| The Forward Air Controller (FAC) is a term for a specially trained military pilot and/or observer whose job is to coordinate air-to-ground attacks made in support of friendly ground troops. FACs have operated both from the ground, in concert with the troops being supported, and from the air in all sorts of aircraft.
The concept of the Forward Air Controller was really hashed out during the Allied invasion of Europe in the latter part of the Second World War. During that conflict, the U.S. Army Air Force and later U.S. Air Force experimented with many different methods of providing air support to the advancing Allied ground troops. The most significant pressure for advancement in such tactics came when it became clear that Allied Forces were being held into a small area of Normandy immediately after D-Day; as the Allies tried increasingly desperate measures to achieve a breakout from the area, they eventually came around to using heavy bombers to provide incredibly concentrated destruction on a small area of entrenched Axis defenders. This was the opening phases of Operation Goodwood.
Later, as fighter-bombers followed advancing ground troops into Europe, the Air Corps began to assign experienced pilots to the front of the advancing columns. This was originally necessary because the Air Corps used a different set of radio frequencies from the ground forces, necessitating an entirely separate communications unit (usually a jeep or truck with mounted radios) to maintain contact. Once that was available, it made sense to have an operator who could speak the same 'language' as the pilots he would be directing.
During the Vietnam War, U.S. Army, Marine and Air Force FACs flew regular duty. Originally, they flew modified civilian aircraft such as the OV-1 and OV-2; during the war, however, the U.S. Armed Forces began to use the purpose-built OV-10 Bronco, a twin-engined prop aircraft in a tractor/pusher arrangement with a twin-tailed fuselage. This airplane, capable of speeds up to 200 MPH, was used by FAC pilots at low altitude, in among hills, along rivers, anywhere they were needed. Equipped with underbelly racks, it could carry both attack ordnance such as 1.75inch rockets or small bombs, and marking ordnance (smoke rockets) used to mark targets for fighter-bombers. FAC pilots were known for performing heroic and occasionally foolhardy acts of support for their brothers on the ground; their mission and operational methods usually meant that they were the first friendly air forces to arrive on the scene when American ground troops were in trouble. As such, it was very difficult for them to simply remain passively in orbit about the scene, and FAC crews began carrying M-16 rifles, grenades and the arms on their aircraft to use in order to blunt attacks so as to give harried, pinned-down ground troops time to retreat or better organize their defense.
In the years after the Vietnam War, American FACs continued to use the OV-10 in preparation for war in Europe which, thankfully, never came. In the 1980s, however, the Air Force began to convert some of its A-10 Thunderbolt ground attack aircraft for use as a FAC platform. Highly survivable, maneuverable and with excellent loiter time, the OA-10A was used to control other A-10s, larger aircraft such as the F-111 and F-117 and eventually other assets such as artillery and theater missiles such as ATACMS.
The FAC's tools evolved over time from tracer rounds for machine guns to smoke rockets, flares, white phosphorus, and finally to the high-tech infrared laser designators and handheld IR laser pointers, as well as the transmission of precise coordinates obtained from GPS and laser rangefinding. | 
