illustrated in FIG. 7, a refrigeration cycle device 1007 is provided. The refrigeration cycle device 1007 is arranged to provide thermal buoyancy by transferring further heat from the oil to the fuel so that the temperature of the fuel is raised more than would be the case with a simple pass through the heat exchanger, and in some cases even beyond the oil temperature. The refrigeration cycle device 1007 may take the form of an additional closed circuit including a refrigerant liquid, an evaporator between oil and refrigerant (to transfer heat from the oil to the refrigerant), and a condenser between fuel and refrigerant (to transfer heat from the refrigerant to the fuel. The refrigeration cycle device 1007 may additionally include a pump, compressor, and / or expansion valve / meter, and may be embodied in any suitable embodiment known in the art. It should be appreciated that the refrigeration cycle device 1007 may also be referred to as a heat pump-it pumps heat from the oil to the fuel, thus cooling the oil and heating the fuel.FIG. 18 shows an example of a refrigeration cycle device 1007 in detail. The depicted refrigeration cycle device 1007 includes a refrigerant liquid that circulates through the device 1007 during operation. In the described implementation, the flow rate of the refrigerant may be adjusted to vary the amount of heat transferred, and the refrigeration cycle device 1007 may be shut off by stopping the flow of the refrigerant. In other implementations, the refrigeration cycle device 1007 may have a single operating speed and its control may simply consist of turning the refrigeration cycle device 1007 on and off. The refrigerant liquid may be 1,1,1,2-tetrafluoroethane (commercially known as R134a refrigerant), although other refrigerant liquids may also be used.During operation, heat is transferred from the oil in the oil cycle system 2000 to the refrigerant liquid in an evaporator 1007 a-the evaporator 1007 aevaporates liquid in the refrigerant liquid (the refrigerant liquid may be a liquid-gas mixture or a pure liquid at this point in the refrigeration cycle) to form a vapor and, in particular, a saturated vapor.The evaporator 1007a is located in the oil circulation system 2000, at or near where the oil is the hotter-generally immediately after the last engine component to be cooled by the oil in a single pass through the oil circulation system 2000 (this may be the transmission 30). To increase heat transfer, one or more thermally conductive plates may be provided between the oil and the refrigerant (generally as part of the evaporator 1007a); optionally, internal passages for the oil and / or refrigerant flow may be provided within a thermally conductive plate or other structure to improve heat transfer.From the evaporator 1007a, the saturated vapor is then sent to a refrigerant compressor 1007b. The refrigerant compressor 1007 bmay also function as a pump that circulates the refrigerant around the refrigeration cycle device 1007. The control of the compressor 1007b may therefore be used to reduce, increase or stop the flow of refrigerant around the device 1007. Advantageously, therefore, the refrigeration cycle device 1007 may meet a variable oil cooling / fuel heating demand due to the ability to modulate the cooling flow rate using the refrigerant compressor 1007b. In some implementations, a separate / additional pump may be provided for the refrigeration cycle device 1007, but using the compressor 1007 bto circulate the refrigerant may reduce the number of components required and thus the size and weight of the device 1007.The compressor 1007b compresses the saturated vapor into superheated vapor and sends it to a condenser 1007c functioning as a refrigerant-fuel heat exchanger 1007c. The condenser 1007c transfers heat from the compressed vapor to the fuel (in the fuel management system 1500) and condenses at least a portion of the superheated vapor into a liquid. Generally, the condenser 1007c can convert the compressed vapor refrigerant into a saturated liquid. The condenser 1007c is located in the fuel path 1500 through the engine 10, downstream of one or all of the fuel-oil heat exchangers 1004, 1006-generally near the combustor 16. The condenser 1007c may be the last engine component through which the fuel flows before reaching the combustor 16 (not numbered pipes / fuel flow passages). The condenser 1007 cmay be the most downstream component of the heat exchange system 3000 in terms of fuel flow.The refrigeration cycle device 1007 of the embodiment illustrated in FIG. 18 further includes an expansion valve 1007d located between the condenser 1007c and the evaporator 1007a. The expansion valve 1007 dis configured to allow expansion of the refrigerant, thereby cooling the refrigerant. For example, the action of the expansion valve 1007d may convert the saturated liquid to a liquid and vapor mixture of reduced temperature. The expanded / cooled refrigerant is then returned to evaporator 1007a and is ready to absorb further heat from the oil.The refrigeration cycle device 1007 may further include a refrigerant tank (not shown) in some implementations. Such a refrigerant container can serve to compensate for any changes in volume of the refrigerant due to temperature and density changes, so that the refrigerant flow can reliably and consistently perform its cooling / heat pump function during all engine operating conditions.In the implementations shown in the figures, the engine 10 includes a gearbox 30 that receives input from the core shaft 26 and outputs drive to the fan 23 to drive the fan at a lower speed than the core shaft, and the oil circulation system 2000 is configured to supply oil to the gearbox 30. However, it should be appreciated that the implementation shown in FIG. 18 does not require that the oil circuit system 2000 include a transmission 30, and that the same refrigeration circuit device 1007 and its controller may be implemented in a direct drive engine (without a transmission for the mainshaft).FIG. 18 illustrates the connection of the refrigeration cycle device 1007 to both the oil system 2000 and the fuel system 1500 within the engine 10. Heat is transferred from the oil system 2000 to the refrigerant in the evaporator 1007a, and this heat is then transferred from the refrigerant to the fuel system 1500 in the condenser 1007c. FIG. 18 also schematically shows two temperature sensors - an oil temperature sensor 2009, which is configured to provide an indication of the maximum oil temperature in the oil circuit system 2000, and a fuel temperature sensor 1009, which is configured to provide an indication of the temperature of the fuel downstream of the refrigeration circuit device 1007 or as it approaches the combustion chamber 16. Data from these sensors 1009, 2009 may be used to control the refrigeration cycle device 1007-for example, to control one or more of the refrigerant flow rate, the oil flow rate, and the fuel flow rate through the corresponding portion(s) of the refrigeration cycle device 1007.An oil valve may control how much oil flows through the refrigeration cycle device 1007 (and more specifically through the evaporator 1007a), and a fuel valve may control how much fuel flows through the refrigeration cycle device 1007 (and more specifically through the condenser 1007c). The refrigeration cycle device 1007 is generally electrically or mechanically driven (e.g., via a connection to a rotating shaft) to provide this further forced heat transfer. The refrigeration cycle device 1007 is shown in connection with the main fuel-oil heat exchanger (primary fuel-oil heat exchanger) 1006 in the depicted implementation, thus providing an increase in fuel temperature after fuel passes both fuel-oil heat exchangers 1004, 1006, and further cooling of the oil of the primary oil system 2000 before being recirculated to the transmission 30. In various implementations, a bypass or return pipe for the oil and / or fuel may be provided around the refrigeration cycle device 1007.The oil stream in the primary oil circulation system 2000 is then conveyed to a power transmission 30, which may also be referred to as the main transmission 30 of the gas turbine engine 10. The power transmission 30 is arranged to receive input from the core shaft 26 and output to the fan 23 via the fan shaft 42 and includes the gears 28, 32, 38 and bearings (e.g., journal bearings) that can be lubricated and / or cooled by the oil. The engine 10 may also include one or more additional bearings for supporting the shafts 26, 42, which may be journal bearings. The oil can additionally be used to lubricate and / or cool the journal bearings and generally significantly increases temperature when used under cruise conditions, thereby assisting in cooling the bearings and transmission 30, as the oil stream carries heat away from the bearings and transmission 30. The oil may also be used to lubricate one or more other engine components 33, e.g., an auxiliary gear box (AGB) and / or one or more bearing chambers. The AGB 33, also known as a power assist, if present, is a gearbox that forms part of the gas turbine engine 10, although it is not part of the engine core 11 and does not drive the fan 23. The AGB instead drives engine accessories, e.g., fuel pumps, and generally handles large loads. A relatively large amount of heat can therefore be dissipated from the AGB into the oil. One or more bearing chambers may be lubricated by the same oil and may similarly release heat into the oil. Per unit of oil flowing through, the AGB and bearing chambers may add more heat to the oil than the main transmission 30 in many implementations. The oil flow may be split into two or more parallel flows, for example, one flow through the main transmission 30 and one flow through the other engine components, or multiple parallel flows through the main transmission 30 (e.g., via different components of the transmission), and separate flows through the AGB and the or each bearing chamber 33.The oil from the power transmission 30 (and / or other engine components 33, such as the AGB, if present) collects in an oil pan 2008. A return pump 2010 is configured to pump oil from the oil pan 2008 back into the oil tank 2002 ready for reuse.FIG. 8 illustrates the secondary oil circulation system 2000', which is another closed oil circulation system 2000'. The secondary closed oil circuit system 2000' includes a secondary oil tank 2002' adapted to receive a volume of oil. In some implementations, gases are removed from oil within the oil tank 2002' by a breather. A secondary feed pump 2004' is configured to pump oil from the secondary oil tank 2002' to the secondary fuel-oil heat exchanger 1004, which in the illustrated implementation is the IDG fuel-oil heat exchanger 1004. The average temperature of the oil entering the IDG fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the IDG fuel-oil heat exchanger 1004 during cruise conditions. In the IDG fuel-oil heat exchanger 1004, thermal energy is transferred from the oil stream to the fuel stream. In this way, the average temperature of the oil stream exiting the IDG fuel-oil heat exchanger 1004 is lower than the average temperature of the oil stream entering the IDG fuel-oil heat exchanger 1004. Also in this way, the average temperature of the fuel exiting the IDG fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel entering the IDG fuel-oil heat exchanger 1004. The oil stream is then conveyed back to an integrated propulsion generator 2006, where it lubricates and / or cools moving components and is warmed while doing so. In some implementations, the oil may be used primarily as coolant for IDG 2006 and may provide minimal or no lubrication. From the integrated propulsion generator 2006, the oil collects in a secondary oil pan 2008'. A secondary return pump 2010' is configured to pump oil from the secondary oil pan 2008' back into the secondary oil tank 2002' where it is ready for reuse. In some implementations, a refrigeration cycle device may also be provided in the secondary oil cycle system 2000'.FIGS. 7 and 8 each show a serial flow path of the oil, wherein the entire oil stream flows successively through the individual components (wherein, of course, one or more bypass pipes or return pipes, not shown, can also be provided for the oil). In other implementations, the oil stream may be split into two or more parallel streams, e.g., a stream through the main fuel-oil heat exchanger 1006 and a stream through an air-oil heat exchanger 2020 (as described