GPWS (Ground Proximity Warning System)

A ground proximity warning system (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle.

More advanced systems, introduced in 1996, are known as enhanced ground proximity warning systems (EGPWS) .sometimes confusingly called terrain awareness warning systems.

The system monitors an aircraft's height above ground as determined by a radar 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:

1. Excessive descent rate ("PULL UP" "SINKRATE")
2. Excessive terrain closure rate ("TERRAIN" "PULL UP")
3. Altitude loss after take off or with a high power setting ("DON'T SINK")
4. Unsafe terrain clearance ("TOO LOW - TERRAIN" "TOO LOW - GEAR" "TOO LOW - FLAPS")
5. Excessive deviation below glideslope ("GLIDESLOPE")
6. Excessively steep bank angle ("BANK ANGLE")
7. Windshear protection ("WINDSHEAR")

In Commercial and Airline operations there are legally mandated procedures that must be followed should an EGPWS caution or warning occur. Both pilots must respond and act accordingly once the alert has been issued.