The parts of the flight plan of an aircraft corresponding to lateral trajectories at low altitude can comprise passages with limited lateral freedom of deployment either because of risks of collision with the ground, or because of risks of penetration into a forbidden overfly zone. The method makes it possible to signal them for the attention of the crew, on the onboard navigation screen, so that said crew should redouble their attention. To do this it uses a detection procedure analogous to that of TAWS ground collision risk warning systems with protection envelopes related to the aircraft, of shapes suitable for the monitoring of the integrity of the spaces necessary for the aircraft to perform flat turns that are as tight as is permitted, while taking account of the local wind.