Method of activating an electric motor in a hybrid power plant of a multi-engined aircraft, and an aircraft

1. A method of driving rotation of a rotor of an aircraft, the aircraft having at least two fuel-burning engines and an electric motor suitable for driving rotation of the rotor, the aircraft further having a regulator in communication with the engines and the electric motor, the method comprising:

driving the rotor by using the engines together;

generating, by the regulator, an authorization only during the aircraft operating in one of at least one predetermined stage of flight, the authorization authorizing the use of the electric motor to drive the rotor in rotation;

generating, by the regulator, while the authorization is valid, an operation order to require the electric motor to operate if one of the engines is considered as failed;

driving, while the operation order is valid, the rotor by using each engine that is not considered as failed and by using the electric motor; and

inhibiting, by the regulator, the electric motor from being used to drive the rotor other than when both conditions of (i) the aircraft is operating in one of the at least one predetermined stage of flight and (ii) one of the engines is considered as failed occur concurrently.

2. The method according to claim 1 , wherein the regulator has one management system per engine and an avionics system communicating with the management systems, the avionics system configured to generate the authorization and transmit the authorization to each management system, and at least one of the management systems configured to generate the operation order and transmit the operation order to the electric motor.

3. The method according to claim 1 , wherein each predetermined stage of flight is a stage of flight that requires a given power for driving the rotor, the given power being less than the maximum power delivered by the engines when one of the engines is considered as failed.

4. The method according to claim 1 , further comprising inhibiting the authorization from being generated while the aircraft is standing on the ground.

5. The method according to claim 1 , wherein the at least one predetermined stage of flight includes a stage of flight at low speed during which the aircraft has a speed of advance less than a threshold speed of advance.

6. The method according to claim 5 , wherein a “current” speed is determined for the speed of advance of the aircraft and the regulator compares the current speed with the threshold speed of advance, the regulator generating the authorization when the current speed is less than the threshold speed of advance.

7. The method according to claim 1 , wherein a predetermined stage of flight is a stage of flight with little power margin during which at least one engine presents a power margin relative to a predetermined power limit that is less than a power threshold.

8. The method according to claim 1 , wherein a predetermined stage of flight is a single-engined stage of flight during which only a single one of the engines is in operation and is operating at a contingency overpower rating usable for only a predetermined duration.

9. The method according to claim 1 , wherein an engine is considered as failed when the engine is delivering no power.

10. The method according to claim 1 , wherein an engine is considered as failed when the engine is stopping or has stopped.

11. The method according to claim 1 , wherein an engine is considered as failed when the engine is idling.

12. The method according to claim 1 , wherein an engine is considered as failed when the power developed by the engine is frozen as a result of a malfunction of a system for regulating the flow of fuel and when the torque developed by the engine is less than a torque threshold.

13. The method according to claim 1 , wherein an engine is considered as failed when the difference between the torque being developed by one engine and the torque being developed by another one of the engines is greater than a predetermined difference.

14. An aircraft comprising:

at least one rotor;

at least two fuel-burning engines suitable for driving the rotor in rotation;

an electric motor suitable for driving the rotor in rotation; and

a regulator in communication with the engines and the electric motor, the regulator configured to

drive the rotor by using the engines together;

generate an authorization only during the aircraft operating in one of at least one predetermined stage of flight, the authorization authorizing the use of the electric motor to drive the rotor in rotation;

generate, while the authorization is valid, an operation order to require the electric motor to operate if one of the engines is considered as failed;

drive, while the operation order is valid, the rotor by using each engine that is not considered as failed and by using the electric motor; and

inhibit the electric motor from being used to drive the rotor other than when both conditions of (i) the aircraft is operating in one of the at least one predetermined stage of flight and (ii) one of the engines is considered as failed occur concurrently.

15. The aircraft according to claim 14 , wherein the regulator includes one management system per engine, each management system configured to control one engine, the regulator further including an avionics system communicating with the management systems, the avionics system configured to generate the authorization and transmit the authorization to each management system, and at least one of the management systems configured to generate the operation order and transmit the operation order to the electric motor.

16. The aircraft according to claim 15 , wherein the regulator includes at least one member selected from the following list: a measurement system for determining a speed of advance of the aircraft, one measurement device per engine for determining the torque being developed by the engine, one measurement device per engine for determining the power being developed by the engine, one sensor per engine for determining the speed of rotation of a member of the engine, and a touch device for determining whether the aircraft is standing on ground.