Method and device for optimising the use of a motor

The method involves determining (22) and displaying (24) a possible continuous utilization duration (DELTA-T) of a first power e.g. maximum takeoff power, when an engine e.g. turboshaft engine, of a power plant of an aircraft develops a second power greater than a third power e.g. maximum continuous power, and less than or equal to the first power, where the utilization duration passes at a variable speed depending on the second power. An alert is triggered when the utilization duration becomes shorter than a predetermined threshold. An independent claim is also included for a unit for controlling a power plant of an aircraft.