A method of predictive determination of a power needed by a rotary-wing aircraft to perform hovering flight in order to land. After measuring the current atmospheric pressure and the current temperature outside the aircraft, the method serves to estimate a predicted atmospheric pressure and a predicted temperature at the altitude where the hovering flight is to be performed. Thereafter, both a maximum power available from a power plant of the aircraft once the aircraft has reached the point where the hovering flight is to be performed, and also a power needed from the power plant in order to perform the hovering flight are calculated. Finally, the present invention serves to display the available maximum power and the power needed on an instrument of the aircraft and thereby warn the pilot of the aircraft when the power needed is close to or even greater than the maximum power available.