A device comprises means for computing the air-craft (A) current position, means for determining at least one maximum permitted deviation (E1) around a set position of the flight path of the flight plan according to accuracy and integrity performances of said current position computation and to the restriction of a flight range authorized in a flight corridor (6A, 6B), and a display system (7) for displaying at least one a distance scale (9) on a viewing screen (8), at least one a fixed symbol (10) displaying the current position and two movable pointers (13, 14) displaying the limits of said maximum permitted deviation (E1).