below). It is understood that although a bypass pipe provides a parallel arrangement from a technical point of view, alternatively a flow path, a branching path as described herein with respect to parallel flows, differs in that each branching route of the parallel arrangement comprises a heat exchanger, whereas the bypass pipe is only a pipe without (appreciable) heat exchange.FIG. 9 schematically illustrates an alternative example portion of the primary closed loop oil system 2000 shown in FIG. 7. In this section, an oil flow is pumped by the feed pump 2004 through a valve 2016. The valve 2016 may be operated to split the oil flow between the main fuel oil heat exchanger 1006 and a first air oil heat exchanger 2020, the first air oil heat exchanger 2020 being arranged in parallel with the main fuel oil heat exchanger 1006. The oil flow path may be described as a branch, with the main fuel-oil heat exchanger 1006 on one branch and the first air-oil heat exchanger 2020 on the other branch arranged in a parallel configuration such that oil may flow over one branch or the other, but the same portion of the oil may not pass through both in the same cycle - the flow is split. The valve 2016 modulates the flow through the two heat exchangers 1006, 2020 and can therefore be described as a modulation valve 2016. The oil streams are then merged again and conveyed to the power transmission 30 and / or other engine components 33.Any suitable percentage of oil may flow through each of the first air-oil heat exchanger 2020 and the main fuel-oil heat exchanger 1006. In some examples, the valve 2016 is operable to vary the flow of oil into the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020 as desired. In various examples, an oil-to-oil heat exchanger 2030 (not shown in FIG. 7, but present in FIG. 11 ) may be provided, for example, arranged in series with the first air-to-oil heat exchanger 2020 on this branch of the parallel split. The oil-to-oil heat exchanger 2030 may allow heat exchange between the primary and secondary closed oil loop systems 2000, 2000'.FIG. 9 also illustrates details of the air-oil heat exchanger 2020, which are not shown in other figures for the sake of clarity, and points in particular with dashed arrows towards the air flow. The air-oil heat exchanger 2020 has an air inlet 2020a and an air outlet 2020b. The air inlet or inlet 2020a may be configured to receive air in the bypass duct 22 of the engine downstream of an outlet vane of the fan 23. The heat exchanger exhaust / air outlet 2020b may be configured to discharge air back into the bypass duct 22 downstream of the inlet 2020a or directly into the outside atmosphere (e.g., if provided with a special outlet nozzle).A valve 2022, referred to as an air valve, is used to control the flow rate of air through the air-oil heat exchanger 2020. In the depicted example, air valve 2022 is provided at or near inlet 2020a to heat exchanger 2020 and may therefore be referred to as an air inlet valve 2022. In other implementations, the air valve 2022 may be provided at or near the outlet 2020 bof the heat exchanger 2020 and may therefore be referred to as an air outlet valve. Any suitable position may be selected for the valve 2022, provided that the valve 2022 can be adjusted so that the flow of air through the air-oil heat exchanger 2020 can be controlled. In the example described, the air valve 2022 is continuously adjustable between a fully closed position (no air flow through the heat exchanger) and a fully open position (maximum air flow through the heat exchanger). In other examples, rather than being continuously adjustable, the air valve 2022 may be adjustable between a plurality of discrete positions, for example, six, five, four, three, or two different positions. In implementations with only two valve positions for the air valve 2022, these positions may be "open" and "closed"-the valve 2022 may be repeatedly opened and closed to provide pulsating air flow when a medium cooling level is desired in some of these implementations.FIG. 10 shows an exemplary arrangement and interaction of the first closed oil circuit system 2000, the second closed oil circuit system 2000', and the fuel system 1000, wherein the fuel flow is shown in thick black lines and the oil flow is shown in thinner black lines. The hatched thick black line indicates a return path 6011 through which only a part of the fuel passes. The combination of the parts of the fuel system 1000 and the oil systems 2000, 2000' together forms the heat exchange system 3000. The primary closed loop oil system 2000 of this exemplary arrangement is arranged as shown in FIG. 7. The closed loop secondary oil system 2000' of this exemplary arrangement is arranged as shown in FIG. 8. The fuel system 1000 of this exemplary arrangement is arranged as shown in FIG. 5, but with an additional recirculation valve 6010 and pipe 6011 as described above. One or more bypass pipes 1005, 2005 may likewise be present, but are not shown for the sake of clarity.In use, fuel is pumped from the fuel tank 50 by the low pressure fuel pump 1002. The fuel then flows through the secondary fuel-oil heat exchanger 1004. The secondary closed oil loop system 2000' is configured such that its recirculating oil stream also flows through the secondary fuel-oil heat exchanger 1004. During normal operation of the engine 10, both in cruise conditions and idle, the average temperature of the oil flow entering the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel flow entering the secondary fuel-oil heat exchanger 1004. The secondary fuel-oil heat exchanger 1004 is configured to transfer heat from the oil stream to the fuel stream. In this way, the average temperature of the oil stream exiting the secondary fuel-oil heat exchanger 1004 is lower than the average temperature of the oil stream entering the secondary fuel-oil heat exchanger 1004. Similarly, the average temperature of the fuel stream exiting the secondary fuel-oil heat exchanger 1004 is higher than the average temperature of the fuel stream entering the secondary fuel-oil heat exchanger 1004.The fuel then flows through the main fuel-oil heat exchanger 1006 and, in addition, through the refrigeration cycle device 1007. The primary closed oil cycle system 2000 is configured such that its oil return flow also flows through the main fuel-oil heat exchanger 1006 and the refrigeration cycle device 1007. During normal operation of the engine 10, both in cruise conditions and idle, the average temperature of the oil flow entering the main fuel-oil heat exchanger 1006 is higher than the average temperature of the fuel flow entering the main fuel-oil heat exchanger 1006. The main fuel-oil heat exchanger 1006 is configured to transfer heat from the oil stream to the fuel stream. In this way, the average temperature of the oil stream exiting the main fuel-oil heat exchanger 1006 is lower than the average temperature of the oil stream entering the main fuel-oil heat exchanger 1006. Similarly, the average temperature of the fuel stream exiting the main fuel-oil heat exchanger 1006 is higher than the average temperature entering the main fuel-oil heat exchanger 1006.The refrigeration cycle device 1007 may, when active (i.e., when energized / energized and used to actively transfer heat from the oil to the fuel), provide a further increase in fuel temperature and a further decrease in oil temperature, respectively, where the fuel is optionally raised above the oil temperature. After passing through the main fuel-oil heat exchanger 1006, the fuel flows to the engine fuel pump 1003, which in the examples shown is downstream of the primary and secondary fuel-oil heat exchangers 1006, 1004 and is arranged to supply fuel to the combustor 16 of the gas turbine engine 10.During cruise conditions, the average temperature of the oil stream flowing through the secondary fuel-oil heat exchanger 1004 may be lower than the average temperature of the oil stream flowing through the main fuel-oil heat exchanger 1006. In this manner, the fuel first passes through the heat exchanger 1004 having a lower average oil flow temperature before passing through the heat exchanger 1006 having a higher average oil flow temperature.In addition to the diverging oil streams 2000, 2000', the heat exchange system 3000 may further include diverging fuel return paths 6020 such that fuel from where fuel enters the gas turbine engine 10 to the combustor 16 is returned to the main fuel path at at least two different locations - for example, upstream or downstream of the main engine pump 1003 as shown in FIG. 6, with a diverging path 6020 branching the main fuel flow path at the exit 1006b of the primary heat exchanger 1006 and opening back into the main fuel flow path downstream of the pump 1003. At least one valve (not shown) may be provided to control a division of fuel flow from heat exchanger 1006 back to the main fuel path through engine 10. The valve may be controlled based on fuel temperature - for example, to direct less fuel via pump 1003 / more fuel to a position downstream of the pump when fuel temperature is relatively high and the likelihood of damage to the pump seals or other components is higher. Control of fuel flow through the diverging fuel return paths may be based on measurement of fuel temperature (e.g., using a temperature sensor at a location downstream of the heat exchanger 1006 and fuel, possibly also using a temperature measurement upstream of the fuel-oil heat exchanger). Recirculation of the recirculated fuel upstream of the fuel-oil heat exchanger 1006 may allow for reduced heat transfer from the oil to the fuel and thus dampen transient overshoot that may occur, for example, at the beginning of the descent phase when the same amount of heat is generated in the oil system, but fuel flow is reduced, which typically results in a temperature increase.FIG. 11 schematically shows an exemplary configuration of the primary closed loop oil system 2000 and the secondary closed loop oil system 2000' where the two independent recirculating oil streams are brought into a heat exchange relationship by an oil-to-oil heat exchanger 2030. Both oil circulation systems 2000, 2000' have in this example a branched arrangement of parallel pipes / heat exchangers.In the example shown in FIG. 11, the primary closed loop oil system 2000 is configured such that the oil return flow from the feed pump 2004 is pumped through a valve 2016, which may be referred to as a modulation valve. The valve 2016 may be operated to split the oil stream such that a portion of the oil stream flows to the main fuel-oil heat exchanger 1006 and a first air-oil heat exchanger 2020, respectively. The valve 2016 may be controllable to adjust the amount of oil routed across each heat exchanger. In the illustrated implementation, the first air-oil heat exchanger 2020 is connected in series with the oil-oil heat exchanger 2030, and the arrangement of the air-oil heat exchanger 2020 and the oil-oil heat exchanger 2030 is connected in parallel with the main fuel-oil heat exchanger 1006. The modulation valve 2016 determines which portion of the oil flows through each branch of the parallel arrangement. In various implementations, any suitable portion of the oil stream may be diverted between the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. In some examples, the valve 2016 is operable to redirect a fixed portion of the oil stream to each of the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020. In other examples, the valve 2016 is operable to direct a variable portion of the oil stream to the main fuel-oil heat exchanger 1006 and the first air-oil heat exchanger 2020, respectively, for example, using temperature sensor data as feedback to control the variable portion and / or based on one or more fuel characteristics, as described below. The valve 2016 may be adjustable between a discrete number of fixed positions or continuously. In some implementations, for example, when the temperature of the fuel entering the combustion chamber 16 is relatively low compared to the maximum operating temperature of the fuel (e.g., based on knowledge of the fuel type or thermal stability), no oil may be directed to the air-oil heat exchanger 2020 and all of the oil may be directed via the fuel-oil heat exchanger 1006.After passing through the heat exchangers 1006, 2020, 2030, the oil stream in the primary closed loop system 2000 is then merged again and conveyed to the power transmission 30 (and / or other engine components 33 such as