The subject of the present invention is a method for improving route and 4D prediction calculations by FMS in the framework of ATC tactical flight instructions, which method enables the FMS of an aircraft to carry out its usual predictions as precisely as possible when it has left or anticipates leaving its initial flight plan following an instruction from the air traffic controller, and has no instructions telling it where and when to return to the initial flight plan, and the invention is characterized in that it consists in transmitting to the aircraft, from an air traffic control centre, information enabling the FMS system to be aware of traffic crossing points (X1) and control sector changes, and to use this information to predict the coordinates of a point (X2) at which it is supposed to rejoin its initial flight plan at the earliest opportunity after the crossing point that required an alteration of trajectory and at the latest opportunity on the last point of the current sector.