Method and system for assisting with the piloting of a rotary wing aircraft in a fuel economy mode

A method for assisting the piloting of a rotary-wing aircraft, including two engines, a first engine of which is put in standby to ensure the operation of the aircraft in a fuel economy mode, the so-called ECO mode, the second engine remaining active in said ECO mode,

• the method being characterized in that to allow the activation by a pilot of the ECO mode, a flight computer of the aircraft checks, in real time, the fulfilment of the following conditions for authorization of entry into ECO mode:
• the sum of the powers supplied by the engines is less than a maximum continuous power,
• the rotation speed N2 of a power turbine of the second engine is greater than a determined speed threshold which is between 80% and 90% of the maximum speed of the engine,
• the altitude of the aircraft is greater than a minimum value allowing a transient phase of autorotation during the reactivation of the engine in standby in the event of a fault of the active engine,
• and no detected critical fault exists.