the AGB) and then to the oil pan 2008. The return pump 2010 then pumps the oil from the oil pan 2008 into the oil tank 2002 for reuse.The oil stream within the secondary closed loop oil system 2000' is arranged to be brought into heat exchange relationship with the separate oil stream within the primary closed loop oil system 2000 by the oil-to-oil heat exchanger 2030. In the oil-to-oil heat exchanger 2030, the oil flow within the primary closed loop oil system 2000 does not mix with the oil flow within the secondary closed loop oil system 2000'. The oil-to-oil heat exchanger 2030 is configured such that heat transfer can take place between the two separate oil streams. In this way, heat from a hotter oil stream may be transferred into the cooler oil stream within the oil-to-oil heat exchanger 2030. No air-to-oil heat exchanger is shown in the depicted secondary closed oil circuit system 2000', however, in other examples, an air-to-oil heat exchanger may be provided - e.g., in series with the oil-to-oil heat exchanger 2030 or on a third parallel branch.In the implementation shown in FIG. 11, the secondary closed loop oil system 2000' is configured such that the oil return flow from the secondary feed pump 2004' is pumped through a valve 2016', which may be referred to as a secondary modulation valve. The valve 2016' is operable to redirect at least a portion of the oil flow between the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030, the oil-oil heat exchanger 2030 being disposed parallel to the IDG fuel-oil heat exchanger 1004. In other implementations, the secondary closed oil loop system 2000' may not have a branched configuration. For example, the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030 may be arranged in series such that in a particular cycle, at least a majority of the oil flowing through the one heat exchanger also flows through the other heat exchanger.In other implementations, an air-to-oil heat exchanger 2020 may be present in each closed loop system or only in the secondary closed loop system and / or no oil-to-oil heat exchanger may be present.In examples, any suitable portion of the oil stream may be diverted between the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. In examples, the valve 2016' is operable to redirect a fixed portion of the oil stream to each of the IDG fuel-oil heat exchanger 1004 and the oil-oil heat exchanger 2030. In examples, valve 2016' is operable to redirect a variable portion of the oil stream to fuel-oil heat exchanger 1004 and oil-oil heat exchanger 2030, respectively. The valve 2016' may be adjustable between a discrete number of fixed positions or continuously. After flowing through the heat exchangers 1004, 2030, the oil flow is then conveyed to the integrated drive generator 2006 and then to the secondary oil pan 2008'. The secondary return pump 2010' then pumps the oil from the secondary oil pan 2008' into the secondary oil tank 2002' for reuse.One or more temperature sensors 1009 may be provided, which are configured, for example, to detect the fuel temperature when approaching the fuel pump 1003 or the combustion chamber 16. The data from one or more sensors, optionally in combination with other data (e.g., fuel characteristics as described above), may be used by a controller 58 of the heat exchange system 3000 to control the flow of fuel and / or oil through and around the heat exchangers 1004, 1006, 2020, 2030 and / or to control the flow of air through the air-oil heat exchanger 2020. Controller 58 may be part of or provided by an EEC, or it may be a separate unit. The control can be automated, for example by an EEC.Figure 16 shows an oil circulation system comprising two circuits 2000, 2000', but without heat exchangers arranged on parallel branches of each oil system as shown in Figure 11. Instead, each oil circuit 2000, 2000' provides a series connected main oil flow path through all heat exchangers in that circuit. In order to adjust the oil flow through the heat exchangers, instead of branching paths to different heat exchangers, one or more bypass pipes 2005, 2005', 2005a are provided. It should be appreciated that in some implementations, a combination of parallel arrangements of heat exchangers and bypass tubes may be used and that FIGS. 11 (both circuits have parallel branches, no bypass tubes) and 16 (all heat exchangers parallel, multiple bypass tubes) may be considered to show two different ends of a design spectrum.In the example shown in FIG. 16, the fuel flow is shown with a thick black line to provide a context for the interaction between the fuel and oil systems. The primary oil circulation system 2000 illustrated in FIG. 16 provides a series-connected oil flow path from a tank 2002 via an oil pump 2004, further through the primary fuel-oil heat exchanger 1006, and then the air-oil heat exchanger 2020, before entering the transmission 30 (and optionally also into other components to be cooled and lubricated) and then being collected in an oil pan 2008 and then pumped back into the tank 2002 by the oil pump 2010. The two heat exchangers 1006, 2020 are thus present in a series circuit. The order of the two heat exchangers 1006, 2020 may be reversed such that the fuel-oil heat exchanger 1006 is after the air-oil heat exchanger 2020 in alternative implementations.The primary oil circulation system 2000 illustrated in FIG. 16 comprises two bypass pipes for oil 2005, 2005 a. The first bypass pipe 2005 is disposed such that a part of the oil can bypass the fuel-oil heat exchanger 1006, and is controlled by a first bypass valve 2007. The first bypass pipe 2005 guides the oil from upstream of the inlet to the primary heat exchanger 1006 and returns it to the main oil flow path before the air-oil heat exchanger 2020. The second bypass pipe 2005 ais disposed such that a part of the oil can bypass the air-oil heat exchanger 2020, and is controlled by a second bypass valve 2007 a. The second bypass pipe 2005 guides the oil from upstream of the inlet to the air-to-oil heat exchanger 2020 and returns it to the main oil flow path before the path reaches the transmission 30 (and optionally also other components to be cooled and lubricated). In implementations of the primary oil circulation system 2000 with only one bypass pipe for oil 2005 a, the selected position may be that of the second bypass pipe 2005 a, such that there is a bypass for the air-oil heat exchanger 2020 and not for the fuel-oil heat exchanger 1006. As a result, as much heat as possible can be reliably dissipated from the oil into the fuel, and by reducing the heat loss to the environment, the thermal efficiency of the engine can be kept high. In implementations with the air-oil heat exchanger 2020 prior to the fuel-oil heat exchanger 1006, the oil flow rate through the air-oil heat exchanger 2020 may be adjusted to provide an appropriate oil temperature for the fuel-oil heat exchanger 1006. The presence of a bypass pipe 2005 on the fuel-oil heat exchanger 1006 may facilitate a quick adjustment of the oil flow ratio when there is a risk of it falling below the desired value (for example, depending on the fuel properties determined, a lower limit for the ratio may be set significantly above zero).For some fuels (and also for some oils), depending on their properties, severe temperature limits may apply, so that a quick adjustment of the flow rates may be required to keep the temperatures within the desired limits. The presence of a bypass pipe 2005, 2005 aon both heat exchangers 1006, 2020 can prevent the oil from becoming too cold under certain circumstances, and thus avoid the risk of excessive oil clotting. It will be appreciated that this may pose a greater problem at take-off or ground idle on a cold day than at cruise.The secondary oil circulation system 2000' shown in FIG. 16 includes a single heat exchanger 1004, which is the secondary fuel-oil heat exchanger, and a single bypass pipe 2005' arranged so that a part of the oil can bypass this heat exchanger 1004. In other implementations, no bypass pipe may be provided on the secondary oil circulation system 2000', or there may be more than one heat exchanger (e.g., an air-oil and / or oil-oil heat exchanger in addition to the secondary fuel-oil heat exchanger 1004), and more than one bypass pipe may be provided, optionally one for each heat exchanger. A bypass valve 2007' is again provided to control the flow of oil through the bypass pipe 2005'.The inventors have recognized that the use of fuels different from conventional kerosine-based jet fuels, such as persistent flying fuels, may result in different fuel properties, and that decisions for the desired heat transfer - and accordingly for the determined liquid flow rates through one or more heat exchangers - may be adjusted in operation to take advantage of the different fuel properties. In particular, the use of one or more controllable valves to regulate oil and / or airflow and the implementation of careful control of these valves may enable a method that provides improved oil cooling (as fuel may draw more heat) and may also improve overall engine heat efficiency as less heat is lost to the environment while still ensuring safe operation. The controllable heat exchange system 3000 and in particular one or more controllable valves 2016, 2022, 2007, 2007', 2007a is key to the regulation of the liquid flow, and therefore to the heat transfer in such engines 10.Although cruise conditions generally account for a much greater proportion of engine operating time, the inventors have recognized that idle operation is also important. Since the fuel flow rate is much lower at idle than at cruise, even a relatively low heat load on the fuel can result in a high temperature rise. The use of non-conventional fuels may therefore have an even greater impact on optimal approaches to thermal management under idle conditions. The methods 100, 200 of FIGS. 12 and 13 deal with these two scenarios of aircraft operation. FIG. 12 illustrates a method 100 that implements these considerations in cruise conditions and FIG. 13 illustrates a method 200 that implements these considerations in idle. This method 200 can be carried out when the aircraft 1 is on the ground, i.e. in ground idle, e.g. while the aircraft is starting, in operation at standstill times during boarding and rolling (in the direction of a runway or a hangar or between other ground-bound locations) or in flight idle, e.g. at the beginning of the descent.Each method 100, 200 is configured for performance in a gas turbine engine 10 comprising an engine core 11 comprising a turbine 19, a compressor 14, a combustor 16 configured to combust a fuel, and a core shaft 26 connecting the turbine to the compressor; a fan 23 disposed upstream of the engine core; a gearbox 30 receiving input from the first core shaft and outputting drive for driving the fan to drive the fan at a lower speed than the first core shaft; an oil circulation system 2000 configured to supply the gearbox with oil; and a heat exchange system 3000. The heat exchange system 3000 includes an air-oil heat exchanger 2020 through which the oil in the oil cycle system flows; a fuel-oil heat exchanger 1006 through which the oil in the oil cycle system and the fuel flow so as to transfer heat between the oil and the fuel; and at least one valve 2016, 2007, 2007 a(which may be referred to as an oil valve) configured so that a part of the oil guided via at least one of the heat exchangers 1006, 2020 may be varied.The at least one oil valve 2016, 2007, 2007 ais adjustable / controllable and optionally controllable by a control unit 58, which can form part of the EEC of the aircraft / can be provided by it. A control device 58 can therefore be provided for implementing this control. The oil valve 2016, 2007, 2007 acan be adjustable between a discrete number of fixed positions or can be continuously adjustable.The two heat exchangers 1006, 2020 may be arranged in the oil circulation system 2000 in a parallel arrangement or in a serial arrangement. Moreover, in some implementations, more than two heat exchangers may be provided - for example, when the oil circuit system 2000 includes multiple fuel-oil heat exchangers 1006, multiple air-oil heat exchangers 2020, and / or one or more additional heat exchangers, such as one or more oil-oil heat exchangers for heat exchange between separate oil circuit systems 2000, 2000'.In parallel arrangements, the oil circuit system 2000 may branch such that a portion of the oil may flow along each branch, and the