Definition
An onboard avionics system that monitors an aircraft's height above terrain using radio altimeter data and other inputs, and issues aural and visual alerts to the flight crew when the aircraft is in danger of unintentional contact with the ground or obstacles. Typical alerts cover excessive sink rate, excessive terrain closure rate, altitude loss after takeoff or go-around, unsafe terrain clearance while not in landing configuration, and excessive deviation below the glideslope.
Plain English
A safety system in the cockpit that watches how close the aircraft is to the ground and warns the pilots — usually with a loud spoken alert — if it looks like they're about to hit terrain without meaning to.
Context Anchor
Seen in aircraft equipment descriptions and heard as cockpit alerts during low-altitude flight, approaches, night flying, or flight in poor visibility.
Derivation
“Proximity” comes from a Latin word meaning “nearness.” In this term, it means nearness to the ground, so the phrase points to a system that warns about dangerous closeness to terrain.
Why Pilots Care
It provides the last line of defense against controlled flight into terrain, one of the leading causes of fatal accidents.
Grounding Statement
Picture descending in cloud toward rising ground: the GWPS is the cockpit system that warns you before the terrain is visible.
Intuition Check
Do not read “ground proximity” as a normal reminder that the airplane is near the ground during landing. It means the system has detected a possibly unsafe closeness to terrain or an unsafe descent toward it.
Example Sentence 1
After the GPWS called 'TOO LOW, TERRAIN,' the captain added full power and pitched up until the alert stopped.
Example Sentence 2
Before departure the pilot verified that the GWPS was armed and receiving valid altitude data.