Navigation aid method for determining, by an instruction giver, the path of an aircraft initially following a predetermined path in an approach phase and upon which are defined a safety altitude and an alarm limit, said aircraft including a navigation system of the INS/GNSS type including a satellite information receiver and at least one inertial unit producing position information, said method including determining a future path; estimating predicted protection radii on the future path are estimated, starting at the calculation time in the case of a critical situation, in terms of position information, which starts at the calculation time; calculating a limit time after which the predicted protection radius is greater than or equal to the alarm limit; and calculating a limit time after which the predicted protection radius is greater than or equal to the alarm limit.