ground proximity warning system

A ground proximity warning system is an automated instrument that monitors aircraft altitude, descent rate, and terrain ahead, then alerts the flight crew if the plane is in danger of striking the ground or an obstacle. Modern variants are called terrain awareness and warning systems (TAWS), though the older term GPWS persists in maintenance documentation. The system uses radio altimetry, barometric data, and in newer versions, a terrain database to compute safe flight envelopes and trigger warnings before a collision becomes inevitable.

The system generates two classes of alert: a "whoop whoop, pull up" voice warning paired with a ground-proximity aural tone, or a quieter "terrain" advisory depending on the severity and type of threat detected. Radio altimeter input provides altitude above ground in the range of 0 to 2,500 feet, critical for detecting low-altitude flight into rising terrain that barometric altimetry alone would miss. Descent rate monitoring catches uncommanded descents; if the aircraft drops faster than expected at a given altitude, the system triggers even if terrain plotting shows clearance.

GPWS installation became mandatory on large transport aircraft after a series of controlled flight-into-terrain (CFIT) accidents in the 1960s and 1970s, many occurring to skilled crews in visual meteorological conditions. Early systems were prone to nuisance warnings during legitimate approaches to high-altitude or mountain-proximity airports, generating pilot distrust. Modern TAWS systems incorporate terrain databases covering the entire flight path and allow pilots to inhibit warnings during known approaches, significantly reducing false alerts while maintaining protection against inadvertent deviations.

The system depends entirely on radio altimeter function; if the altimeter fails or becomes unreliable in heavy precipitation or over water, warnings may be suppressed or delayed. Maintenance checks verify altimeter output against barometric altitude at multiple heights and confirm the warning logic operates correctly. Mode-C transponder data is also integrated on equipped aircraft, allowing ground-based conflict alerts (traffic collision avoidance systems) to coordinate with onboard terrain warnings.

A ground proximity warning system is distinct from windshear detection, which monitors vertical air motion, and from traffic collision avoidance systems, which detect other aircraft. GPWS or TAWS remains mandatory equipment on commercial aircraft over 12,500 pounds maximum takeoff weight in most jurisdictions, and its presence is verified before every flight as part of pre-departure systems checks.