Air Observation Post

Air Observation Post (AOP) is the term used by the Royal Air Force and other services of the United Kingdom and Commonwealth air forces for an aeroplane or helicopter used in the role of artillery spotter. In this role, either the pilot of the aircraft or a passenger acts both just as an observer watching for targets on the ground, and as a Forward Observation Officer directing the fire, by radio, of artillery on the ground (or calling in tactical ground-attack aircraft).

In the interwar period the role of artillery spotting was mixed with reconnaissance and ground attack to be served by Army Co-operation (AC) squadrons. At the beginning of World War II their standard aircraft was the Westland Lysander. The enormous losses of athe British Expeditionary Force showed both the aircraft and the concept to be a failure, and around 1941 AC squadrons were converted to fighter-bomber aircraft, dropping the liaison and artillery spotting roles. These roles have been take over by numbers 651 to 663 Squadrons of the RAF, the all-new Air Observation Post units working closely with Army. A further three of these squadrons - Nos. 664, 665 and 666 - were RCAF AOP squadrons manned by Canadian and British personnel.

For the remainder of the World War II, AOP units used light, fixed-wing aircraft, notably several marks of Auster aircraft, flown by RAF or British Army pilots. In 1957, the newly formed Army Air Corps took over the role, aeroplanes eventually being replaced by helicopters.