Power plant, a helicopter including such a power plant, and a method implemented by said power plant

A method of driving a rotary wing and an anti-torque rotor of a helicopter, said helicopter having a power plant provided with a single engine and with a main gearbox (MGB) driving said rotary wing and with a tail gearbox (TGB) driving said anti-torque rotor, said power plant including a first electric motor mechanically connected to said main gearbox (MGB) so as to be capable of driving said main gearbox (MGB), and a second electric motor mechanically connected to said tail gearbox (TGB) so as to be capable of driving said tail gearbox (TGB), in which method at least one of the following actions is performed:

• in a normal mode of operation, the main gearbox (MGB) and the tail gearbox (TGB) are driven using the sole engine;
• in a first emergency mode, in the event of overspeed of the engine, the first electric motor is caused to operate in electricity generator mode and the second electric motor is caused to operate in electricity generator mode to slow down said engine during a first predetermined duration, and then if the engine is still in overspeed at the end of said first duration, the engine is stopped and the first electric motor is caused to operate in electric motor mode in order to drive said main gearbox (MGB), and said second electric motor is caused to operate in electric motor mode in order to drive said tail gearbox (TGB);
• in a second emergency mode, said engine is no longer capable of driving said main gearbox (MGB) at a given first speed, and the first electric motor is caused to operate in electric motor mode in order at least to contribute to driving said main gearbox (MGB);
• in a third emergency mode, said engine is no longer capable of driving said tail gearbox (TGB) at a given second speed, and said second electric motor is caused to operate in electric motor mode in order at least to contribute to driving said tail gearbox (TGB);
• and
• in a fourth emergency mode, said engine is no longer capable of driving said main gearbox (MGB) at a given first speed and is no longer capable of driving said tail gearbox (TGB) at a given second speed, and the first electric motor is caused to operate in electric motor mode in order at least to contribute to driving the main gearbox (MGB), and the second electric motor is caused to operate in electric motor mode in order at least to contribute to driving said tail gearbox (TGB).