Aside from its rugged construction, the A-10’s survival in the forward area is abetted by its low-speed maneuverability, which allows it to fly safely as low as 100 feet above ground level. Flying at low altitude conveys two main advantages: first, radar has trouble detecting the aircraft against background clutter; second, the pilot can exploit the air defense concept of "intervisbility" by means of terrain masking tactics.
Terrain-Masking
What is intervisibility? This concept simply put, defines how neither radar, optical systems, nor the naked eye can see through solid objects like hills, or even dense foliage. Though air defense units are normally posted on hilltops or ridgelines that provide long lines of sight, in rolling hill country (such as is found in Germany) the chances of detecting a very low-level target exposied for a brief instant (say, an A-10A at 100 feet) are very small indeed. Statistical analyses done in the Fulda Gap area (the principal Soviet invasion route into the US sector of the NATO Central Front) indicate that the median unmask range for an aircraft flying at 30 meters (100 feet) is on the order of 5 km. Of equal or greater importance, statistics show that even at that range a target normally remains unmasked for less than 10 seconds. Because all existing air defense systems require an uninterrupted line of sight, this means that even if an air defense weapon does acquire and track a target, it will not have time to launch its weapon; even if it does, that weapon will not have the time to cover the distance to the target before the aircraft remasks by passing behind another hill.
Consider the following example: an air defense missile system detects an aircraft at 5 km range and 30 meters height, flying towards the launcher at 400 km/hr. (110 m/sec). It has a radar with a 2-second scan period, and requires two scans to establish a track. Its fire control system requires another 2 seconds to establish a solution and present it to the operator. The launcher takes another second to slew onto the target and launch the missile. The missile, with an average velocity of 700 m/sec., requires 5 seconds to fly 3,500 meters. Total system reaction and flyout time is therefore 4+2+1+5=12 seconds. Under average conditions, therefore, there is a 50% chance that the missile system will never be able to launch, and an almost 100% certainty that if launched, the missile will lose lock before intercept (when the target re-masks). This is, of course, a very sterile example; in reality, many different systems with interlocking fields of fire would increase significantly the chances of a successful engagment. Still, the use of low-level terrain masking allows an aircraft like the A-10 to approach quite close to its targets without being observed or engaged, and then to escape destruction after its attack run.
As originally conceived (in the mid-1970s), the A-10’s basic mission profile was derived from World War II “Cab Rank” close air support tactics as used by the RAF and USAAF in 1944-45. Flights of four aircraft would loiter at medium altitude (5,000-10,000 feet) just behind the front lines, until called upon by a Forward Air Controller (FAC) on the ground or in a spotter aircraft. They would then strike those targets using low-level strafing, bombing or rocket attacks. Using this type of mission profile, the A-10 would take off from a base perhaps 250 n.mi. behind the front, cruise to the battle area at high altitude (25,000 feet) to conserve fuel, then loiter for as long as 2 hours at medium altitude over or just behind the FEBA. The primary weapon would be the GAU-8, supplemented by up to 18 Mk.82 500-lb HE bombs, or a similar number of Mk.20 Rockeye cluster bombs.
By the time the Warthog entered service in the early 1980s, the forward area threat had become far more severe, necessitating a reevaluation of tactics. The current mission profile for a high-threat environment calls for takeoff some 250 n.mi behind the front, followed by a 25,000 foot cruise to an initial point approximately 40 n.mi. behind the front. At this point, the A-10 descends to a height of less than 250 feet (often less than 100 feet) above ground level for penetration of the forward area. Any loitering is done at this height, which reduces time on station to approximately 30 minutes. The A-10 then exits the battle area at very low level before climbing to 25,000 feet for the return to base. Flying for such extended periods at near-zero feet requires considerable agility, as provided through low wing loading and a reasonable thrust-to-weight ratio. This can only be achieved by reducing the payload to much less than the theoretical maximum. A typical loadout for this mission profile would be the GAU-8 with 1,174 rounds, and up to 6 AGM-65 Mavericks, plus an ALQ-119 jamming pod. Much more emphasis is placed on standoff weapons in order to avoid the lethal envelopes of air defense weapons systems like the Shilka, Tunguska, Gecko and Tor.
