Reduced-engine operation technique for rotorcraft

1. An aircraft comprising:

multiple engines; and

a flight control system coupled to the multiple engines with a multi-engine interface,

wherein the flight control system is configured to shutdown at least one engine of the multiple engines during a reduced-engine operation by continuously calculating altitude for the reduced-engine operation based on one or more of an aircraft descent rate of the aircraft and an engine restart time of the at least one engine.

2. The aircraft of claim 1 , wherein:

the multiple engines comprise two or more engines, and

the reduced-engine operation comprises a high-efficiency mode of operation for shutting-down at least one engine of the two or more engines.

3. The aircraft of claim 1 , wherein:

the multiple engines comprise two engines, and

the reduced-engine operation comprises a single-engine cruise mode of operation for shutting-down at least one engine of the two engines.

4. The aircraft of claim 1 , wherein:

the aircraft descent rate is based on current ambient conditions related to the altitude including outside ambient temperature during autorotation of the aircraft with the at least one engine of the multiple engines shutdown, and

the engine restart time is a function of the current ambient conditions related to one or more of the altitude, outside ambient temperature, and measured gas temperature of the at least one engine.

5. The aircraft of claim 1 , wherein:

the flight control system continuously calculates the altitude as a minimum altitude needed for the reduced-engine operation based on a pre-determined balance between the aircraft descent rate and the engine restart time.

6. The aircraft of claim 1 , wherein:

the flight control system provides visual indication of the altitude to a pilot with a gauge including current altitude versus a minimum altitude for the reduced-engine operation,

the flight control system is configured to calculate the altitude based on one or more of pressure altitude, density altitude and height above-ground-level, and

the flight control system is configured to calculate the altitude with an altitude margin built-in for safety.

7. The aircraft of claim 6 , wherein:

the gauge refers to a dedicated reduced-engine operation display control panel that includes an altitude gauge, one or more push buttons to engage/disengage the reduced-engine operation, and a visual display that displays status messages and caution-warning-advisory messages related to reduced-engine operation, and

the flight control system is configured to prevent activation of the reduced-engine operation when the caution-warning-advisory messages from full autonomy digital engine control (FADEC) are active.

8. The aircraft of claim 6 , wherein:

the gauge provides a mode message that indicates availability of the reduced-engine operation based on the current altitude versus the minimum altitude, or

the gauge provides an engaged-disengaged message that indicates the reduced-engine operation is currently engaged or disengaged.

9. The aircraft of claim 1 , wherein:

the flight control system is configured to selectively shutdown at least one engine of the multiple engines based on input from a pilot, and

the flight control system is configured to provide visual indication of the at least one engine selectively shutdown by the pilot.

10. The aircraft of claim 1 , wherein:

while in the reduced-engine operation during flight, if the altitude as a current altitude drops below a predetermined altitude, then the flight control system is configured to automatically restart the shutdown engine and provide corresponding advisory messages to a pilot.

11. The aircraft of claim 1 , wherein:

when the reduced-engine operation is selectively activated by a pilot during flight, the flight control system is configured to calculate and determine whether sufficient altitude is available or unavailable for reduced-engine operation, and

if the sufficient altitude is determined, the flight control system is configured to allow engine shutdown of the at least one engine during the reduced-engine operation and also provide an indication message to the pilot that the reduced-engine operation is available and active, and

if insufficient altitude is determined, the flight control system is configured to prevent engine shutdown of the at least one engine during flight and also provide a warning message to the pilot that the reduced-engine operation is unavailable and inactive.

12. The aircraft of claim 1 , wherein:

upon selective activation of the reduced-engine operation by a pilot, the flight control system is configured to adjust operation of the at least one engine of the multiple engines by throttling the at least one engine to IDLE, sub-IDLE, or completely OFF.

13. The aircraft of claim 1 , wherein:

upon activation of the reduced-engine operation by a pilot, the flight control system is configured to reduce airspeed prior to shutdown of at least one engine of the multiple engines so as to prevent over-torque or over-temperature on at least one active engine of the multiple engines when the at least one engine is shutdown.

14. The aircraft of claim 1 , wherein:

the aircraft comprises a rotorcraft, and

the multiple engines comprise multiple turbo-shaft engines.

15. An aircraft comprising:

multiple engines; and

a flight control system coupled to the multiple engines with a multi-engine interface,

wherein the flight control system is configured to shutdown at least one engine of the multiple engines during a reduced-engine cruise by continuously calculating altitude based on one or more of an aircraft descent rate and an engine restart time.

16. The aircraft of claim 15 , wherein:

the aircraft descent rate is based on current ambient conditions related to the altitude including outside ambient temperature during autorotation of the aircraft with the at least one engine of the multiple engines shutdown,

the engine restart time is a function of the current ambient conditions related to one or more of the altitude, outside ambient temperature, and measured gas temperature of the at least one engine, and

the altitude is continuously calculated as a minimum altitude needed for the reduced-engine cruise based on a pre-determined balance between the aircraft descent rate and the engine restart time.

17. A method comprising:

interfacing with multiple engines in a rotorcraft;

continuously calculating altitude of the rotorcraft for a reduced-engine operation based on one or more of an aircraft descent rate and an engine restart time;

shutting-down at least one engine of the multiple engines if the altitude is determined to be a sufficient altitude for the reduced-engine operation; and

restarting the at least one engine of the multiple engines if the altitude is determined to be an insufficient altitude for the reduced-engine operation.

18. The method of claim 17 , wherein:

the multiple engines comprise two or more engines, and

the reduced-engine operation comprises a high-efficiency mode of operation for shutting-down at least one engine of the two or more engines.

19. The method of claim 17 , wherein:

the multiple engines comprise two engines, and

the reduced-engine operation comprises a single-engine cruise mode of operation for shutting-down at least one engine of the two engines.

20. The method of claim 17 , wherein:

the aircraft descent rate is based on current ambient conditions related to the altitude including outside ambient temperature during autorotation of the aircraft with the at least one engine of the multiple engines shutdown,

the engine restart time is a function of the current ambient conditions related to one or more of the altitude, outside ambient temperature, and measured gas temperature of the at least one engine, and

the altitude is continuously calculated as a minimum altitude needed for the reduced-engine operation based on a pre-determined balance between the aircraft descent rate and the engine restart time.