Unlike the permanent airfields built in the United Kingdom and designed for the strategic bombardment of Germany, the tactical combat airfields on the continent were temporary, often improvised airfields to be used by the tactical air forces to support the advancing ground armies engaged on the battlefield. Once the front line moved out of range for the aircraft, the groups and squadrons moved up to newly built ALGs closer to the ground forces and left the ones in the rear for other support uses, or simply abandoned them.
When the Allies invaded Normandy on D-Day, Royal Air Force Airfield Construction Service engineers were among those in the initial assault waves. Their mission was to rapidly construct forward operating airfields, known as Advanced Landing Grounds (ALGs), on the European continent. As the Allied armies advanced across France and into Germany, several hundred airfields were built or rehabilitated for use by the allied air forces.
For security reasons, the airstrips were referred to by a coded number instead of location. In the United Kingdom, USAAF installations were identified by three digit (AAF) numbers ranging from AAF-101 to AAF-925. After D-Day, continental airfields in the European Theater of Operations (ETO) were also assigned coded numbers. American airfields were given A-, Y-, or R-, prefixes and numbered consecutively from 1 to 99. Both "A" and "Y" designated airfields could be found in France, however many "Y" fields would also be in France, as well as the Netherlands; Belgium and occupied areas of Germany. "R" coded fields were usually located in occupied Germany. British airfields on the continent were also consecutively numbered, but with a B-prefix.
The numbering system for airfields was sequentially assigned as airfields were allocated, not by location or by date of operational use. A-1, Saint Pierre du Mont, was declared operational on 13 June 1944; A-3 Cardonville on 14 June. However A-2, Cricqueville-en-Bessin, was declared operational a few days later on 19 June.
Also many of these airfields had no combat air group or squadron attached to them. They were designed for casualty evacuation and supply transport and consisted of a quickly built runway manned only by a small complement of station personnel with little or no infrastructure other than tents. As the ground forces moved east, wounded would be sent to the airfield to be picked up by C-47s and taken to hospitals in England or other rear areas. Also supplies would be airlifted to the fields and unloaded, to be quickly transported to the front line units. These were normally known as S&E Fields (Supply and Evacuation).
Once completed, airfields were usually utilised by the combat groups or squadrons within a day or so of being declared operational for military use by the IX Engineering command engineers. They would be used for perhaps a few days to a week, to several months, depending on the location, use, and operational requirements. Once the combat units moved up to the next assigned ALG, they could be utilised as S&E Fields, or deconstructed quickly and abandoned, with the land being released back to the landowners or civil authorities in the area.