Ground Proximity Warning System
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Ground Proximity Warning System (GPWS) monitors an aircraft's height above ground as determined by a 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 decent 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")
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 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-Dependant navigational systems, or a combination of the above. Onboard 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.