air-to-oil heat exchangers 2020 and fuel-to-oil heat exchangers 1006 may be arranged in a parallel configuration on the various branches of the oil circuit system, as shown in FIGS. 9 and 11. In such implementations, the at least one valve configured to allow for the change in the proportion of oil routed via at least one of the heat exchangers 1006, 2020 may be or may include a modulation valve 2016 configured to allow for the change in the proportion of oil routed via each branch.In series arrangements, as shown in FIG. 16, the oil circulation system 2000 includes one or more bypass pipes 2005, 2005a, each configured such that a portion of the oil bypasses one or more heat exchangers. In such implementations, the at least one valve configured to allow for the change in the proportion of oil routed via at least one of the heat exchangers 1006, 2020 may be or include one or more bypass valves 2007, 2007a. It is understood that bypass pipes 2005 and bypass valves 2007 can also be provided in parallel arrangements-in such implementations, the at least one valve, which is configured such that the proportion of the oil conducted via at least one of the heat exchangers 1006, 2020 can be varied, can be or comprise a modulation valve 2016 and a bypass valve 2007, 2007a. The method 100, 200 can therefore comprise controlling 102, 202 a plurality of valves 2016, 2007, 2007 a-for example a bypass valve 2007 aand a modulation valve 2016 or two bypass valves 2007, 2007 aor two bypass valves 2007, 2007 aand a modulation valve 2016. Further, the heat exchange system 3000 may comprise a plurality of separate oil circuits 2000, 2000', and the method 100, 200 may further comprise controlling oil valves for additional heat exchangers 1004, 2030 of the second oil circuit 2000' or, in the case of an oil-oil heat exchanger 2030, both circuits.In implementations in which a bypass pipe 2005 ais provided via the air-oil heat exchanger 2020, the controlled bypass valve may be the valve 2007 afor the bypass pipe 2005 avia the air-oil heat exchanger 2020. In some of these implementations, this bypass pipe 2007 acan be the only oil bypass pipe in the primary oil circulation system 2000-a bypass pipe may not be provided for the fuel-oil heat exchanger 1006.In alternative implementations, such as that shown in FIG. 16, the heat exchange system 3000 includes at least two bypass pipes for oil and optionally three or more, each bypass pipe 2005, 2005', 2005a being arranged such that oil can bypass one of the heat exchangers 1006, 1004, 2020. The method 100, 200 may include modulating the amount of oil directed across each bypass pipe 2005, 2005', 2005a. In implementations with multiple bypass pipes within the same closed oil circuit system (e.g., with two bypass pipes in the primary oil circuit system 2000 as shown in FIG. 16 ), the same bypass valve 2007-which may be a three-way valve-may be used to control the flow through both bypass pipes 2005, 2005 a, or a different bypass valve 2007, 2007 amay be provided for each as shown in FIG. 16.In some implementations, the heat exchange system 3000 further includes a refrigeration cycle device 1007 configured to provide thermal buoyancy by transferring additional heat from the oil to the fuel in addition to the heat transferred from the fuel-oil heat exchanger 1006. The oil flow through the refrigeration cycle device 1007 may also be adjusted as part of the method 100, 200. To transfer heat from the oil to the fuel, and thus take advantage of the increased heat capacity of many recent flying fuels, a positive thermal gradient is usually required between oil and fuel (i.e., the oil is hotter than the fuel). Due to the increased thermal stability of the fuel, newer fuels may potentially have suitable operating temperatures that are higher than the temperature that the oil reaches upon exiting the transmission 30 or other components. Therefore, thermal lift may be desirable to further increase fuel temperature. Thermal buoyancy is the ability to transfer heat from a colder to a warmer liquid (generally via a refrigeration circuit).In some implementations, heat exchange system 3000 further includes branching fuel return paths 6020 and at least one valve that controls a split of the fuel flow as described above. The valve may be controlled based on feedback from one or more temperature sensors and / or based on one or more fuel characteristics. The method 100, 200 may include adjusting fuel flow along each branch based on the oil flow ratio and / or temperature of the fuel as it exits the fuel-oil heat exchanger 1006. Engine 10 may therefore be capable of conducting heat (in the form of heated fuel) into fuel system 1500 at various locations (e.g., upstream or downstream of fuel pump 1003 or another component). It should be appreciated that as the fuel temperature increases, the heat capacity / heat resistance of the fuel system components may become a limiting factor and that returning hot fuel to a main fuel flow path downstream of a pump 1003 rather than upstream of the pump 1003 may facilitate exploiting the increased thermal stability / higher usable operating temperature of new fuels.With respect to the method 100, which is to be performed in particular during cruising, the method 100 comprises controlling 102 the at least one valve 2016, 2007, 2007 asuch that under cruising conditions an oil flow ratio of: is in the range of from 0 to 0.59 and optionally from 0 to 0.50, from 0 to 0.40, from 0 to 0.30, from 0 to 0.20, from 0 to 0.10, or from 0 to 0.05 and optionally from 0 to 0.01. Optionally, the oil flow ratio may be in the range of 0.05 to 0.55. The oil valve 2016, 2007, 2007 acan be controlled such that the oil flow ratio is equal to zero-for example, by preventing any oil flow into the air-oil heat exchanger 2020 in some implementations.It will be appreciated that even for a particular engine 10 operating on a particular fuel, a range of values for that ratio is common in cruise operation due to different conditions, e.g., fuel temperature, ambient temperature, thrust demand, etc. For example, an upper limit of the range may apply on a cold low altitude day (ISA-30 conditions) (cruise: 35,000 ft) with low heat buildup of the oil system, and a lower limit may apply on a hot high altitude day (ISA+40 conditions) (cruise: 39,000 ft) with high heat buildup of the oil system.The method 100 may further include receiving 104 data to enable calculation or derivation of the oil flow ratio, e.g., pump speed data, fuel flow rate data, and / or oil flow rate data. Such data may be received 104 and used 106 by a controller 58 to adjust the controller 102 of the one or more oil valves in cruise operation to maintain the oil flow ratio at a desired level or within desired limits. This checking and adjustment / correction 106 may be performed at regular intervals or in response to predetermined stimuli (e.g., a change in the temperature or flow rate of the fuel or oil, or a change in engine operation, e.g., thrust demand or flight altitude). These steps 104, 106 may alternatively be considered part of controlling 102 the one or more oil valves 2007, 2007 a, 2016. The method 100 may also be configured to use other information, e.g., temperature data (of the oil, fuel, and / or ambient temperature of an environment surrounding the aircraft 1), flow rate data (of the oil and / or fuel), and / or one or more fuel characteristics, in determining 106 which control actions to take.The step of controlling 102 the at least one valve 2016, 2007, 2007 ato adjust the oil flow ratio comprises reducing the amount of oil conducted via the at least one air-oil heat exchanger 2020 if the oil flow ratio is too high in some implementations; optionally fully closing the valve / closing a passage of a three-way valve in some implementations such that the oil flow rate into the air-oil heat exchanger 2020 falls to zero. In some implementations, a minimum but non-zero oil flow rate through the air-to-oil heat exchanger 2020 may be maintained throughout operation to prevent clotting of the oil in the air-to-oil heat exchanger 2020.In some implementations, one or more temperature sensors 1009 may be provided, and data regarding fuel temperature, optionally upon entry into the combustion chamber 16, may be used for fine tuning control decisions. The method 100 may include controlling 102 the one or more oil valves 2007, 2007a, 2016 under cruise conditions such that the oil flow ratio is in the range of 0 to 0.35 and optionally 0 to 0.25 if the fuel temperature at entry into the combustion chamber 16 is at least 140° C.; and / or such that the oil flow ratio is in the range of 0 to 0.20 and optionally 0 to 0.15 if the fuel temperature at entry into the combustion chamber 16 is at least 160° C.; and / or such that the oil flow ratio is in the range of 0 to 0.1 and optionally 0 to 0.075 if the fuel temperature at entry into the combustion chamber 16 is at least 180° C.In some implementations, one or more fuel characteristics may be considered when determining how to control 102 the one or more oil valves 2007, 2007a, 2016 (the fuel characteristics may be determined using one of the approaches described above). For example, the method 100 may include controlling 102 the at least one oil valve 2007, 2007a, 2016 under cruise conditions such that the oil flow ratio is in the range from 0 to 0.20 and optionally from 0 to 0.15 if the fuel consists of at least 70% of sustainable flight fuel; and / or such that the oil flow ratio is in the range from 0 to 0.1 if the fuel consists of at least 80% of sustainable flight fuel.With respect to the method 200, which is to be performed in particular at idle, the method 200 comprises controlling 202 the at least one oil valve 2016, 2007, 2007 asuch that under idle conditions an oil flow ratio of: is in the range of from 0.62 to 5.29, and optionally from 0.62 to 5.00, from 0.62 to 4.50, from 0.62 to 4.00, from 0.62 to 3.50, from 0.62 to 3.50, or from 0.62 to 2.50. Optionally, the oil flow ratio may be in the range of 0.67 to 4.67. The method 200 may include controlling 202 the at least one oil valve such that, under idle conditions, the oil flow ratio is above 1.0 and optionally above 1.5 or 2.0. A controller 58 - optionally either a stand-alone unit or a portion of an EEC - may be provided to implement this controller 202.With respect to method 100 performed during cruise operation, method 200 executing at idle may further include receiving 204 data to enable calculation or derivation of the oil flow ratio, e.g., pump speed data, fuel flow rate data, and / or oil flow rate data. Such data may be received 204 and used 206 by a controller 58 to adjust the controller 202 of the one or more oil valves 2007, 2007a, 2016 at idle so that the oil flow ratio is maintained at a desired level or within desired limits. This checking and adjustment / correction 206 may be performed at regular intervals or in response to predetermined stimuli (e.g., a change in the temperature or flow rate of the fuel or oil, or a change in engine operation, e.g., the start of coasting). These steps 204, 206 can alternatively be considered as part of the control 202 of the at least one oil valve 2007, 2007 a, 2016. The method 200 may also be configured to use other information, e.g., temperature data (of the oil, fuel, and / or ambient temperature of an environment surrounding the aircraft 1), flow rate data (of the oil and / or fuel), and / or one or more fuel characteristics, in determining 206 which control actions to take.Similarly, the step of controlling 202 the at least one valve 2016, 2007, 2007 ato adjust the oil flow ratio may include reducing the amount of oil directed across the at least one air-to-oil heat exchanger 2020 when the oil flow ratio is too high; optionally, fully closing the valve / closing a passage of a three-way valve in some implementations such that the oil flow rate into the air-to-oil heat exchanger 2020 decreases to zero. In some implementations, during idle operation, a minimum but non-zero oil flow rate through the air-to-oil heat exchanger 2020 may be maintained to prevent oil from coagulating in the air-to-oil heat exchanger 2020. However, it should be noted that ambient temperatures in ground idle are generally higher than in cruise altitude, so that this may not be required or may be required only to a lesser extent in ground idle. On cold days or on airports or runways in cold climate regions, it may nevertheless be desirable to keep the oil flowing in order to reduce the risk of