Method for optimising the performance of an aircraft, device and aircraft

The method involves defining two levels of performance of an aircraft (1), and determining a power margin based on a rotational speed of a gas generator (2'). Another power margin is determined based on a temperature of the gas. Each power margin is compared to a threshold to determine whether a target level of performance is equal to one of the levels of performance, and the target level of performance is displayed so that a pilot controls the aircraft depending on the target level of performance established before the flight. An independent claim is also included for a device for optimizing the performance of a rotary wing aircraft.