Method of assistance with the navigation of a multi-engine rotorcraft in the event, called the engine failure event, of failure of one of the main engines (M1, M2) of a power plant fitted to the rotorcraft, said power plant delivering the mechanical power required at least for driving in rotation at least one main rotor of the rotorcraft providing at least the lift of the rotorcraft, the main rotor, outside the engine failure event, being driven by the power plant in accordance with the application of a speed setpoint (CNr), called the NR setpoint, the value of which is calculated variable by a flight control unit according to the current flight conditions of the rotorcraft in a range of NR setpoint values proportional to the value of a predefined nominal speed (NRnom) for driving the main rotor, the flight control unit supplying said NR setpoint to a regulation unit for regulating the individual operation of the main engines (M1, M2) for driving the main rotor at a speed, called the NR speed, in accordance with the application of the NR setpoint (CNr), the regulation unit applying various regulation ratings of the individual operation of the main engines (M1, M2) according to the current flight state of the rotorcraft, including: - according to a current flight state of the rotorcraft outside the engine failure event, first regulation ratings, called AEO ratings, defining a maximum authorized rating of the main engines (M1, M2) for respective predefined durations at each of the AEO ratings, - according to a current flight state of the rotorcraft in the engine failure event, second regulation ratings, called OEI ratings, defining an emergency rating authorized for at least one of the main engines (M1, M2) remaining operational for respective predefined durations at each of the OEI ratings, - the rotorcraft being fitted with a unit, called a display unit, using a display screen for displaying at least one value relating to a mechanical power margin authorized for use by the pilot, called the authorized margin (Ma), deduced by calculation means according to at least the current regulation rating of the main engines (M1, M2) while taking account at least of limit criteria identifying the operating state of the main engines (M1, M2), characterized in that, outside the engine failure event, the value displayed by the screen relating to the authorized margin (Ma), then called the limited margin (Ml), is the value of the authorized margin (Ma) reduced by a computer by a predefined value, called the safety margin (Ms), on condition at least of a driving of the main rotor at an NR speed, referred to as "low", controlled by the flight control unit and identified below a predefined speed threshold for driving the main rotor, called the NR speed threshold, such that, in the engine failure event and in the event of manoeuvring of the rotorcraft, prior to the engine failure event, at a drive NR speed of the main rotor below the NR speed threshold, the pilot of the rotorcraft has a mechanical power reserve making it easier for the pilot to take action on the behaviour of the rotorcraft in order to re-establish control quickly over its progress, while avoiding any significant drop in the number of revolutions per second of rotation of the main rotor. - according to a current flight state of the rotorcraft outside the engine failure event, first regulation ratings, called AEO ratings, defining a maximum authorized rating of the main engines (M1, M2) for respective predefined durations at each of the AEO ratings, - according to a current flight state of the rotorcraft in the engine failure event, second regulation ratings, called OEI ratings, defining an emergency rating authorized for at least one of the main engines (M1, M2) remaining operational for respective predefined durations at each of the OEI ratings, - the rotorcraft being fitted with a unit, called a display unit, using a display screen for displaying at least one value relating to a mechanical power margin authorized for use by the pilot, called the authorized margin (Ma), deduced by calculation means according to at least the current regulation rating of the main engines (M1, M2) while taking account at least of limit criteria identifying the operating state of the main engines (M1, M2),