clotting. The outdoor / ambient temperature may therefore be used as an input to control the oil flow rate and one or more temperature sensors may be provided accordingly.In some implementations, one or more temperature sensors 1009 may be provided, and data regarding fuel temperature, optionally upon entry into the combustion chamber 16, may be used for fine tuning control decisions. The method 200 may include controlling 202 the one or more oil valves 2007, 2007a, 2016 under idle conditions such that the oil flow ratio is in the range of from 0.62 to 4.0 or from 0.62 to 3.0 if the fuel temperature at entry into the combustion chamber 16 is at least 160° C.; and / or such that the oil flow ratio is in the range of from 0.62 to 3.0 or from 0.62 to 2.0 if the fuel temperature at entry into the combustion chamber 16 is at least 180° C.In some implementations, one or more fuel characteristics may be considered when determining how to control 202 the one or more oil valves 2007, 2007a, 2016 (the fuel characteristics may be determined using one of the approaches described above). For example, the method 200 may include controlling 202 the at least one oil valve 2007, 2007a, 2016 under idling conditions such that the oil flow ratio is in the range from 0.62 to 3.67 and optionally from 0.62 to 3.67 if the fuel consists of at least 70% sustainable flight fuel; and / or such that the oil flow ratio is in the range from 0.62 to 2.67 or from 0.62 to 1.67 if the fuel consists of at least 80% sustainable flight fuel.While the methods 100, 200 described with respect to FIGS. 12 and 13 optionally use knowledge of one or more fuel characteristics to adjust the specific controller 102, 202, other methods, such as the 300 described with respect to FIG. 14, focus more on determining and using fuel characteristics to improve performance of the engine 10 while exploiting the different characteristics of different flying fuels.The method 300 shown in FIG. 14 is performed in a gas turbine engine 10 substantially as described for the preceding methods 100, 200, however, the one or more controllable oil valves 2007, 2007 a, 2016 are replaced by or provided in addition to a controllable air valve 2022 configured to control a flow rate of air through the air-oil heat exchanger 2020.Method 300 includes determining 302 at least one fuel property of the fuel to be burned by combustion chamber 16. The at least one fuel property may be or include one of the examples mentioned above - for example aromatic content (e.g. xylene, toluene, benzene and / or phenol content), paraffin content (e.g. isoparaffins, n-paraffins, cycloparaffins), concentration of hetero-atom species (e.g. sulfur-based compounds / sulfur content and / or % nitrogen-based compounds such as anilines or indoles), % SAF and hydrogen content (generally indicated as H / C molar ratio). The determination 302 may be made by one or more of the approaches described above, e.g., by chemical and / or physical detection and determination, by analyzing engine performance using the fuel, and / or by receiving fuel data (e.g., electronically or by manual data input), optionally using a fuel composition determination module 57.The method 300 then includes controlling 304 the air valve 2022 based on the at least one fuel property to adjust the flow rate of air through the air-oil heat exchanger 2020. The air valve 2022 may be adjustable between a discrete number of fixed positions or may be continuously adjustable in various implementations. The controller 304 may therefore include a discrete or continuous setting. To implement this controller 302, a controller 58 may be provided, which may be a dedicated controller for the valve 2022 and / or for the heat exchange system 3000, generally or part of a more general EEC. The method 300 may also be configured to use other information in determining which control actions to take, e.g., temperature data (of the oil, fuel, and / or ambient temperature of an environment surrounding the aircraft 1) and / or flow rate data (of the air, oil, and / or fuel). The controller 58 or other processing module may make this determination of the appropriate control action(s).As indicated by the dashed line in FIG. 14, this method 300 may optionally be repeated. In some implementations, for example, in implementations with only one fuel on board the aircraft 1, the method 300 may be performed only once during a flight cycle, e.g., when refueling or when reaching a flight altitude. In other implementations, such as implementations with multiple fuel sources, the fuel supplied to the combustion chamber 16 may vary over a flight. The determination 302 may therefore be performed more than once - e.g.: (i) once for each fuel source 50, 53 at fueling / commencement of a flight in implementations with multiple fuel sources, only one or the other of which is used at any particular time (and knowing from which fuel source fuel is drawn may cause selection of the appropriate stored fuel property); or (ii) multiple times during a flight, e.g., in response to a change in fuel source(s) from which fuel is drawn (note that the fuel provided to the combustion chamber 16 may be a mixture of fuels from different sources in some implementations), or at regular intervals.The air valve 2022 may therefore be configured to remain in a fixed position during operation of the engine 10, the fixed position being determined at start-up of the engine 10 based on the at least one determined fuel characteristic (option (i) above). Alternatively, the air valve 2022 may be configured to adjust its position during operation, thereby changing the flow of air to the air-to-oil heat exchanger 2020 during operation of the engine 10 during a single flight (option (ii) above). The air valve 2022 may therefore be actively controlled to vary the air flow rate through the air-to-oil heat exchanger 2020, particularly in implementations where the aircraft 1 carries multiple different fuels in different tanks, and may change which fuel (or fuel mixture) is used during flight. The active control of the air valve 2022 may be automated and performed by the controller 58.It should be understood that a fixed position of the air valve 2022 does not necessarily mean a fixed air flow rate, as the air speed may vary relative to the aircraft 1. During operation on the ground (e.g. at idle), deviations can be attributed exclusively to the wind speed. During flight, aircraft speed may also affect the air flow rate into the air-to-oil heat exchanger 2020. In some implementations, a controller 58 may take into account wind speed and / or relative motion between the aircraft 1 and ambient air when determining an appropriate air valve position. Therefore, the position of the air valve may be changed to maintain the air flow rate more constant in some scenarios.The method 300 can be executed at any point in time during the operation of the aircraft 1, for example in ground idling, in flight idling and / or in travel operation.The air valve 2022 may be controlled 304 such that a relatively large reduction in airflow occurs as compared to a fully open valve 2022, provided that the determining step 302 indicates that the fuel is capable of absorbing the additional heat that is then no longer lost to the environment. For example, subject to the suitability of the at least one determined fuel property, method 300 may include controlling 304 air valve 2022 such that the air flow rate into air-oil heat exchanger 2020 is reduced to less than 60%, 50%, or 40% of the flow rate that would prevail when the engine 10 is operating under idle conditions when the valve is fully open. Even greater reductions are possible during cruise operation - for example, the air flow rate into the air-to-oil heat exchanger 2020 can be reduced to less than 20% of the flow rate that would prevail when the valve is fully open when the engine 10 is running under cruise conditions, provided that the fuel is suitable for receiving the additional heat, and during cruise conditions, the air flow rate can even be reduced to zero (by closing the valve 2022). From the other perspective, if the engine 10 is then operated with a fuel that is less able to absorb the additional heat, correspondingly large changes can be made in the re-opening of the valve 2022. It will be appreciated that the airflow rates in ground idle are generally much lower than in cruise or flight idle and that the same percentage change in airflow may therefore correspond to a much greater difference in airflow while the aircraft 1 is in air than when on the ground. Data on the current operating state of the aircraft (e.g. flight altitude and / or speed of the aircraft) and optionally on the weather (e.g. wind speed) can therefore likewise be taken into account in the valve control.In an example of a transmission engine 10 in which the air-oil heat exchanger 2020 is located upstream of the fuel-oil heat exchanger 1006 and flow modulation is performed only on the air side of the air-oil heat exchanger 2020 (i.e., no oil flow modulation), the following percentage air flow modulations may be common, where "% of the air flow modulation" is the percentage of the total air flow that would flow through the heat exchanger 2020 if the valve 2022 were fully opened (or indeed no valve would be present), simply based on the pressure ratio at the heat exchanger inlet 2020a and outlet 2020b:• During cruise operation, the percentage of airflow modulation for fuel with a temperature limit of 120° C. may be 20% or 17%. This value can then be linearly decreased to 0% (i.e., no air flow / valve closed for at least a portion of the travel duration) with increasing temperature limit for fuel having a temperature limit of 170° C. For fuel with a temperature limit of 260° C. or more, the percentage of airflow modulation during the entire cruise mode may be 0%, and the valve 2022 may be fully closed during the entire cruise mode.• At idle, and particularly at ground idle, the percentage of airflow modulation for fuel with a temperature limit of 120° C. may be 58%. This value may then be linearly decreased to 38% as the temperature limit increases as the fuel temperature limit reaches 250° C., and further linearly decreased to 28% as the temperature limit increases as the fuel temperature limit reaches 315 ° C.The percentages may depend on the fuel temperature limit. The fuel temperature limit is the highest fuel temperature that is considered safe for the operation of the aircraft for this fuel and engine 10, and may depend on fuel properties such as thermal stability and also on the thermal compatibility of the engine components.In some implementations, the at least one fuel characteristic may be or include thermal stability, and in cruise operation, the air valve 2022 may be adapted to reduce the air flow to less than 15% of the flow rate that would be present at fully open valve 2022, provided that the fuel is stable in operation at temperatures above 160° C. and / or to less than 5% of the flow rate that would be present at fully open valve 2022, provided that the fuel is stable in operation at temperatures above 180° C. In some implementations where the valve 2022 is continuously variable, the air flow rate may be continuously changed in proportion to thermal stability (optionally, linearly with thermal stability).In some implementations, the at least one fuel property may be or include the aromatic hydrocarbon content of the fuel, and the air valve 2022 may be adapted to reduce the airflow during cruise operation to less than 5% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a molar proportion of aromatic hydrocarbons of less than 12%, and optionally less than 10% or less than 5%.In some implementations, the at least one fuel characteristic may be or include the percentage of Persistent Flying Fuel, SAF, in the fuel, and the air valve 2022 may be adapted to reduce airflow during cruise operation to less than 5% of the flow rate that would prevail at the valve fully open provided that the fuel has a SAF content of over 60% and / or to less than 2% of the flow rate that would prevail at the valve fully open provided that the fuel has a SAF content of over 80%.Auto-oxidation of conventional flying fuels from fossil fuels-which may lead to fouling or coking of combustion chamber nozzles and other fuel passages-generally begins to increase exponentially when the fuel temperature is anywhere in the range between 100°C and 150°C (depending on fuel specifics). The inventors have recognized that SAF-based