Terrain awareness and warning system

A terrain awareness and warning system (TAWS) aims to prevent "controlled flight into terrain" (CFIT) accidents. The actual systems in current use are known as ground proximity warning system and enhanced GPWS. The U.S. Federal Aviation Administration (FAA) developed the TAWS term to encompass all current and future systems which meet the relevant FAA standards. New systems, with different names than GPWS and EGPWS, may be developed which meet TAWS objectives. As well, the term TAWS is sometimes used to refer to second-generation EGPWS systems as opposed to first generation GPWS systems, although this usage can be confusing.

As of 2007, 5% of the world's commercial airlines still lack a TAWS, leading to a prediction of two CFIT accidents in 2009, even though they were completely preventable.

Several factors can still place aircraft at risk for CFIT accidents: older TAWS systems, deactivation of the EGPWS system, or ignoring TAWS warnings when an airport is not in the TAWS database.

Beginning in the early 1970s, a number of studies looked at the occurrence of CFIT accidents, where a properly functioning airplane under the control of a fully qualified and certificated crew is flown into terrain (or water or obstacles) with no apparent awareness on the part of the crew.

Findings from these studies indicated that many such accidents could have been avoided if a warning device called a ground proximity warning system (GPWS) had been used. As a result of these studies and recommendations from the U.S. National Transportation Safety Board (NTSB), in 1974 the FAA required all Part 121 certificate holders (that is, those operating large turbine-powered airplanes) and some Part 135 certificate holders (that is, those operating large turbojet airplanes) to install TSO-approved GPWS equipment.

In 1978, the FAA extended the GPWS requirement to Part 135 certificate holders operating smaller airplanes: turbojet-powered airplanes with ten or more passenger seats. These operators were required to install TSO-approved GPWS equipment or alternative ground proximity advisory systems that provide routine altitude callouts whether or not there is any imminent danger. This requirement was considered necessary because of the complexity, size, speed, and flight performance characteristics of these airplanes. The GPWS equipment was considered essential in helping the pilots of these airplanes to regain altitude quickly and avoid what could have been a CFIT accident.

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