A gas
turbine engine for an aircraft is disclosed. The gas
turbine engine comprises:a
combustor, comprising a
combustion chamber and a plurality of
fuel spray nozzles configured to inject fuel into the
combustion chamber, wherein the plurality of
fuel spray nozzles comprises a first subset of
fuel spray nozzles and a second subset of fuel spray nozzles, wherein the
combustor is operable in a condition in which each of the fuel spray nozzles of the first subset of fuel spray nozzles is supplied with fuel at a greater fuel flow rate than each of the fuel spray nozzles of the second subset of fuel spray nozzles, wherein a ratio of the number of fuel spray nozzles in the first subset of fuel spray nozzles to the number of fuel spray nozzles in the second subset of fuel spray nozzles is in the range of 1:3 to 1:6. A fuel-flow nvPM emissions index ratio is defined as:EIidle×Wf,idleEImaxTO×Wf,maxTOwhere: EIidle is the nvPM emissions index in mg / kg of the gas
turbine engine if operating at around 7% available thrust for given operating conditions; EImaxTO is the nvPM emissions index in mg / kg of the gas turbine engine if operating at around 100% available thrust for the given operating conditions; Wf,idle is the rate of fuel flow to the fuel spray nozzles in kg / s at around 7% available thrust for the given operating conditions; and Wf,maxTO is the rate of fuel flow to the fuel spray nozzles in kg / s at around 100% available thrust for the given operating conditions. The fuel-flow nvPM emissions index ratio of the gas turbine engine is less than 0.3. The gas turbine engine is configured to provide fuel comprising a sustainable
aviation fuel (SAF) to the plurality of fuel spray nozzles. Also disclosed are methods of operating a gas turbine engine.