Terrain awareness and warning system

In aviation, a terrain awareness and warning system (TAWS) is generally an on-board system aimed at preventing unintentional impacts with the ground, termed "controlled flight into terrain" accidents, or CFIT. The specific systems currently in use are the ground proximity warning system (GPWS) and the enhanced ground proximity warning system (EGPWS). The U.S. Federal Aviation Administration (FAA) introduced the generic term TAWS to encompass all terrain-avoidance systems that meet the relevant FAA standards, which include GPWS, EGPWS and any future system that might replace them. As of 2007, 5% of the world's commercial airlines still lacked a TAWS. A study by the International Air Transport Association examined 51 accidents and incidents and found that pilots did not adequately respond to a TAWS warning in 47% of cases. 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.

History

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. In the 1960s and 70s, there was an average of one CFIT accident per month, and CFIT was the single largest cause of air travel fatalities during that time. C. Donald Bateman, an engineer at Honeywell, is credited with developing the first ground proximity warning system (GPWS); in an early test, conducted after the 1971 crash of Alaska Airlines Flight 1866, the device provided sufficient warning for a small plane to avoid the terrain, but not enough for the larger Boeing 727 jetliner involved. Bateman's earliest devices, developed in the 1960s, used radio waves to measure altitude and triggered an alarm when the aircraft was too low, but it was not aimed forward and could not provide sufficient warning of steeply rising terrain ahead.

Early GPWS mandates Findings from these early studies indicated that many such accidents could have been avoided if a 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 14 CFR 121 (Part 121) certificate holders (that is, those operating large turbine-powered airplanes) and some 14 CFR 135 (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. Installation of GPWS or alternative FAA-approved advisory systems was not required on turbo-propeller powered (turboprop) airplanes operated under Part 135 because, at that time, the general consensus was that the performance characteristics of turboprop airplanes made them less susceptible to CFIT accidents. For example, it was thought that turboprop airplanes had a greater ability to respond quickly in situations where altitude control was inadvertently neglected, as compared to turbojet airplanes. However, later studies, including investigations by the NTSB, analyzed CFIT accidents involving turboprop airplanes and found that many of these accidents could have been avoided if GPWS equipment had been used. Some of these studies also compared the effectiveness of the alternative ground proximity advisory system to the GPWS. GPWS was found to be superior in that it would warn only when necessary, provide maximum warning time with minimal unwanted alarms, and use command-type warnings. Based on these reports and NTSB recommendations, in 1992 the FAA amended §135.153 to require GPWS equipment on all turbine-powered airplanes with ten or more passenger seats.

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