Ground-Controlled Approach
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In aviation a Ground-Controlled Approach, or GCA, is a type of precision instrument approach, used to guide aircraft to a safe landing in adverse weather conditions.
[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 seige of Berlin around 1946. It does involve close communication between ground-based air traffic controllers and pilots in approaching aircraft : actually one only pilot is guided at a time. The controllers monitor special 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 down to a landing. The instructions include both altitude and course corrections necessary to follow the correct approach path. In fact there are two tracks , an horizontal (azimuth or runway extension or final track) and a vertical one (glide path) that a landing traffic should be on to complete a landing. Controllers issue position information and/or correction for both of them every 15 seconds. The guidance is stopped at about 2 miles from the runway threshold where pilots must get visual contact with the ground. Pilots and GCA conttrollers must complete a minimum number of such approaches in a year to maintain competency.
GCAs are no longer in widespread use, although military air traffic controllers in the United States are required to maintain currency in their use. NATO has kept GCA active for a long period while civil aviation reverted early to the ILS system. In actual fact modern ILS is more precise that GCA and can serve many aircraft at the same time without requiring any previous pilot training but this type of approach is : a) quite useful when the approaching aircraft is not equipped with sophisticated navigation aids and b) may also become a life saver when an aircraft has experienced electrical problems and no longer has functioning navaids, as long as one radio still works and/or under poor visibility, in the absence of any other landing aid.
