Ground-controlled approach
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In aviation a ground-controlled approach (GCA), is a type of service provided by air-traffic controllers whereby they guide aircraft to a safe landing in adverse weather conditions based on radar images. Most commonly a GCA uses information from either a Precision Approach Radar (PAR) (for precision approaches) or an Airport Surveillance Radar (ASR) (for a non-precision approach).
[edit] Overview
Ground-control approach is the oldest air traffic technique to fully implement radar to service a plane - it was largely used during the Berlin airlift in 1948-49. It requires close communication between ground-based air traffic controllers and pilots in approaching aircraft. Only one pilot is guided at a time (max. 2 under certain circumstances). The controllers monitor dedicated radar displays, such as those of precision approach radar systems, to determine the precise heading and altitude of approaching aircraft. The controllers then provide verbal instructions by radio to the pilots to guide them to a landing. The instructions include both altitude and course corrections necessary to follow the correct approach path. In fact there are two tracks: a horizontal one (azimuth or runway extension or final track) and a vertical one (glide path) that a landing aircraft should be on to complete a landing. Controllers issue position information and/or correction for both of them every five seconds. The guidance is stopped at landing threshold, but pilots must get visual contact with the ground by decision height (about three quarters of a mile from the threshold). Pilots and GCA controllers must complete a minimum number of such approaches in a year to maintain competency.
GCAs are no longer in widespread use. However, military air traffic controllers in the United States are required to maintain currency in their use, while the Belgian Air Force still uses the PAR for ground-controlled approaches on a daily basis. NATO has kept GCA active for a long period while civil aviation adopted the instrument landing system (ILS) early. Modern ILS eliminates the possibility of human error from the controller, and can serve many aircraft at the same time without requiring any previous pilot training. The ground-controlled approach is quite useful when the approaching aircraft is not equipped with sophisticated navigation aids, and may also become a life saver when an aircraft has electrical problems and has no functioning navigational aids, as long as one radio works. Sometimes the PAR-based ground-controlled approach is also requested by qualified pilots when they are dealing with an emergency on-board to lighten their workload.
