A method for managing a power plant intended for a rotary-wing aircraft, said power plant comprising at least two main engines (11, 12), at least one secondary engine and a main power transmission gearbox, said main (11, 12) and secondary engines mechanically driving said main power transmission gearbox in order to cause at least one main output shaft of said main power transmission gearbox to rotate, said main output shaft being integral in rotation with a main rotor of said aircraft, each main engine (11, 12) continuously delivering a first main mechanical power which is less than or equal to a maximum continuous main mechanical power PMC P and temporarily delivering a contingency second main mechanical power which is greater than or equal to a guaranteed minimum contingency main mechanical power POEI MINI, each secondary engine continuously delivering a secondary mechanical power which is less than or equal to a maximum continuous secondary mechanical power PMC S, characterised in that - an engine health check is carried out on each main engine (11, 21) to determine a level of deterioration of each main engine (11, 12) and a main mechanical power margin CSM P11, CSM P12 of each main engine (11, 12), - a coefficient K is determined which is greater than or equal to the value zero such that K = | min [0, CSM P11, CSM P12] |, CSM P11, CSM P12 being respectively said main mechanical power margin of each main engine (11, 12), "0" being the value zero, " min " being a function which gives as a result the minimum value of a series of values, and "| |" being the absolute value function, - each main engine (11, 12) is controlled to provide a main mechanical power, and - each secondary engine is controlled to provide two separate mechanical power levels, said secondary mechanical power provided by each secondary engine enabling each main engine (11, 12) and each secondary engine jointly to supply a mechanical power which is greater than or equal to a sufficient mechanical power PMS in order to enable flying of said aircraft using said main rotor, said two secondary mechanical power levels being a first secondary mechanical power PM S1 and a second secondary mechanical power PM S2, said second secondary mechanical power PM S2 being greater than said first secondary mechanical power PM S1 and such that PM S2 = PM S1 + K.P det, P det being a value of a predetermined main mechanical power and ". " being the multiplication function, said second secondary mechanical power PM S2 thus making it possible to compensate for a loss of main mechanical power from at least one main engine (11, 21). - an engine health check is carried out on each main engine (11, 21) to determine a level of deterioration of each main engine (11, 12) and a main mechanical power margin CSM P11, CSM P12 of each main engine (11, 12), - a coefficient K is determined which is greater than or equal to the value zero such that K = | min [0, CSM P11, CSM P12] |, CSM P11, CSM P12 being respectively said main mechanical power margin of each main engine (11, 12), "0" being the value zero, " min " being a function which gives as a result the minimum value of a series of values, and "| |" being the absolute value function, - each main engine (11, 12) is controlled to provide a main mechanical power, and - each secondary engine is controlled to provide two separate mechanical power levels, said secondary mechanical power provided by each secondary engine enabling each main engine (11, 12) and each secondary engine jointly to supply a mechanical power which is greater than or equal to a sufficient mechanical power PMS in order to enable flying of said aircraft using said main rotor, said two secondary mechanical power levels being a first secondary mechanical power PM S1 and a second secondary mechanical power PM S2, said second secondary mechanical power PM S2 being greater than said first secondary mechanical power PM S1 and such that PM S2 = PM S1 + K.P det, P det being a value of a predetermined main mechanical power and ". " being the multiplication function, said second secondary mechanical power PM S2 thus making it possible to compensate for a loss of main mechanical power from at least one main engine (11, 21).