Aircraft operation

A method of determining at least one characteristic of a fuel provided to a gas turbine engine of an aircraft, the gas turbine engine forming part of a propulsion system and comprising:

• a combustor arranged to combust the fuel and having an exit, and wherein a combustor exit pressure—P40—is defined as the total pressure at the combustor exit at cruise conditions;
• a turbine comprising a rotor having a leading edge and a trailing edge, and wherein a turbine rotor entry pressure—P41—is defined as the total pressure at the leading edge of the rotor of the turbine at the cruise conditions;
• and
• a compressor having an exit, wherein a compressor exit pressure—P30—is defined as the total pressure at the exit from the compressor at the cruise conditions;
• wherein the method comprises:
• changing the fuel supplied to the gas turbine engine;
• and
• determining the at least one fuel characteristic of the fuel based on a change in at least one of P30, P40 and P41,
• wherein the at least one fuel characteristic comprises at least one of:
• i. a percentage of sustainable aviation fuel in the fuel;
• and
• ii. an indication that the fuel is a fossil fuel.

A propulsion system for an aircraft comprising:

• a gas turbine engine comprising:
• a combustor arranged to combust a fuel and having an exit, and wherein a combustor exit temperature—T40—is defined as an average temperature of flow at the combustor exit at cruise conditions;
• a turbine comprising a rotor having a leading edge and a trailing edge, and wherein a turbine rotor entry temperature—T41—is defined as an average temperature of flow at the leading edge of the rotor of the turbine at the cruise conditions;
• and
• a compressor having an exit, wherein a compressor exit temperature—T30—is defined as an average temperature of flow at the exit from the compressor at the cruise conditions;
• a fuel tank arranged to contain the fuel to power the gas turbine engine;
• and
• a processor programmed to:
• change the fuel supplied to the gas turbine engine;
• receive data corresponding to a change in at least one of T30, T40 and T41;
• and
• determine at least one fuel characteristic of the fuel based on the change in the at least one temperature, the at least one fuel characteristic comprising at least one of:
• i. a percentage of sustainable aviation fuel in the fuel;
• and
• ii. an indication that the fuel is a fossil fuel.

A propulsion system for an aircraft comprising:

• a gas turbine engine comprising:
• a combustor arranged to combust a fuel and having an exit, and wherein a combustor exit pressure—P40—is defined as the total pressure at the combustor exit at cruise conditions;
• a turbine comprising a rotor having a leading edge and a trailing edge, and wherein a turbine rotor entry pressure—P41—is defined as the total pressure at the leading edge of the rotor of the turbine at the cruise conditions;
• and
• a compressor having an exit, wherein a compressor exit pressure—P30—is defined as the total pressure at the exit from the compressor at the cruise conditions;
• a fuel tank arranged to contain the fuel to power the gas turbine engine;
• and
• a processor programmed to:
• change the fuel supplied to the gas turbine engine;
• receive data corresponding to at least one of P30, P40 and P41;
• and
• determine at least one fuel characteristic of the fuel based on a change in the at least one of P30, P40 and P41, the at least one fuel characteristic comprising at least one of:
• i. a percentage of sustainable aviation fuel in the fuel;
• and
• ii. an indication that the fuel is a fossil fuel.