Gas turbine engine

A method of operating a gas turbine engine, the gas turbine engine comprising:

• 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 1:2 to 1:5;
• and wherein:
• a MTO nvPM emissions index ratio is defined as:
• EI
• max
• ⁢
• TO
• ,
• SAF
• EI
• max
• ⁢
• TO
• ,
• FF
• where:
• EImaxTO,SAF is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine when operating at 100% available thrust for given operating conditions if a fuel provided to the plurality of fuel spray nozzles comprises a sustainable aviation fuel (SAF);
• and
• EImaxTO,FF is the system loss corrected nvPM emissions index in mg/kg of the gas turbine engine when operating at 100% available thrust for the given operating conditions if a fuel provided to the plurality of fuel spray nozzles is a fossil-based hydrocarbon fuel;
• and
• the MTO nvPM emissions index ratio of the gas turbine engine is less than 1;
• and
• the method comprises providing fuel comprising a sustainable aviation fuel to the plurality of fuel spray nozzles.