Method for optimising the performance of an aircraft having a rotary wing set in motion by a power plant comprising at least one turbine engine, each turbine engine having a gas generator and an assembly comprising at least one turbine (2", 2"'), in which method: - during a definition step, a minimum first performance level (LEND) of the aircraft and a maximum second performance level (HEND) of the aircraft are defined by the manufacturer, and - during a check step, at least one health check of each turbine engine is performed by: ∘ determining a current power delivered by each turbine engine, ∘ measuring a current speed of rotation (Ng) and a current temperature (TET, T45) of the gases of said assembly, ∘ determining a first power margin between said current power and a first minimum power defined by the manufacturer for guaranteeing said first performance level at said current speed of rotation (Ng), ∘ determining a second power margin between said current power and a second minimum power defined by the manufacturer for guaranteeing said first performance level at said current temperature (TET, T45), and - during an evaluation step, each power margin is compared with a first threshold (SH1, SH2), a target performance level being the second performance level when the first and second power margins of each turbine engine are respectively greater than or equal to a first high threshold and a second high threshold, said target performance level being the first performance level when the first power margin of a turbine engine is less than the first high threshold and/or when the second power margin of a turbine engine is less than the second high threshold, and - the target performance level is displayed, and the aircraft is piloted during a flight in accordance with the target performance level established before the flight. - during a definition step, a minimum first performance level (LEND) of the aircraft and a maximum second performance level (HEND) of the aircraft are defined by the manufacturer, and - during a check step, at least one health check of each turbine engine is performed by: ∘ determining a current power delivered by each turbine engine, ∘ measuring a current speed of rotation (Ng) and a current temperature (TET, T45) of the gases of said assembly, ∘ determining a first power margin between said current power and a first minimum power defined by the manufacturer for guaranteeing said first performance level at said current speed of rotation (Ng), ∘ determining a second power margin between said current power and a second minimum power defined by the manufacturer for guaranteeing said first performance level at said current temperature (TET, T45), and ∘ determining a current power delivered by each turbine engine, ∘ measuring a current speed of rotation (Ng) and a current temperature (TET, T45) of the gases of said assembly, ∘ determining a first power margin between said current power and a first minimum power defined by the manufacturer for guaranteeing said first performance level at said current speed of rotation (Ng), ∘ determining a second power margin between said current power and a second minimum power defined by the manufacturer for guaranteeing said first performance level at said current temperature (TET, T45), and - during an evaluation step, each power margin is compared with a first threshold (SH1, SH2), a target performance level being the second performance level when the first and second power margins of each turbine engine are respectively greater than or equal to a first high threshold and a second high threshold, said target performance level being the first performance level when the first power margin of a turbine engine is less than the first high threshold and/or when the second power margin of a turbine engine is less than the second high threshold, and - the target performance level is displayed, and the aircraft is piloted during a flight in accordance with the target performance level established before the flight.