An aircraft terrain avoidance system include a device having a first unit knowing a profile of the terrain that is located at the front of the aircraft, a second unit for determining an avoidance trajectory, a third unit which is connected to the first and second units and used to verify if there is a terrain collision risk for the aircraft, a fourth unit for emitting an alarm signal in the event of detection of a collision risk by the third unit, at least one aircraft performance database relating to an avoidance maneuvering gradient which can be flown by the aircraft according to particular flight parameters, and a fifth unit for determining the effective values of the particular parameters during the flight of the aircraft. The third unit is formed such that it is possible to determine the avoidance trajectory according to information received from the database and the fifth unit.