Gas turbine engine spool coupling

1. A system comprising:

a gas turbine engine comprising a low speed spool and a high speed spool;

a spool coupling system comprising an electro-mechanical actuator and a clutch configured to mechanically link the low speed spool and the high speed spool; and

a controller operable to:

determine a mode of operation of the gas turbine engine;

monitor for a spool coupling activation condition associated with the mode of operation;

activate the electro-mechanical actuator of the spool coupling system based on the controller detecting the spool coupling activation condition, wherein engagement of the clutch and power transfer between the low speed spool and the high speed spool occurs based on activation of the electro-mechanical actuator of the spool coupling system and reaching an engagement condition of the clutch of the spool coupling system; and

deactivate the electro-mechanical actuator of the spool coupling system after disengagement of the clutch and the spool coupling activation condition no longer being detected.

2. The system of claim 1 , wherein disengagement of the spool coupling system occurs based on the gas turbine engine reaching a disengagement condition.

3. The system of claim 1 , wherein determining the mode of operation of the gas turbine engine includes distinguishing between ground-based operation and flight operation.

4. The system of claim 1 , wherein monitoring for the spool coupling activation condition comprises detecting a reduction in speed of the low speed spool below an idle condition prior to a windmill condition while the mode of operation is an in-flight mode.

5. The system of claim 1 , wherein the engagement condition of the spool coupling system comprises a gear ratio level of the spool coupling system aligning with a ratio of high speed spool speed to low speed spool speed.

6. The system of claim 5 , wherein a stabilization condition is reached after engagement of the spool coupling system resulting in fan windmill power being transferred to the high speed spool, and wherein the low speed spool provides power to the high speed spool until the spool coupling system reaches a disengagement condition.

7. The system of claim 1 , wherein the mode of operation of the gas turbine engine is a sub-idle mode that activates, engages, disengages, and deactivates the spool coupling system below an idle level of operation of the gas turbine engine during flight.

8. The system of claim 1 , wherein the mode of operation of the gas turbine engine is a low spool power-assisted idle that results in the high speed spool at idle when the spool coupling system is engaged.

9. The system of claim 1 , wherein the spool coupling system comprises a variable transmission system with multiple gear ratios and multiple levels of spool power transfer and engagement speeds.

10. A method comprising:

determining, by a controller, a mode of operation of a gas turbine engine, the gas turbine engine comprising a low speed spool and a high speed spool;

monitoring, by the controller, for a spool coupling activation condition associated with the mode of operation;

activating an electro-mechanical actuator of a spool coupling system based on the controller detecting the spool coupling activation condition, wherein engagement of a clutch of the spool coupling system and power transfer between the low speed spool and the high speed spool occurs based on activation of the electro-mechanical actuator of the spool coupling system and reaching an engagement condition of the clutch of the spool coupling system, wherein the spool coupling system is configured to mechanically link the low speed spool and the high speed spool when the clutch is engaged; and

deactivating the electro-mechanical actuator of the spool coupling system after disengagement of the clutch and the spool coupling activation condition no longer being detected.

11. The method of claim 10 , wherein disengagement of the spool coupling system occurs based on the gas turbine engine reaching a disengagement condition.

12. The method of claim 10 , wherein determining the mode of operation of the gas turbine engine includes distinguishing between ground-based operation and flight operation.

13. The method of claim 10 , wherein monitoring for the spool coupling activation condition comprises detecting a reduction in speed of the low speed spool below an idle condition prior to a windmill condition while the mode of operation is an in-flight mode.

14. The method of claim 10 , wherein the engagement condition of the spool coupling system comprises a gear ratio level of the spool coupling system aligning with a ratio of high speed spool speed to low speed spool speed.

15. The method of claim 14 , wherein a stabilization condition is reached after engagement of the spool coupling system resulting in fan windmill power being transferred to the high speed spool, and wherein the low speed spool provides power to the high speed spool until the spool coupling system reaches a disengagement condition.

16. The method of claim 10 , wherein the mode of operation of the gas turbine engine is a sub-idle mode that activates, engages, disengages, and deactivates the spool coupling system below an idle level of operation of the gas turbine engine during flight.

17. The method of claim 10 , wherein the mode of operation of the gas turbine engine is a low spool power-assisted idle that results in the high speed spool at idle when the spool coupling system is engaged.

18. The method of claim 10 , wherein the spool coupling system comprises a variable transmission system with multiple gear ratios and multiple levels of spool power transfer and engagement speeds.