A method of assisting the piloting of an aircraft (1) comprising a first rotor (5) having a plurality of first blades (6) with a first variable collective pitch, and a second rotor (7) having a plurality of second blades (8) with a second variable collective pitch. According to the method, a computer determines a power limit curve (70) and displays said limit curve (70) in a diagram (60) to inform the pilot of the first and second collective pitch margins available before reaching the limits of at least one operating rating of the power plant of the aircraft.