fuels could provide the ability to achieve fuel temperatures above 150° C., and optionally 200° C., or even 250° C. or more, without significant auto-oxidation occurring. The maximum operating temperature of the fuel depends on the fuel composition; therefore, it is important to determine 302 at least one fuel property. Fuel mixtures of SAF and conventional fossil fuels in turn have different thermal stability and thus different temperature limits, depending on the particular properties of the mixture. Testing SAF blends with the conventional aviation fuel Jet-A has determined that increasing the SAF percentage in the blend with Jet-A increases thermal stability, but in a non-linear manner. For this reason, preset discrete steps may be preferable to continuously change the air flow rate in proportion to the SAF content.In some implementations, the at least one fuel characteristic may be or include the heating value of the fuel, and the air valve 2022 may be adapted to reduce the airflow during cruise operation to less than 4% of the flow rate that would prevail when the valve is fully open, provided that the fuel has a heating value of at least 43.5 MJ / kg, optionally subject to the suitability of another fuel characteristic (e.g., thermal stability).In some implementations, multiple fuel characteristics may be checked together using, for example, a lower threshold for one fuel characteristic when another fuel characteristic is within a certain range or above / below a certain threshold.More air may be usually required and the valve 2022 may be controlled 304 to be opened further as the thermal stability of the fuel decreases, the sulfur concentration of the fuel increases (an example of the concentration of heteroatomic species generally associated with the thermal stability because an increased concentration generally decreases the thermal stability), or when the heating value of the fuel exceeds a threshold (for higher heating value fuels, the flow rate is generally reduced to achieve the same level of thrust without waste of fuel - the fuel in the fuel-oil heat exchanger 1006 may therefore experience an increased temperature increase due to its reduced flow rate. However, newer fuels (e.g., SAF) with higher thermal stability also have higher heating values in many cases. The increased temperature rise of the fuel due to the lower flow rate may therefore be acceptable in some scenarios; this demonstrates how useful to take multiple fuel characteristics into account in common rather than only a single fuel characteristic alone).The inventors have recognized that it may adversely affect engine performance and may even be dangerous when improper fuels reach these higher temperatures. Excessive thermal decomposition / deposition of a fuel may cause safety problems by blocking passageways and causing component malfunctions (e.g., jamming valves and clogged nozzles, ports, valves, etc.), which may result in loss of thrust control. Therefore, the method 400 illustrated in FIG. 15 is provided as a safety provision.The method 400 shown in FIG. 15 may be performed in a gas turbine engine 10 substantially as described for the foregoing methods 100, 200 of oil flow ratio control, but the one or more controllable oil valve(s) 2007, 2007a, 2016 configured such that a portion of the oil directed across at least one of the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006 may be varied is replaced with at least one controllable oil or air valve 2016, 2007, 2007a, 2022 configured such that at least one of oil flow rate and air flow rate may be varied by at least one of the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006. Therefore, air flow control, oil flow control, or both may be provided. In implementations where a controllable air valve 2022 is provided (optionally in addition to a controllable oil valve 2016, 2007, 2007 a), the same engine 10 as used for the method 300 for air flow control may be used. Engine 10 for use with this method 400 also requires a temperature sensor 1009 configured to provide an indication of fuel temperature - optionally upon entry into combustion chamber 16. This temperature sensor 1009 is generally located at or near an inlet to the combustor 16 and, more specifically, near a nozzle inlet of the combustor 16 to directly sense the fuel temperature as it enters the combustor, although it will be appreciated that variations in fuel temperature between the outlet 1006b of the fuel-oil heat exchanger 1006 and the combustor 16 are likely small and, unless deemed insignificant, are calculable from knowledge of the engine 10 when the fuel temperature is to be measured elsewhere than the sensor 1009. In some implementations where the fuel temperature is used upon entering the combustor 16, the sensor 1009 may be located elsewhere and the fuel temperature upon entering the combustor may be calculated based on the temperature output and knowledge about the engine 10.The method 400 includes determining 402 whether the fuel temperature has increased above a set threshold during cruise conditions based on an output of the temperature sensor 1009. For example, a direct comparison may be made between the data received from temperature sensor 1009 and one or more temperature thresholds stored in memory. This determination 402 may therefore be performed automatically, optionally by a controller 58 (which may be or include a dedicated processing module or may be provided by a more general EEC).The method 400 further comprises, responsive to determining that the fuel temperature has increased above the set threshold during cruise conditions, controlling 404 the at least one valve 2016, 2007, 2007a, 2022 to change the at least one flow rate (air or oil flow rate) through the at least one heat exchanger 1006, 2020 as needed to decrease the fuel temperature. For example, an oil valve 2016, 2007 may be adjusted such that less oil is conducted through the heating oil heat exchanger 1006. For example, the oil may be diverted from a first branch of an oil circulation system 2000 that includes the fuel-oil heat exchanger 1006 and directed via a parallel branch that instead includes an air-oil heat exchanger 2020 in parallel configurations (as shown in FIGS. 9 and 11 ) using a modulation valve 2016, or may simply be directed via a bypass pipe 2005 to bypass the fuel-oil heat exchanger 1006 and combine with the remaining oil that has flowed through the fuel-oil heat exchanger 1006 at or downstream of the oil exit from that heat exchanger 1006 using a bypass valve 2007 as shown in FIG. 16. In a parallel configuration as shown in FIG. 11, reducing the oil flow through the fuel-oil heat exchanger 1006 by controlling the modulation valve 2016 may automatically increase the oil flow through the air-oil heat exchanger 2020 (unless a bypass pipe is present around the air-oil heat exchanger 2020 and is used for balancing). Bypass valves 2007 and modulation valves 2016 may be used together in some implementations.As indicated by the dashed line in FIG. 15, this method 400 may be repeated. For safety reasons, frequent checks may be desirable, optionally on a regular schedule and / or when triggered by a change in engine operation (e.g., a change in flight altitude or thrust demand). The oil valve 2016, 2007, 2007 aor the air valve 2022 can therefore be actively controlled 404 in order to vary the oil or air flow rate through the respective heat exchanger 1006, 2020. The active control of the valve 2016, 2007, 2007 a, 2022 can be automated and implemented by the control unit 58. For example, the active control of the modulation valve 2016 may be automated and implemented by a controller 58 of the heat exchange system 3000, which may be a dedicated controller or a portion of a more general EEC. In various implementations, the control of the modulation valve 2016 may be a closed or open loop depending on the availability of inline measurements-in particular, a closed loop approach may be preferable when feedback data (e.g., from oil flow rate sensors) is available.In implementations with both an air valve 2022 and an oil valve 2016, 2007 a, the air flow through the air-to-oil heat exchanger 2020 can be increased as more oil is directed over the air-to-oil heat exchanger 2020; the increase can be made linearly with the increasing oil flow rate. Expressed in percent of the airflow, the increase may be large--for example, from a fully closed to a fully open air valve 2022, or from a first position with 10% of the maximum air flow rate high to 90% or 100% of the maximum air flow rate if the fuel temperature is deemed too high. The method 400 may include, responsive to determining 402 that the fuel temperature has increased above a set threshold during cruise conditions, controlling 404 aof the air valve 2022 to pass more air through the air-to-oil heat exchanger 2020, and controlling 404 bof the oil valve 2016, 2007 to pass less oil through the fuel-to-oil heat exchanger 1006. The control / adjustment 404 aof the air valve 2022 may be performed simultaneously with the control / adjustment 404 bof the oil valve 2016, 2007.In some implementations, the flow of air through the air-to-oil heat exchanger 2020 may not be actively controlled. In such implementations, the air flow is likely to naturally change under different operating conditions based on the pressure ratio between the heat exchanger air inlet 2020 aand the heat exchanger exhaust / air outlet 2020 b. In this configuration, even if the oil-to-oil heat exchanger 2020 on the oil side is completely bypassed and if the oil flow rate through the heat exchanger 2020 is zero, air continues to flow through the air passages of the air-to-oil heat exchanger 2020. In alternative implementations, active control of the air flow through the air-oil heat exchanger 2020 is provided, for example, as described for the method 300 shown in FIG. 14. For example, the air-to-oil heat exchanger 2020 and the fuel-to-oil heat exchanger 1006 may be connected in series, with no bypass on the oil side of the air-to-oil heat exchanger 2020, but with an air valve 2022 on the air side that controls the flow of air (in other implementations, both an oil bypass valve 2007' and an air flow valve 2022 may be provided). In such an in-line arrangement, the air-to-oil heat exchanger 2020 may be upstream or downstream of the fuel-to-oil heat exchanger 1006 with respect to the oil stream. In implementations with such an in-line arrangement, the location of the air-oil heat exchanger 2020 upstream of the fuel-oil heat exchanger 1006 with respect to the oil stream may help avoid overheating of the fuel - the oil may be cooled by increasing the air flow (and optionally also by recirculating oil through the air-oil heat exchanger 2020) any amount before reaching the fuel-oil heat exchanger 1006.In some implementations, an oil bypass pipe 2005 aand a corresponding control valve 2007 amay be implemented only for the air-oil heat exchanger 2020. Optionally, in the described method 400, only the oil flow rate through the air-to-oil heat exchanger 2020 may be actively controlled.In various implementations, the following applies:• A bypass pipe 2005 ais provided only on the oil side of the air-oil heat exchanger 2020 (no oil bypass of the fuel-oil heat exchanger and no active air flow control);• A bypass pipe 2005a, 2005 (no active air flow control) may be provided for both the air-oil heat exchanger 2020 and the fuel-oil heat exchanger 1006; or• The air flow rate through the air-to-oil heat exchanger 2020 may be controllable and there may be no oil bypass pipes present.Generally, control over at least one flow through the air-oil heat exchanger 2020 may be considered more important than control of flow through the fuel-oil heat exchanger 1006. However, in some implementations, an oil bypass pipe 2005 and / or a fuel bypass pipe 1005 (and corresponding control valve) may be provided to the fuel-oil heat exchanger 1006, optionally in addition to one or more further controllable valves.A combination of air flow control and oil flow control may be implemented in both parallel and serial arrangements. It should be appreciated, however, that while all of the various control options may be used together, the increased complexity and number of components, and thus the increased weight of the overall heat exchange system 3000, may not be desirable. Thus, in many implementations, selection of a subset of the control options may be preferable. For example, either air flow control or oil flow control may be used