A method for determining a switch-over vertical point from which an aircraft, having a current position, and flying a current vertical trajectory according to a manual piloting mode having an altitude setpoint, denoted target altitude, loaded by the pilot, switches to a piloting mode guided by a flight management system, in order to rejoin a predefined flight plan having a set of initial altitude constraints, comprising: calculating a first predicted vertical trajectory, determining a first point of intersection between the first predicted trajectory and the target altitude, determining a second predicted trajectory, determining the switch-over vertical point belonging to the first predicted vertical trajectory, based on any incompatible constraints, and as the intersection between the first predicted vertical trajectory and a predicted vertical trajectory calculated by integration of the dynamic flight equations by applying calculation hypotheses for a piloting mode guided by a flight management system.