1. An aircraft having a rotary wing and at least one main gearbox for driving rotation of said rotary wing, said aircraft having a first main engine and a second main engine driving said main gearbox, said aircraft being provided with a main regulation system regulating the first main engine and the second main engine in application of a setpoint that is variable, wherein the aircraft is provided with a secondary engine driving said main gearbox, said aircraft having a secondary regulation system that regulates the secondary engine in application of a setpoint that is constant, the secondary regulation system being independent of said main regulation system.