to adjust heat transfer in the air-to-oil heat exchanger 2020, such that control of both may be deemed superfluous for a particular heat exchanger 2020. Similarly, in an implementation where the oil flows through the air-oil heat exchanger 2020 prior to the fuel-oil heat exchanger 1006, the oil temperature adjustment in the air-oil heat exchanger 2020 (by controlling the air and / or oil flow in this heat exchanger) may be used as an alternative to the oil flow adjustment through the fuel-oil heat exchanger 1006 to change the amount of heat transferred to the fuel in the fuel-oil heat exchanger 1006.Additionally or alternatively, in implementations where the heat exchange system 3000 includes a refrigeration cycle device 1007 configured to transfer additional heat from the oil to the fuel beyond the heat transferred from the fuel-oil heat exchanger 1006, the method 400 may further include controlling the refrigeration cycle device 1007 to reduce the amount of the additional heat transferred to the fuel in response to determining 402 that the fuel temperature has increased above the set threshold during cruise conditions. This control may be performed by reducing the flow of oil through the refrigeration cycle device 1007, or by reducing the power to the refrigeration cycle device 1007 (e.g., by reducing the flow rate of the refrigerant liquid), or by deactivating the refrigeration cycle device 1007. The set threshold for deactivation of the refrigeration cycle device 1007 may be lower than the set threshold for changing a flow rate of oil and / or air through a heat exchanger 1006, 2020.Additionally or alternatively, in implementations where the heat exchange system 3000 includes a secondary oil circulation system 2000' that also includes a fuel-oil heat exchanger 1004, the method may further include reducing the oil flow through the secondary fuel-oil heat exchanger 1004 when the fuel temperature has increased above a predetermined threshold (which may be different than the predetermined threshold at which the oil flow is reduced by the primary fuel-oil heat exchanger 1006).The set threshold for the control step 404 may be in the range of 140° C. to 300° C., and optionally 200° C. to 300° C., and further optionally 250° C. to 300° C. The predetermined threshold may be, for example, 140° C., 150° C., 160° C., 170° C., 180° C., 190° C., 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C., 270° C., 280° C., 290° C., or 300° C.The method 400 of some implementations further comprises determining 401 the set threshold value to which the output of the temperature sensor is compared. This determination 401 can be carried out on the wing and optionally also in flight. The determination 401 may be or include calculating a threshold or identifying an appropriate threshold from a pre-stored set of thresholds stored in memory.The inventors have recognized that knowledge of one or more fuel properties may be used in the determination 401 to ensure safety while exploiting the properties that vary between flying fuels. The determination 401 may therefore be based on at least one fuel property of the fuel. The at least one fuel property may be or include one of the examples discussed above, such as the thermal stability of the fuel, the nitrogen content of the fuel, the sulfur content of the fuel, and / or the sustainable flying fuel (SAF) (% SAF) content of the fuel. The at least one fuel property may be determined by one or more of the approaches discussed above.The step of determining 401 the established threshold value may comprise increasing the established threshold value, optionally linearly, with increasing thermal stability of the fuel. The set threshold for controlling the thermal modulation may therefore be increased linearly with the thermal stability of the fuel. Thermal stability is the temperature at which the fuel begins to decompose and form deposits which can lead to blockages and malfunctions of components. It will be appreciated that this is generally not an immediate change in reaching a particular temperature; aviation fuels include various components that can decompose at different temperatures and residence time at high temperatures is also a factor for fuel degradation. The threshold may be set based on a comparison of the maximum allowable deposition rate by fuel decomposition with the thermal stability of the fuel used. A fuel may be deemed stable at a particular temperature if its decomposition rate at that temperature is below a particular threshold. Method 400 may further include using a clock or timer or fuel flow rate sensor - a higher fuel temperature may be acceptable at a higher pump rate, for example, because the faster flowing fuel spends less time at that temperature before being burned.Alternatively or additionally, the step of determining 401 the fixed threshold may comprise increasing the fixed threshold as the SAF content of the fuel increases (optionally linearly) for fuels having a SAF content of over 70%. As discussed above, it has been found that increasing % SAF when mixed with a fossil based aviation fuel increases thermal stability, but in a non-linear manner. Thus, pre-set discrete thresholds based on the % SAF may be preferable to a continuous change of the set threshold proportional to the SAF content, or a more complex (non-linear) but continuous relationship may be established.Alternatively or additionally, the step of determining 401 the fixed threshold value may comprise decreasing the fixed threshold value as the content of the heteroatomic species of the fuel increases (optionally linearly). For example, it has been found that the thermal stability of Jet-A decreases with increasing nitrogen content (the nitrogen content is a measure of the amount of nitrogenous species present). It is known that the interaction between sulfur-containing and nitrogen-containing species in the fuel can make an important contribution to the decomposition rate of the fuel. Thus, consideration of the fuel composition with respect to multiple heteroatomic species may be implemented to account for these interactions.In some implementations, particularly implementations where a direct measure of the thermal stability of the fuel is not available, multiple fuel properties may be determined and used in valve control decisions. The one or more determined fuel properties may, in fact, be converted to a measure of thermal stability.The inventors have recognized that in addition to carefully controlling the heat exchangers 1004, 1006, 2020, 2030 as described above to control heat transfer within an engine 10, the inclusion of a refrigeration circuit 1007 in the heat exchange system 3000 may allow for still further advantages to be gained from newer fuels. In particular, thermal lift provided by a refrigeration cycle device 1007 may be utilized to bring the fuel to a temperature above that of the oil, thereby improving oil cooling and overall engine performance. It will be appreciated that power supply is required to transfer heat from a cooler to a warmer liquid. In the described implementation, therefore, an active vaporization and condensation process with driven components is used. In particular, the oil circulation system 2000 may be configured to transfer heat from the oil to the refrigerant liquid in an evaporator 1007 athat evaporates liquid in the refrigerant liquid (which may be fully liquid or a liquid-gas mixture at this stage of the cycle) to form a saturated vapor.Subsequently, a compressor 1007 bis provided to compress the saturated steam and generate superheated steam. This hot vapor is then directed to a refrigerant-liquid-fuel heat exchanger 1007c; the heat exchanger is a condenser 1007c, and the heat transfer from the compressed vapor to the fuel at least partially reconverts (and optionally converts) the superheated vapor to a liquid. The cooler refrigerant is then returned to evaporator 2007a and optionally passed via expansion valve 2007d (between the condenser and evaporator / the oil interface) to convert the saturated liquid to a lower temperature liquid and vapor mixture before returning to evaporator 1007a.By controlling such a refrigeration cycle device 1007, the thermal lift provided may be adjusted according to fuel and engine operating conditions.Referring now to FIG. 17, a method 500 for controlling a gas turbine engine 10 is shown to take advantage of thermal lift in controlling engine performance. Method 500 is performed in an engine 10 substantially as described herein (note that the same principles may be applied to a direct drive engine as well as a transmission engine) and that includes a refrigeration cycle device 1007. The method 500 includes controlling 502 of the refrigeration cycle device 1007 such that the fuel temperature entering the combustion chamber 16 is higher than a maximum oil temperature in the oil cycle system 2000.The controller 502 may be performed at regular intervals -- with the heat transfer to the fuel corrected as needed -- or may be applied continuously to maintain the fuel temperature above the highest oil temperature. The controller 502 may be applied at any stage of engine operation-e.g., idle and / or cruising-and optionally throughout operation.The controller 502 may include transferring 502 aof the heat from the oil to the refrigerant liquid using the evaporator 1007 a. The flow rate of the oil and / or refrigerant liquid through the evaporator 1007a may be adjusted as desired, generally using one or more valves. A bypass pipe and / or a return pipe may be provided around the evaporator 1007a for the oil, so that the oil flow rate through the evaporator 1007a can be adjusted without changing the oil flow rate by other components. Alternatively or additionally, the evaporator 1007 amay be located on a separate parallel branch of the oil system 2000 and the modulation valve 2016 may be used to adjust the proportion of oil directed through the evaporator 1007 a. The pump speed of oil pump 2004 may be adjusted to change the oil flow rate in some implementations. The flow rate of the refrigerant liquid through the evaporator 1007 acan be controlled by controlling a pumping rate of the refrigerant around the refrigeration cycle device 1007.The controller 502 may then include compressing 502 bof the saturated refrigerant vapor generated in the evaporator 1007 ato form superheated vapor using the refrigerant compressor 1007 b. Here again, the flow rate of the refrigerant may be controlled by controlling a pumping rate of the refrigerant around the refrigeration cycle device 1007-the compressor 1007b fulfills the role of the pump in the implementation shown in FIG. 18, but a separate refrigerant pump may be provided in addition to the compressor 1007b in other implementations. The superheated steam formed in the compressor 1007b is then sent to the condenser 1007c.The controller 502 may then include transferring 502 cof the heat from the superheated steam to the fuel using the condenser 1007 c. By this heat transfer, the refrigerant vapor is at least partially converted into a liquid (i.e., condensed). A saturated liquid may form in the condenser 1007c.In the implementation shown in FIG. 18, the refrigeration cycle device 1007 further includes an expansion valve 1007 dwhich is located downstream (from the perspective of refrigerant flow) of the condenser 1007 cand upstream of the evaporator 1007 a. In such implementations, the method 500 includes using 502 dof the expansion valve 1007 dto convert the refrigerant from the condenser 1007 cto a reduced temperature liquid and vapor mixture, which is then returned to the evaporator 1007 a. The expansion valve 1007d may be actively controlled 502d as part of the control step 502.In the implementation shown in FIG. 18, the engine 10 includes a temperature sensor 1009 that senses the temperature of the fuel downstream of the refrigeration cycle device 1007 (i.e., as it approaches the combustor 16), and a temperature sensor 2009 that provides an indication of the maximum oil temperature within the oil circuit system 2000 (e.g., that is immediately downstream of the most downstream oil cooled engine component). The method 500 may further include comparing 501 the oil and fuel temperatures and adjusting the controller 502 of the refrigeration cycle device 1007 based on the comparison. For example, if the fuel temperature is lower than the maximum oil temperature, the pump speed of the refrigeration cycle device 1007 (and therefore the refrigerant flow rate) may be increased and / or the