Abstract: An apparatus and method for adjusting set clearance altitude of an aircraft to control the aircraft to fly along a predetermined path relative to the terrain below. A terrain following radar system and a stored terrain elevation data base are used to determine the respective positions and elevations of selected portions of the terrain downrange from the aircraft. This information is used along with pre-stored information relating to the ability of a ground threat to detect the aircraft and the roughness of the terrain to compute three different altitudes. The first altitude (Ia) represents the maximum height at which the aircraft can fly without being detected by the ground threat. The second altitude (Da) represents the maximum desired altitude of the aircraft at any time during a flight. The third altitude (Ra) represents the minimum altitude at which the aircraft is allowed to fly.