A method and device including a unit to extract a linear terrain profile stored in memory, a terrain height for a current distance of the aircraft, a unit for determining a current height of the aircraft with respect to the terrain, using at least one measurement taken by a radar altimeter, a unit for computing a first current altitude of the aircraft, using the terrain height and the current height, a unit for computing a second current altitude of the aircraft, corresponding to an altitude on an approach profile of the current position of the aircraft and a unit for computing the difference between the first and second current altitudes, the difference being transmitted to a guidance unit to vertically guide the aircraft.