fraction of oil directed via the evaporator 1007 amay be increased (e.g., by adjusting an oil valve).The method 500 may also include controlling 504 one or more other components of the heat exchange system 3000-for example, the liquid flow through the fuel-oil heat exchanger 1006 may be adjusted 504 to increase heat transfer to the fuel by increasing the oil flow rate through the fuel-oil heat exchanger 1006 (e.g., using a bypass or modulation valve). Additionally or alternatively, the liquid flow through the fuel-oil heat exchanger 1006 may be adjusted 504 to increase heat transfer to the fuel by recirculating the fuel through the fuel-oil heat exchanger 1006 (e.g., using a recirculation valve 6010). Additionally or alternatively, heat transfer to fuel in the fuel-oil heat exchanger 1006 may be increased 504 by decreasing air cooling of the oil before the oil enters the fuel-oil heat exchanger 1006 (e.g., using an air valve 2022). It should be appreciated that the opposing adjustments 504 may be made when the fuel temperature is too high. While the control step 504 is shown for components other than the refrigeration cycle device 1007 after the control step 502 for the refrigeration cycle device 1007 in FIG. 17, it should be appreciated that these steps 502, 504 may be performed in any order or simultaneously. Moreover, in some implementations, only the refrigeration cycle device 1007 may be controlled 502 in response to the comparison 501 of fuel and oil temperatures.The temperature comparison step 501 may be performed at regular intervals throughout operation or in response to certain stimuli, e.g., a change in engine mode, a change in flight altitude, a change in thrust demand, a change in fuel, or a change in fuel or oil temperature.In various implementations, the method 500 includes controlling 502 the refrigeration cycle device 1007 such that the fuel temperature upon entering the combustion chamber 16 is at least 2° C., and optionally at least 5° C., 10° C., 15° C., 20° C., or 25° C. higher than the maximum oil temperature in the oil cycle system 2000. The refrigeration cycle device 1007 may be controlled 502 such that the fuel temperature entering the combustion chamber 16 is between 2° C. and 50° C. higher than the maximum oil temperature in the oil cycle system 2000.The method 500 may include controlling 502, 504 the engine 10 to run for at least 10%, 20%, 30%, or 50% of the time in cruise operation with a fuel temperature entering the combustor 16 that is higher than the maximum oil temperature. In some implementations, the fuel temperature upon entering the combustion chamber 16 may be maintained higher than the maximum oil temperature throughout the cruise operation.In some implementations, the controller 502 is based on one or more fuel characteristics of the fuel (e.g., one of the example fuel characteristics described above, which may be determined by one of the methods described above).The method 500 may include controlling 502 the refrigeration cycle device 1007 (and optionally also controlling 504 one or more other components of the heat exchange system 3000) such that the fuel temperature upon entering the combustion chamber 16 is higher than the maximum oil temperature within the oil cycle system 2000 by an amount determined based on the one or more fuel characteristics of the fuel. For example, a greater increase in fuel temperature beyond oil temperature may be possible as the thermal stability and / or heating value of the fuel is higher. In some implementations, two or more fuel characteristics may be evaluated together. The method 500 may therefore include sensing one or more fuel characteristics and adjusting the controller 502, 504 based on these characteristics, optionally in addition to other data - e.g., operating mode, flight altitude, ambient temperature, oil type, oil and / or fuel flow rates, refrigerant type, etc.The method 500 is independent of engine architecture and may be applied to both transmission gas turbine engines 10 and direct drive gas turbine engines. The oil circulation system is arranged to provide oil for cooling and optionally also lubricating one or more engine components - in transmission engines 10, the cooled one or more engine components may be or comprise the power transmission 30. In transmission or direct drive engines, the cooled one or more engine components may be or include an auxiliary transmission 33, one or more bearings (e.g., for a shaft), power electronics, one or more components of an integrated drive generator, and / or any other parts of the engine to be cooled.It should be understood that the invention is not limited to the above-described embodiments, and that various modifications and improvements may be made without departing from the concepts described herein. Except in the case of mutual exclusion, each of the features may be employed separately or in combination with any other features, and the disclosure extends to and includes all combinations and sub-combinations of one or more features described herein.

Claims

A method (200) of operating a gas turbine engine (10) for an aircraft (1), the gas turbine engine (10) comprising: an engine core (11) comprising a turbine (19), a compressor (14), a combustor (16) arranged to combust a fuel, and a core shaft (26) connecting the turbine to the compressor; a fan (23) arranged upstream of the engine core (11); a gearbox (30) receiving input from the core shaft (26) and outputting drive to the fan (23) to drive the fan at a lower speed than the core shaft, an oil circulation system (2000) arranged to supply the gearbox (30) with oil; and a heat exchange system (3000) comprising: an air-to-oil heat exchanger (2020) through which the oil in the oil circulation system flows; a fuel-to-oil heat exchanger (1006) through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; and at least one valve (2016, 2007, 2007a) configured such that a portion of the oil that is conducted via at least one of the heat exchangers (1006, 2020) can be varied, wherein the method (200) comprises controlling (202) the at least one valve (2016, 2007, 2007a) such that, under idling conditions, an oil flow ratio of: rate of O%0020̈Lflow into the air - O%0020̈Lw a%0020̈M exchanger (m 3 s-1) rate of O%0020̈Lflow into the fuel - O%0020̈Lw a%0020̈M exchanger (m 3 s-1) is in the range of 0.62 to 5.29.The method (200) of claim 1, wherein the oil circulation system (2000) branches such that a portion of the oil may flow along each branch, and the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in a parallel configuration on different branches of the oil circulation system, and wherein the valve (2016) arranged such that the proportion of the oil routed via at least one of the heat exchangers (1006, 2020) may be varied is a modulation valve (2016) arranged such that the proportion of the oil routed via each branch may be varied.The method (200) of claim 1, wherein the oil circulation system (2000) further comprises at least one bypass pipe (2005, 2005a) configured such that a portion of the oil can bypass at least one of the air-oil heat exchanger (2020) and the fuel-oil heat exchanger (1006), and wherein the valve (2007, 2007a) configured such that the amount of the oil conducted via at least one of the heat exchangers (1006, 2020) can be varied is a bypass valve (2007, 2007a) configured such that a portion of the oil can bypass the at least one heat exchanger (2020, 1006).The method (200) of claim 3, wherein the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in series in the oil circulation system (2000).The method (200) of claim 3, wherein the air-oil and fuel-oil heat exchangers (2020, 1006) are arranged in parallel on different branches of the oil circulation system (2000), and wherein the method (200) comprises controlling both the bypass valve (2007, 2007a) and a modulation valve (2016) arranged such that the proportion of oil conducted via each branch can be varied.The method (200) of claim 1, comprising controlling (202) the at least one valve (2016, 2007, 2007a) such that the oil flow ratio is below 5.50 under idling conditions; optionally such that the oil flow ratio is below 5.0 under idling conditions; further optionally such that the oil flow ratio is below 4.5 under idling conditions.The method (200) of claim 1, wherein controlling (202) the at least one valve (2016, 2007, 2007a) to adjust the oil flow ratio comprises decreasing the amount of oil routed via the at least one air-oil heat exchanger (2020) when the oil flow ratio is too high.The method (200) of claim 1, wherein the method (200) comprises controlling (202) the at least one valve (2016, 2007, 2007a) under idle conditions such that the heat transfer ratio is in the range of 0.62 to 3.00 when the fuel temperature at entry into the combustion chamber (16) is above 160°C; and / or such that the heat transfer ratio is in the range of 0.62 to 2.00 when the fuel temperature at entry into the combustion chamber (16) is above 180°C.The method (200) of claim 1, wherein the method (200) comprises controlling (202) the at least one valve (2016, 2007, 2007a) such that the heat transfer ratio is in the range of 0.62 to 3.67 when the fuel is at least 70% sustainable flying fuel.The method (200) of claim 1, wherein the heat exchange system (3000) further comprises a refrigeration cycle device (1007) configured to provide thermal buoyancy by transferring additional heat from the oil to the fuel in addition to the heat transferred from the fuel-oil heat exchanger (1006), and wherein the method (200) further comprises controlling the refrigeration cycle device (1007) to adjust the amount of the heat additionally transferred to the fuel.The method (200) of claim 1, wherein the heat exchange system (3000) comprises a plurality of bypass pipes (2005, 2005a, 2005'), each configured to allow oil to bypass a heat exchanger (1006, 1004, 2020), and wherein the step of controlling (202) the at least one valve (2016, 2007, 2007a) comprises controlling (202) at least two bypass valves.A gas turbine engine (10) for an aircraft (1) comprising: an engine core (11) comprising a turbine (19), a compressor (14), and a core shaft (26) connecting the turbine to the compressor; a fan (23) disposed upstream of the engine core; and a gearbox (30) receiving input from the core shaft and outputting drive to the fan to drive the fan at a lower speed than the core shaft; an oil circulation system (2000) disposed to supply the gearbox (30) with oil; and a heat exchange system (3000) comprising: an air-to-oil heat exchanger (2020) through which the oil in the oil circulation system flows; a fuel-to-oil heat exchanger (1006) through which the oil in the oil circulation system and the fuel flow so that heat is transferred between the oil and the fuel; and at least one valve (2016, 2007, 2007a) configured such that a portion of the oil that is conducted via at least one of the heat exchangers (1006, 2020) can be varied, and wherein the at least one valve (2016, 2007, 2007a) is configured to be controlled such that, under idling conditions, an oil flow ratio of: rate of O%0020̈lflow into the air - O%0020̈ lw a%0020̈mé exchanger (m 3 s-1) rate of O%0020̈lflow into the fuel - O%0020̈ lw a%0020̈mé exchanger (m 3 s-1) is in the range of 0.62 to 5.29.The gas turbine engine (10) of claim 12, wherein the heat exchange system (3000) further comprises a refrigeration cycle device (1007) configured to provide thermal lift by the transfer of further heat from the oil to the fuel such that the fuel temperature is raised above the oil temperature.The gas turbine engine (10) of claim 12, wherein: the turbine (19) is a first turbine, the compressor (14) is a first compressor, and the core shaft (26) is a first core shaft; the engine core further comprises a second turbine (17), a second compressor (15), and a second core shaft (27) connecting the second turbine to the second compressor; and the second turbine, the second compressor, and the second core shaft are arranged to rotate at a higher speed than the first core shaft.The gas turbine engine (10) of claim 12, wherein the heat exchange system (3000) further comprises branching fuel return paths and at least one valve that controls a split of the fuel flow, the branching paths configured to return fuel from the heat exchange system at at least two different locations along a main fuel path leading from the location where the fuel enters the gas turbine engine (10) to the combustor (16).

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