Gas turbine engine system bowed rotor start mitigation and wear reduction

1. An engine system for an aircraft, the engine system comprising:

a first gas turbine engine;

a second gas turbine engine;

a control system configured to:

operate the first gas turbine engine with an idle fuel burn schedule in a taxi mode of the aircraft;

dry crank the second gas turbine engine in a first pre-takeoff portion of the taxi mode to cool the second gas turbine engine absent fuel burn by the second gas turbine engine;

operate the second gas turbine engine with a sub-idle fuel burn schedule in a second pre-takeoff portion of the taxi mode of the aircraft, the sub-idle fuel burn schedule comprising a reduction of the idle fuel burn schedule; and

increase a fuel flow of the first gas turbine engine and the second gas turbine engine above the idle fuel burn schedule prior to takeoff of the aircraft.

2. The engine system of claim 1 , wherein the control system is further configured to:

transition the second gas turbine engine from the sub-idle fuel burn schedule to the idle fuel burn schedule in a prepare-for-takeoff mode prior to acceleration of the first gas turbine engine and the second gas turbine engine for takeoff of the aircraft.

3. The engine system of claim 1 , wherein the control system is further configured to:

operate the first gas turbine engine with the sub-idle fuel burn schedule in a first post-landing portion of the taxi mode based on detecting landing of the aircraft;

dry crank the first gas turbine engine in a second post-landing portion of the taxi mode to cool the first gas turbine engine absent fuel burn by the first gas turbine engine; and

operate the second gas turbine engine with the idle fuel burn schedule based on detecting landing of the aircraft.

4. The engine system of claim 3 , wherein dry cranking is driven by one or more of: an electric motor, a pneumatic drive, and a hydraulic drive powered by one or more of: an auxiliary power unit, a battery system, an electric generator, a hydraulic source, and a pneumatic source.

5. The engine system of claim 1 , wherein dry crank of the second gas turbine engine is initiated prior to starting the first gas turbine engine.

6. The engine system of claim 1 , wherein the control system is further configured to alternate a designation of the first gas turbine engine and the second gas turbine engine between two or more gas turbine engines of the aircraft, and the control system is further configured to track a sub-idle time and/or a dry crank time of the two or more gas turbine engines and alternate the designation of the first gas turbine engine and the second gas turbine engine to substantially balance the sub-idle time and/or the dry crank time of the two or more gas turbine engines for the aircraft.

7. The engine system of claim 1 , wherein at least one engine bleed schedule is adjusted corresponding to the sub-idle fuel burn schedule to control a compressor work distribution during sub-idle operation, and wherein adjustment to the at least one engine bleed schedule increases an engine bleed flow to at least one environment control system of the aircraft during sub-idle operation.

8. The engine system of claim 1 , wherein dry crank is active for a dry crank time based on one or more of: a predetermined period of time, a computed period of time, and/or a comparison between sensor data and a threshold condition.

9. A gas turbine engine for an aircraft, the gas turbine engine comprising:

a fan section;

a compressor section operably coupled to the fan section;

a combustor section;

a turbine section operably coupled to the compressor section; and

an engine controller configured to:

meter a fuel flow to the combustor section according to an idle fuel burn schedule in a first taxi mode of the aircraft;

meter the fuel flow to the combustor section according to a sub-idle fuel burn schedule in a second taxi mode of the aircraft, the sub-idle fuel burn schedule comprising a reduction of the idle fuel burn schedule; and

dry crank the gas turbine engine prior to metering the fuel flow according to the sub-idle fuel burn schedule prior to takeoff of the aircraft or dry crank the gas turbine engine after metering the fuel flow according to the sub-idle fuel burn schedule after landing of the aircraft.

10. The gas turbine engine of claim 9 , wherein the first taxi mode comprises a taxi-to-flight-line mode prior to takeoff of the aircraft and the second taxi mode comprises a taxi-to-gate mode after landing of the aircraft.

11. The gas turbine engine of claim 9 , wherein the first taxi mode comprises a taxi-to-gate mode after landing of the aircraft and the second taxi mode comprises a taxi-to-flight-line mode prior to takeoff of the aircraft.

12. The gas turbine engine of claim 11 , wherein the engine controller is further configured to transition from the sub-idle fuel burn schedule to the idle fuel burn schedule in a prepare-for-takeoff mode prior to acceleration for takeoff of the aircraft.

13. The gas turbine engine of claim 9 , wherein the engine controller is further configured to adjust at least one engine bleed schedule of the compressor section corresponding to the sub-idle fuel burn schedule to control a compressor work distribution during sub-idle operation of the gas turbine engine.

14. A method of engine system control, the method comprising:

operating a first gas turbine engine with an idle fuel burn schedule in a taxi mode of an aircraft;

dry cranking a second gas turbine engine in a first pre-takeoff portion of the taxi mode to cool the second gas turbine engine absent fuel burn by the second gas turbine engine;

operating a second gas turbine engine with a sub-idle fuel burn schedule in a second pre-takeoff portion of the taxi mode of the aircraft, the sub-idle fuel burn schedule comprising a reduction of the idle fuel burn schedule; and

increasing a fuel flow of the first gas turbine engine and the second gas turbine engine above the idle fuel burn schedule prior to takeoff of the aircraft.

15. The method of claim 14 , further comprising:

transitioning the second gas turbine engine from the sub-idle fuel burn schedule to the idle fuel burn schedule in a prepare-for-takeoff mode prior to acceleration of the first gas turbine engine and the second gas turbine engine for takeoff of the aircraft.

16. The method of claim 14 , further comprising:

operating the first gas turbine engine with the sub-idle fuel burn schedule in a first post-landing portion of the taxi mode based on detecting landing of the aircraft;

dry cranking the first gas turbine engine in a second post-landing portion of the taxi mode to cool the first gas turbine engine absent fuel burn by the first gas turbine engine; and

operating the second gas turbine engine with the idle fuel burn schedule based on detecting landing of the aircraft.

17. The method of claim 14 , wherein dry crank of the second gas turbine engine is initiated prior to starting the first gas turbine engine.

18. The method of claim 14 , further comprising:

alternating a designation of the first gas turbine engine and the second gas turbine engine between two or more gas turbine engines of the aircraft; and

tracking a sub-idle time and/or a dry crank time of the two or more gas turbine engines and alternate the designation of the first gas turbine engine and the second gas turbine engine to substantially balance the sub-idle time and/or the dry crank time of the two or more gas turbine engines for the aircraft.

19. The method of claim 14 , further comprising:

adjusting at least one engine bleed schedule corresponding to the sub-idle fuel burn schedule to control a compressor work distribution during sub-idle operation, wherein adjusting the at least one engine bleed schedule increases an engine bleed flow to at least one environment control system of the aircraft during sub-idle operation.

20. The method of claim 14 , wherein dry crank is active for a dry crank time based on one or more of: a predetermined period of time, a computed period of time, and/or a comparison between sensor data and a threshold condition.