Ground Proximity Warning System

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Ground Proximity Warning System (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground. Another common name for such a system is Ground-Collision Warning System (GCWS).

The system monitors an aircraft's height above ground as determined by radio altimeter. A computer then keeps track of these readings, calculates trends, and will warn the captain with visual and audio messages if the aircraft is in certain defined flying configurations ("modes").

The modes are:

Excessive descent rate ("PULL UP" "SINKRATE")
Excessive terrain closure rate ("TERRAIN" "PULL UP")
Altitude loss after take off ("DON'T SINK")
Unsafe terrain clearance ("TOO LOW - TERRAIN" "TOO LOW - GEAR" "TOO LOW - FLAPS")
Excessive deviation below glideslope ("GLIDESLOPE")

Don Bateman, a Canadian born engineer is credited with the invention of GPWS. He spearheaded the development of GPWS in the late 1960s after a series of controlled flight into terrain (CFIT) accidents killed hundreds of people.

Prior to the development of GPWS there were an average of over 10 CFIT crashes a year. Since the U.S. Federal Aviation Administration required large airplanes to carry such equipment in 1974, the number of CFIT accidents have reduced significantly. In 2000 the FAA extended this requirement to smaller commuter planes as well.

Traditional GPWS does have a blind spot. Since it can only gather data from directly below the aircraft, it must predict future terrain features. If there is a dramatic change in terrain, such as a steep slope, GPWS will not detect the aircraft closure rate until it is too late for evasive action.

A new technology, the Enhanced Ground Proximity Warning System (EGPWS) solves this problem by combining a worldwide digital terrain database with a Long-Range Navigation System such as Global Positioning System, INS (Inertial Navigation System), Radio-Dependent navigational systems, or a combination of the above. On-board computers can compare its current location with a database of the Earth's terrain. Pilots will receive much more timely cautions and warnings of any obstructions to the aircraft's path.

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