Hybrid electric idle and braking for an aircraft

1. A method comprising:

detecting a braking condition of an aircraft comprising a gas turbine engine, the gas turbine engine comprising a low spool and a high spool;

determining a storage capacity state of an energy storage system of the aircraft;

transferring power from a low-spool generator to the energy storage system based on the storage capacity state of the energy storage system;

transferring power to the high spool through a high-spool electric motor to support combustion in the gas turbine engine while a rotational speed of the low spool is reduced responsive to the low-spool generator extracting energy from the low spool, wherein the low-spool generator is used as a brake of the low spool by selectively adding a load to the low spool; and

splitting energy output of the low-spool generator between the energy storage system and the high-spool electric motor, using a controller, based on the storage capacity state and a target speed of the high spool wherein the storage capacity state indicates a capacity to receive a power surge from the low-spool generator with a non-linear mapping having a first charging rate and a second charging rate, different from the first charging rate, wherein the first charging rate is defined over a first storage capacity range and the second charging rate is defined over a second storage capacity range, the first storage capacity range being different from the second storage capacity range;

wherein the controller is configured to split the energy output of the low-spool generator to charge the energy storage system at the first charging rate and the second charging rate over the first storage capacity range and over the second storage capacity range, respectively, based on the storage capacity state.

2. The method of claim 1 , further comprising:

detecting the braking condition based on a thrust command and an operating mode of the aircraft.

3. The method of claim 2 , wherein power transfer is performed based on determining that the operating mode of the aircraft is a taxi mode or a landing mode.

4. The method of claim 1 , wherein the low-spool generator is a low-spool electric machine configurable between a low-spool generator mode and a low-spool motor mode of operation, and the high-spool electric motor is a high-spool electric machine configurable between a high-spool generator mode and a high-spool motor mode of operation.

5. The method of claim 1 , further comprising:

determining a demand associated with one or more accessories driven by the high spool and one or more target engine pressures; and

controlling the high-spool electric motor to adjust a rotational speed of the high spool during the braking condition based on the demand associated with the one or more accessories driven by the high spool and the one or more target engine pressures.

6. The method of claim 1 , further comprising:

monitoring for a flutter condition during landing of the aircraft; and

adjusting operation of the low-spool generator and/or the high-spool electric motor to reduce the flutter condition.

7. The method of claim 1 , wherein the braking condition comprises operating the gas turbine engine at a minimum fuel limit, and the low-spool generator reduces a speed and thrust output of the low spool below nominal minimum values associated with the minimum fuel limit by adding a load to the low spool.

8. The method of claim 1 , wherein the gas turbine engine further comprises a thrust reverser, and power transfer to the high spool through the high-spool electric motor is performed when the thrust reverser is deployed.

9. An engine system of an aircraft, the engine system comprising:

an energy storage system;

a gas turbine engine comprising a low spool, a high spool, a low-spool generator operably coupled to the low spool, and a high-spool electric motor operably coupled to the high spool; and

a controller configured to:

detect a braking condition of the aircraft;

determine a storage capacity state of the energy storage system;

transfer power from the low-spool generator to the energy storage system based on the storage capacity state of the energy storage system;

transfer power to the high spool through the high-spool electric motor to support combustion in the gas turbine engine while a rotational speed of the low spool is reduced responsive to the low-spool generator extracting energy from the low spool, wherein the low-spool generator is used as a brake of the low spool by selectively adding a load to the low spool;

split energy output of the low-spool generator between the energy storage system and the high-spool electric motor based on the storage capacity state and a target speed of the high spool, wherein the storage capacity state indicates a capacity to receive a power surge from the low-spool generator with a non-linear mapping having a first charging rate and a second charging rate, different from the first charging rate, wherein the first charging rate is defined over a first storage capacity range and the second charging rate is defined over a second storage capacity range, the first storage capacity range being different from the second storage capacity range; and

charge the energy storage system at the first charging rate and the second charging rate over the first storage capacity range and over the second storage capacity range, respectively, based on the storage capacity state.

10. The engine system of claim 9 , wherein the controller is further configured to detect the braking condition based on a thrust command and an operating mode of the aircraft.

11. The engine system of claim 10 , wherein power transfer is performed based on determining that the operating mode of the aircraft is a taxi mode or a landing mode.

12. The engine system of claim 9 , wherein the low-spool generator is a low-spool electric machine configurable between a low-spool generator mode and a low-spool motor mode of operation, and the high-spool electric motor is a high-spool electric machine configurable between a high-spool generator mode and a high-spool motor mode of operation.

13. The engine system of claim 9 , wherein the controller is configured to:

determine a demand associated with one or more accessories driven by the high spool and one or more target engine pressures; and

control the high-spool electric motor to adjust a rotational speed of the high spool during the braking condition based on the demand associated with the one or more accessories driven by the high spool and the one or more target engine pressures.

14. The engine system of claim 9 , wherein the controller is configured to monitor for a flutter condition during landing of the aircraft and adjust operation of the low-spool generator and/or the high-spool electric motor to reduce the flutter condition.

15. The engine system of claim 9 , wherein the braking condition comprises operating the gas turbine engine at a minimum fuel limit, and the low-spool generator reduces a speed and thrust output of the low spool below nominal minimum values associated with the minimum fuel limit by adding a load to the low spool.

16. The engine system of claim 9 , wherein the gas turbine engine further comprises a thrust reverser, and power transfer to the high spool through the high-spool electric motor is performed when the thrust reverser is deployed.