Definition
An onboard safety system that monitors the aircraft's height above terrain using a radar altimeter and other inputs, and alerts the flight crew with audible and visual warnings when the aircraft is in danger of unintended contact with the ground. Typical alerts cover excessive sink rate, excessive terrain closure rate, altitude loss after takeoff, unsafe terrain clearance while not in the landing configuration, and excessive deviation below an instrument glideslope.
Plain English
A system that watches how close the aircraft is to the ground and shouts a warning if it looks like the airplane is about to hit terrain when it shouldn't.
Context Anchor
Seen in instrument flying, approach, and low-altitude operations, especially when discussing aircraft warning systems and terrain safety.
Derivation
Ground means the earth’s surface. Proximity comes from a Latin word meaning nearness. Warning means an alert before danger. Together, the phrase points to a system that warns when the aircraft is too near the ground.
Why Pilots Care
It provides the last line of defense against controlled flight into terrain, a leading cause of fatal accidents in instrument conditions.
Analogy
It is like a car’s collision warning system, but for the aircraft’s nearness to the ground instead of another car.
Intuition Check
Do not assume GPWS simply shows altitude. It is a warning system that looks for unsafe closeness to the ground or obstacles and alerts the pilot when action may be needed.
Example Sentence 1
On final approach, the GPWS called out 'sink rate, sink rate' and the pilot reduced the descent rate before continuing the landing.
Example Sentence 2
The crew responded immediately to the GPWS warning by initiating a climb.