GPWS / EGPWS — Ground Proximity Warning System / Enhanced Ground Proximity Warning System

What GPWS Does

The Ground Proximity Warning System monitors an aircraft's height above terrain, typically using a radio altimeter, along with parameters such as descent rate and configuration, to generate aural and visual alerts when the aircraft is dangerously close to the ground in situations that suggest an unintended descent. GPWS is fundamentally reactive: it detects an unsafe condition that already exists and warns the crew, who must then respond, usually with a maximum-performance climb.

GPWS Alerts and Warnings

What EGPWS Adds

Enhanced GPWS, also referred to as part of a broader Terrain Awareness and Warning System (TAWS), adds a real-time comparison between the aircraft's GNSS-derived position and an internal digital terrain database. This allows the system to look ahead along the aircraft's flight path and provide predictive terrain warnings before the aircraft is already in a hazardous position, rather than only reacting once it is close to terrain.

Regulatory Requirements

In the United States, 14 CFR 121.354 requires most turbine-powered airplanes operated under Part 121 to be equipped with an approved terrain awareness and warning system, reflecting the safety benefit these systems have demonstrated in reducing controlled-flight-into-terrain accidents. Similar equipage requirements exist under other civil aviation authorities' operating rules.

Why It Matters

Controlled-flight-into-terrain, where an airworthy aircraft under crew control is flown into terrain, water, or an obstacle, was historically one of the leading causes of fatal airline accidents. The introduction of GPWS, and later the forward-looking capability of EGPWS/TAWS, is widely credited by safety authorities and industry bodies with substantially reducing this category of accident on aircraft equipped with the technology.

Crew Response

Because GPWS and EGPWS warnings are time-critical, standard operating procedures require an immediate, defined escape maneuver whenever a warning is triggered, without first trying to visually confirm the terrain threat, given how little margin may remain by the time the warning sounds.