A terrain awareness and warning system (TAWS) aims to prevent "Controlled Flight Into Terrain" (CFIT) accidents.[1] The actual systems in current use are known as ground proximity warning system and enhanced GPWS.[1] The U.S. Federal Aviation Administration (FAA) developed the TAWS term to encompass all current and future systems which meet the relevant FAA standards.[1] 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.[2]
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.[3]
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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.[4]
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.[4][5]
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.[6] 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.[4]
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.[4]
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.[4]
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 (57 FR 9944, March 20, 1992).[4]
After these rules were issued, advances in terrain mapping technology permitted the development of a new type of ground proximity warning system that provides greater situational awareness for flight crews. The FAA has approved certain installations of this type of equipment, known as the enhanced ground proximity warning system (EGPWS). However, in the proposed final rule, the FAA is using the broader term “terrain awareness and warning system” (TAWS) because the FAA expects that a variety of systems may be developed in the near future that would meet the improved standards contained in the proposed final rule.[4]
The TAWS improves on existing GPWS systems by providing the flight crew much earlier aural and visual warning of impending terrain, forward looking capability, and continued operation in the landing configuration. These improvements provide more time for the flight crew to make smoother and gradual corrective action.[4]
In 1998, the FAA issued Notice No. 98-11, Terrain Awareness and Warning System (63 FR 45628, August 26, 1998), proposing that all turbine-powered U.S.-registered airplanes type certificated to have six or more passenger seats (exclusive of pilot and copilot seating), be equipped with an FAA-approved terrain awareness and warning system.[4]
On March 23, 2000, the FAA issued Amendments 91–263, 121–273, and 135-75 (65 FR 16736, March 29, 2000) with an effective date of March 29, 2001. These amendments amended the operating rules to require that all U.S. registered turbine-powered airplanes with six or more passenger seats (exclusive of pilot and copilot seating) be equipped with an FAA-approved TAWS.[4] The mandate only affects aircraft manufactured after March 29, 2002.[7]
A TAWS works by using digital elevation data and airplane instrumental values to predict if a likely future position of the aircraft intersects with the ground.[8] The flight crew is thus provided with "earlier aural and visual warning of impending terrain, forward looking capability, and continued operation in the landing configuration."[9]
Class A TAWS includes all the requirements of Class B TAWS, below, and adds the following additional three alerts and display requirements of:
Class B TAWS is defined by the U.S. FAA[2][11] as:
A class of equipment that is defined in TSO-C151b[12] and RTCA DO-161A. As a minimum, it will provide alerts for the following circumstances:
Class C defines voluntary equipment intended for small general aviation airplanes that are not required to install Class B equipment.[13] This includes minimum operational performance standards intended for piston-powered and turbine-powered airplanes, when configured with less than six passenger seats, excluding any pilot seats. Class C TAWS equipment shall meet all the requirements of a Class B TAWS with the small aircraft modifications described by the FAA.[13] The FAA has developed Class C to make voluntary TAWS usage easier for small aircraft.[14]
In April 2010, a Polish Air Force Tupolev Tu-154M aircraft crashed near Smolensk, Russia, in a possible CFIT accident[15] killing all passengers and crew.[16][17][18][19] The aircraft was equipped with TAWS made by Universal Avionics Systems of Tucson.[16] According to the transcripts of cockpit conversation[20] provided by Russian Interstate Aviation Committee TAWS was turned on.[21] However, the airport where the aircraft was going to land (Smolensk (XUBS)) was not in the TAWS database.[22][23] The transcript shows TAWS gave the crew clear multiple warnings that were apparently ignored. There is a method of setting up the TAWS to prevent false warnings when flying into airports not in the database ("Terrain inhibit" mode) but it was not used in accident. In that mode some of warning signals are not generated (landing requires contact with ground), however excessive rate of descent and excessive airspeed can cause the TAWS to issue a "SINK RATE" followed by "PULL UP" warning,[24] The remains of the TAWS unit from this catastrophe were taken to the US for examination by its manufacturer and apparently no abnormalities were found.
In January 2008 a Polish Air Force Casa C-295M crashed in a CFIT accident near Mirosławiec, Poland, despite being equipped with EGPWS; the EGPWS warning sounds had been disabled, and the pilot-in-command was not properly trained with EGPWS.[25] The U.S. FAA has also conducted a study about civilizing 3D military thrust vectoring to recover jet liners from catastrophes.[26]
In February 2007, an EGPWS-equipped King Air 200 crashed in a CFIT accident near Bozeman, MT. The NTSB report said that the pilot descended below the 9,100-foot minimum obstruction clearance altitude while landing on a dark, overcast night. Damage prevented post-accident testing of the EGPWS unit, which was capable of issuing audible warnings but did not have a terrain display. Damage from the accident and a lack of cockpit voice recorder prevented investigators from determining whether the EGPWS unit issued any warnings, or even if it was switched on. EGPWS maker Honeywell said there is no evidence that the unit in the King Air failed to perform as designed.[27]