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1 . A gas turbine engine for an aircraft, comprising:

a processor;

a staged combustion system having pilot fuel injectors and main fuel injectors, the staged combustion system being operable in a pilot-only range of operation in which fuel-delivery is only to the pilot fuel injectors, and a pilot-and-main range of operation in which fuel-delivery is to at least the main fuel injectors at a relative rate defined by a pilot-and-main staging ratio; and

a fuel delivery regulator arranged to control the fuel-delivery to the pilot and main fuel injectors, the fuel delivery regulator being arranged to receive from a first fuel source a first fuel having a first fuel characteristic and receive from a second fuel source a second fuel having a second fuel characteristic, the second fuel characteristic being different from the first, wherein

the staged combustion system is further operable in a transition range of operation between the pilot-only and the pilot-and-main ranges of operation, and

the processor is programmed to:

within the transition range of operation, execute the fuel-delivery to both the pilot and main fuel injectors at a transition staging ratio which is different from the pilot-and-main staging ratio, and

control the fuel delivery regulator to deliver one of the first and second fuels to one or both of the pilot and main fuel injectors during the transition range of operation and the other of the first and second fuels to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation, wherein the first fuel is delivered to only the pilot fuel injectors during the transition range of operation.

2 . The gas turbine engine according to claim 1 , wherein

the first fuel characteristic is associated with a first level of nvPM production which is less than a second level of nvPM production associated with the second fuel characteristic, and

the processor is programmed to deliver the first fuel to the pilot fuel injectors during the transition range of operation and deliver the second fuel to the one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation.

3 . The gas turbine engine according to claim 2 , wherein the first fuel characteristic corresponds to a greater proportion of SAF within the respective fuel compared to the second fuel characteristic.

4 . The gas turbine engine according to claim 1 , wherein the transition staging ratio has a continuous variation with changing engine power within at least part of the transition range of operation.

5 . The gas turbine engine according to claim 4 , wherein the continuous variation is such that, within the transition range of operation, the proportion of the total fuel flow to the fuel injectors that is attributable to fuel flow to the pilot fuel injectors decreases with increasing engine power and the proportion of the total fuel flow to the fuel injectors that is attributable to fuel flow to the main fuel injectors increases with increasing engine power.

6 . The gas turbine engine according to claim 1 , wherein the transition staging ratio has a constant intermediate value within at least part of the transition range of operation which is different from the pilot-and-main staging ratio, and lies between a pilot-only staging ratio and the pilot-and-main staging ratio.

7 . The gas turbine engine according to claim 6 , wherein the transition staging ratio varies between a series of constant intermediate values, each being different from the pilot-and-main staging ratio, and each lying between the pilot-only staging ratio and the pilot-and-main staging ratio.

8 . The gas turbine engine according to claim 1 , wherein

the fuel delivery regulator comprises a fuel blender arranged to receive a fuel supply from both the first and second fuel sources and output the first fuel from the first fuel source, the second fuel from the second fuel source, or a blend thereof, and

the fuel blender is arranged for the fuel-delivery to the pilot fuel injectors and to the main fuel injectors.

9 . The gas turbine engine according to claim 1 , wherein the proportion of the first fuel delivered from the first fuel source compared to the proportion of the second fuel delivered from the second fuel source is determined according to a target resultant level of nvPM production at a particular fuel flow rate within the transition range of operation.

10 . The gas turbine engine according to claim 1 , wherein the proportion of the first fuel delivered from the first fuel source compared to the proportion of the second fuel delivered from the second fuel source during the transition range of operation is determined at least partly according to any one or more of:

a) the amount of first and second fuels respectively having the first fuel characteristic and the second fuel characteristic available for a proposed flight,

b) the amount of total fuel requirement for the fuel injectors during pilot-only operation for the entire flight in a range of operation in which the first fuel is provided from the first fuel source, and/or

c) a fuel composition limit parameter.

11 . A method of operating a gas turbine engine for an aircraft, the gas turbine engine comprising a staged combustion system having pilot fuel injectors and main fuel injectors, the staged combustion system being operable in a pilot-only range of operation in which fuel-delivery is only to the pilot fuel injectors, and a pilot-and-main range of operation in which fuel-delivery is to at least the main fuel injectors at a relative rate defined by a pilot-and-main staging ratio, the method comprising:

regulating fuel delivery to the pilot and main fuel injectors from a first fuel source containing a first fuel having a first fuel characteristic and a second fuel source containing a second fuel having a second fuel characteristic, the second fuel characteristic being different from the first; and

operating the staged combustion system in a transition range of operation between the pilot-only and the pilot-and-main ranges of operation within which a processor executes the fuel-delivery to both the pilot and main fuel injectors at a transition staging ratio which is different from the pilot-and-main staging ratio,

wherein the regulating of the fuel delivery comprises delivering, under control of the processor, one of the first and second fuels to one or both of the pilot and main fuel injectors during the transition range of operation and the other of the first and second fuels to one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation, wherein the first fuel is delivered to only the pilot fuel injectors during the transition range of operation.

12 . The method according to claim 11 , wherein

the first fuel characteristic is associated with a first level of nvPM production which is less than a second level of nvPM production associated with the second fuel characteristic, and

under the control of the processor, the first fuel is delivered to the pilot fuel injectors during the transition range of operation and the second fuel is delivered to the one or both of the pilot and main fuel injectors during at least part of the pilot-and-main range of operation.

13 . The method according to claim 12 , wherein the first fuel characteristic corresponds to a greater proportion of SAF within the respective fuel compared to the second fuel characteristic.

14 . The method according to claim 11 , wherein the transition staging ratio has a continuous variation with changing engine power within at least part of the transition range of operation.

15 . The method according to claim 14 , wherein the continuous variation is such that, within the transition range of operation, the proportion of the total fuel flow to the fuel injectors that is attributable to fuel flow to the pilot fuel injectors decreases with increasing engine power and the proportion of the total fuel flow to the fuel injectors that is attributable to fuel flow to the main fuel injectors increases with increasing engine power.

16 . The method according to claim 11 , wherein the transition staging ratio has a constant intermediate value within at least part of the transition range of operation which is different from the pilot-and-main staging ratio, and lies between a pilot-only staging ratio and the pilot-and-main staging ratio.

17 . The method according to claim 16 , wherein the transition staging ratio varies between a series of constant intermediate values, each being different from the pilot-and-main staging ratio, and each lying between the pilot-only staging ratio and the pilot-and-main staging ratio.

18 . The method according to claim 11 , wherein

regulating fuel delivery comprises delivering the first fuel from the first fuel source, the second fuel from the second fuel source, or a blend thereof, using a fuel blender, and

the fuel blender is arranged for the fuel-delivery to the pilot fuel injectors and to the main fuel injectors.

19 . The method according to claim 11 , wherein the proportion of the first fuel delivered from the first fuel source compared to the proportion of the second fuel delivered from the second fuel source is determined according to a target resultant level of nvPM production at a particular fuel flow rate within the transition range of operation.

20 . The method according to claim 11 , wherein the proportion of the first fuel delivered from the first fuel source compared to the proportion of the second fuel delivered from the second fuel source during the transition range of operation is determined at least partly according to any one or more of:

a) the amount of first and second fuels respectively having the first fuel characteristic and the second fuel characteristic available for a proposed flight,

b) the amount of total fuel requirement for the fuel injectors during pilot-only operation for the entire flight in a range of operation in which the first fuel is provided from the first fuel source, and/or

c) a fuel composition limit parameter.