Method for assisting in the flight management of an aircraft aiming to reach a constraint point on a predetermined lateral trajectory that is assumed to have to be followed by the aircraft, at a required time of arrival RTA, said aircraft occupying a current position defined by a current altitude with a current horizontal speed. The method including, when it follows a rallying flight plan, calculating a new rallying flight plan following the lateral trajectory to be followed and including a new estimated speed profile different from the estimated speed profile over at least one update area, the new estimated speed profile including, over said update areas, either a value greater than that of the estimated speed profile, if the arrival time difference is positive, or a value less than that of the estimated speed profile, if the arrival time difference is negative.