| CPC F02C 9/28 (2013.01) [F05D 2220/323 (2013.01); F05D 2240/35 (2013.01); F05D 2270/3013 (2013.01); F05D 2270/303 (2013.01); F05D 2270/309 (2013.01)] | 19 Claims |

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1. A method of determining at least one fuel characteristic of a fuel provided to a gas turbine engine of an aircraft, the gas turbine engine forming part of a propulsion system, the method comprising:
making an operational change, the operational change being effected by a controllable component of the propulsion system and arranged to affect operation of the gas turbine engine;
sensing a response to the operational change; and
determining the at least one fuel characteristic based on the response to the operational change,
wherein the at least one fuel characteristic comprises at least one of:
i. percentage of sustainable aviation fuel in the fuel;
ii. sulphur content of the fuel;
iii. oxygen content of the fuel;
iv. thermal stability of the fuel; and
v. an indication that the fuel is a fossil fuel.
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