Method of regulating a power plant of a rotorcraft, and an associated rotorcraft

A rotorcraft including:

• a power plant comprising two engine units and a main gearbox, the two engine units being suitable for mechanically driving the main gearbox in order to drive an outlet main shaft of the main gearbox in rotation, the outlet main shaft being constrained to rotate with a main rotor of the rotorcraft, the main rotor rotating at a speed of rotation, the power plant further comprising a first computer for determining a first setpoint for the speed of rotation of the main rotor, the first setpoint lying in the range 90% of a nominal value to 110% of the nominal value, the two engine units comprising firstly a first engine unit comprising at least one main engine and a first regulation device that regulates operation of the at least one main engine on a current setpoint for the speed of rotation that is equal to the first setpoint for the speed of rotation of the main rotor, and secondly a second engine unit comprising at least one secondary engine and a second regulation device, the second regulation device regulating operation of the at least one secondary engine;
• first identification means allowing to identify that the rotorcraft is flying at a height that is greater than or equal to a predetermined threshold height;
• and
• second identification means allowing to identify that the rotorcraft is flying at a velocity that is greater than or equal to a predetermined threshold velocity;
• wherein if the first identification means identify that the rotorcraft is flying at a height greater than or equal to the predetermined threshold height and if the second identification means simultaneously identify that the rotorcraft is flying at a velocity greater than or equal to the predetermined threshold velocity, the first regulation device is configured to regulate a speed of rotation of the at least one main engine by progressively reducing the current setpoint by an increment i in percentage of the current setpoint per second, from the first setpoint down to a second setpoint that is less than the first setpoint, the current setpoint being reduced so long as a norm of an acceleration vector of the rotorcraft is less than 1 m·s−2.