According to the invention, as a function of the altitude, the vertical speed and the heading of the aircraft, as well as of the deviation of the actual trajectory of the aircraft with respect to the automatic trajectory, a time interval (T) is determined, during which it is tolerable for said aircraft to be able to fly without any control command, both on the part of the stick (1) and on that of said automatic pilot (2) and an alert is emitted for the attention of the crew in the case where it is noted that the absences of manual piloting and of automatic piloting extend simultaneously over a duration at least equal to said tune interval (T).