Ground Controlled Approach

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, including in adverse weather conditions, based on primary radar images. Most commonly a GCA uses information from either a Precision Approach Radar (PAR, for precision approaches with vertical, glidepath guidance) or an Airport Surveillance Radar (ASR, providing a non-precision Surveillance radar approach with no glidepath guidance). The term GCA may refer to any type of ground radar guided approach such as a PAR, PAR without glideslope or ASR. When both vertical and horizontal guidance from the PAR is given, the approach is termed a precision approach. If no PAR glidepath is given, even if PAR equipment is used for lateral guidance, it is considered a non-precision approach

Ground-controlled approach is the oldest air traffic technique to fully implement radar to service a plane. The system was simple, direct, and worked well, even with previously untrained pilots. 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 precision approach radar systems, to determine the precise course 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 descent rate (glidepath) and heading (course) corrections necessary to follow the correct approach path.

Two tracks are displayed on the Precision Approach Radar (PAR) scope:

By following controller commands to keep the landing aircraft on both glidepath and approach centerline, a pilot will arrive precisely over the runway's touchdown zone. In order to insure continuous radio communication integrity, controllers are required to make radio transmissions at certain minimum intervals depending on the type of approach flow and phase of the approach. In order to land, pilots must have the runway or runway environment in sight prior to reaching the "decision height," for PAR approaches (usually 100–400 ft above the runway touchdown zone) or prior to the "Missed Approach Point" for non-precision approaches. The published minimum visibility and decision height/minimum descent altitude vary depending upon the approach and runway lighting, obstacles in the approach corridor, type of aircraft, and other factors. Pilots of revenue flights periodically must demonstrate PAR approach proficiency, and GCA controllers must conduct a minimum number of such approaches in a year to maintain competency.

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