Propulsion system control

1. A method of operating an aircraft comprising a propulsion system, the propulsion system comprising a gas turbine engine and a fuel tank arranged to provide fuel to the gas turbine engine, the method comprising:

supplying the fuel to the gas turbine engine;

determining one or more fuel characteristics of the fuel supplied to the gas turbine engine from performance of the gas turbine engine during at least one of taxi, take-off, and climb of the aircraft; and

controlling the propulsion system during cruise based on the one or more fuel characteristics that have been determined,

wherein the one or more fuel characteristics of the fuel are selected from the group consisting of:

i. percentage of sustainable aviation fuel in the fuel;

ii. aromatic hydrocarbon content of the fuel;

iii. multi-aromatic hydrocarbon content of the fuel;

iv. percentage of nitrogen-containing species in the fuel;

V. presence or percentage of a tracer species or trace element in the fuel;

vi. hydrogen to carbon ratio of the fuel;

vii. hydrocarbon distribution of the fuel;

viii. level of non-volatile particulate matter emissions on combustion;

ix. naphthalene content of the fuel;

X. sulphur content of the fuel;

xi. cycloparaffin content of the fuel;

xii. oxygen content of the fuel;

xiii. thermal stability of the fuel;

xiv. level of coking of the fuel;

XV. an indication that the fuel is a fossil fuel; and

xvi. at least one of density, viscosity, calorific value, and heat capacity.

2. The method of claim 1 , wherein the one or more fuel characteristics of the fuel are selected from the group consisting of:

i. percentage of sustainable aviation fuel in the fuel;

ii. multi-aromatic hydrocarbon content of the fuel;

iii. percentage of nitrogen-containing species in the fuel;

iv. presence or percentage of a tracer species or trace element in the fuel;

V. hydrogen to carbon ratio of the fuel;

vi. hydrocarbon distribution of the fuel;

vii. level of non-volatile particulate matter emissions on combustion;

viii. naphthalene content of the fuel;

ix. sulphur content of the fuel;

X. cycloparaffin content of the fuel;

xi. oxygen content of the fuel;

xii thermal stability of the fuel;

xiii. level of coking of the fuel;

xiv. an indication that the fuel is a fossil fuel; and

XV. at least one of density, viscosity, and heat capacity.

3. The method of claim 1 , further comprising receiving weather data relating to weather conditions around the aircraft or along a planned route of the aircraft, and wherein the received weather data are used in the control of the propulsion system.

4. The method of claim 1 , further comprising detecting weather conditions around the aircraft in flight, and wherein the detected weather conditions are used in the control of the propulsion system.

5. The method of claim 1 , further comprising determining at least one additional fuel characteristic based on detection of at least one fuel property.

6. The method of claim 5 , wherein the detection is performed on-wing.

7. The method of claim 1 , further comprising determining at least one additional fuel characteristic based on received fuel composition data.

8. The method of claim 7 , wherein the fuel composition data are provided to the aircraft on refuelling.

9. The method of claim 1 , wherein the one or more fuel characteristics are determined for fuel in at least one fuel tank of the aircraft.

10. The method of claim 9 , wherein the one or more fuel characteristics are determined for each of a plurality of fuels stored on board the aircraft.

11. The method of claim 1 , wherein the one or more fuel characteristics are determined for fuel immediately before entry into a combustor of the gas turbine engine.

12. The method of claim 11 , wherein the determination of the one or more fuel characteristics of fuel immediately before entry into the combustor of the gas turbine engine is performed a plurality of times during flight to account for changes in fuel composition.

13. The method of claim 1 , wherein the determining of the one or more fuel characteristics of the fuel further comprises:

obtaining at least one fuel characteristic of any fuel already present in the fuel tank prior to refuelling;

determining at least one fuel characteristic of a fuel added to the fuel tank on refuelling; and

calculating at least one fuel characteristic of the resultant fuel in the fuel tank after refuelling.

14. A propulsion system for an aircraft comprising:

a gas turbine engine;

a fuel tank arranged to contain a fuel to be supplied to the gas turbine engine;

a propulsion system controller arranged to:

determine one or more fuel characteristics of the fuel supplied to the gas turbine engine from performance of the gas turbine engine during at least one of taxi, take-off, and climb of the aircraft; and

control the propulsion system during cruise based on the one or more fuel characteristics that have been determined,

wherein the one or more fuel characteristics of the fuel are selected from the group consisting of:

i. percentage of sustainable aviation fuel in the fuel;

ii. aromatic hydrocarbon content of the fuel;

iii. multi-aromatic hydrocarbon content of the fuel;

iv. percentage of nitrogen-containing species in the fuel;

V. presence or percentage of a tracer species or trace element in the fuel;

vi. hydrogen to carbon ratio of the fuel;

vii. hydrocarbon distribution of the fuel;

viii. level of non-volatile particulate matter emissions on combustion;

ix. naphthalene content of the fuel;

X. sulphur content of the fuel;

xi. cycloparaffin content of the fuel;

xii. oxygen content of the fuel;

xiii. thermal stability of the fuel;

xiv. level of coking of the fuel;

XV. an indication that the fuel is a fossil fuel; and

xvi. at least one of density, viscosity, calorific value, and heat capacity.

15. The propulsion system of claim 14 , further comprising a receiver arranged to receive data relating to fuel composition, based on which the one or more fuel characteristics can be determined.

16. The propulsion system of claim 14 , further comprising at least one sensor arranged to provide data relating to the one or more fuel characteristics.

17. The propulsion system of claim 14 , wherein the propulsion system comprises a plurality of fuel tanks arranged to contain different fuels to power the gas turbine engine, and the propulsion system controller is arranged to determine the one or more fuel characteristics of each different fuel.