Terrain Awareness and Warning System (TAWS): Meaning, Function, Difference

Terrain Awareness and Warning System (TAWS) is a passive, electronic, safety net located in the avionics bay of the airplane. By providing the flight crew much earlier aural and visual warning of impending terrain, the system alerts pilots when they approach potentially hazardous ground, and advises the flight crew when inadvertent impact with terrain is predicted.

TAWS is an on-board system aimed at preventing unintentional impacts with the ground by delivering advanced alerts of dangerous terrain conditions ahead of the aircraft, rather than relying only on the radio altimeter reading and terrain closure rates as its predecessor GPWS once did.

Class A TAWS places an airplane symbol on a digital terrain map, whereas the earlier Ground Proximity Warning System (GPWS) is defined by the FAA as a type of terrain awareness and warning system (TAWS).

Expert behind this article

Jim Goodrich

Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.

What does TAWS stand for in aviation?

TAWS stands for Terrain Awareness and Warning Systems, which is an on-board system, and all have been generically identified by ICAO as Terrain Awareness and Warning Systems. The FAA is using the broader term 'terrain awareness and warning system'.

TAWS is a generic term for terrain-avoidance systems - Terrain Awareness and Warning System. It is a safety net that automatically provides distinctive warning to pilots when the aircraft is in a dangerous situation relative to terrain. A TAWS alert indicates an abnormal status in relation to terrain and invites a possible change of flight path or aircraft configuration. Enhanced versions like TAWS+ from ACSS bring advanced terrain prediction and avoidance technology to the cockpit, while Helicopter-TAWS (HTAWS) refers to TAWS intended for helicopter operations and alerts the crew of imminent danger. Because CFIT - controlled flight into terrain - has been a consistent leader of aviation-related fatalities, TAWS is installed on newly manufactured aircraft and is governed by TSO-151c standards. In military use the same terrain awareness warning system principles apply.

How does TAWS work in aviation?

Modern TAWS works by using digital elevation data and airplane instrumental values. It pulls aircraft position, speed and direction data from GPS and monitors altitude. The system compares this data to a database of Earth's terrain and manmade obstacles that results from a combination of radar topography imagery taken by the U.S. space shuttle program, Department of Defense information, and other data. TAWS integrates GPS data, terrain databases, radar altimeters, and aircraft performance information to generate predictive warnings about potential terrain hazards. The system predicts a likely future position of the aircraft intersecting with the ground and calculates the potential risk of controlled flight into terrain (CFIT).

TAWS alerts pilots when it detects a possible conflict with terrain. The system provides distinctive visual and auditory alerts when it detects a possible conflict. These alerts invite prompt review and a possible change of flight path or aircraft configuration. A hard warning indicates that the aircraft is in a dangerous situation. The system monitors the aircraft's position, altitude, and flight path continuously. TAWS uses forward-looking capability and envelope modulation to reduce nuisance mode 2 alerts while maintaining its primary safety function.

When is TAWS required in aviation?

TAWS is required for most aircrafts. Class A TAWS systems are required for all but the smallest commercial air transport aircraft. Class B TAWS systems are required by larger General Aviation aircraft and recommended for smaller commercial or GA aircraft. Class B TAWS equipment is required for turbine-powered airplanes operated under part 91 with six or more passenger seats, and for part 135 airplanes with six to nine passenger seats. Class A equipment is required for part 135 airplanes with ten or more seats. Airplanes manufactured on or before March 29, 2002, must meet Class B requirements for six to nine passengers.

The European regulatory framework mandates HTAWS for helicopters in commercial air transport if heavier than 3,175 kg (7,000 lbs) or with a MOPSC of more than nine.

What is TAWS inhibit in aviation?

TAWS inhibit refers to the terrain inhibit feature, which flight operations management and pilots use to manage risks associated with distraction from continuous alerts and complacency. Inhibiting TAWS prevents the system from issuing alerts. The crew selects the TAWS inhibit switch, a physical switch that is not emulated, while the aircraft is on the ground or in flight so that nuisance call-outs or hard warnings are silenced. While the inhibit switch cuts power to system audio, the visual terrain display remains available. TAWS still provides visual terrain display when inhibited.

For safety of flight, any TAWS aural alert that is already in progress will automatically inhibit all aural TRAFFIC alerts. When any of the TAWS aural alerts are in progress, all aural TRAFFIC alerts are inhibited.

Because the Flight Data Recorder cannot record an alert that was not issued, excessive or prolonged inhibition removes evidence of terrain conflict. When no alert is issued FDR cannot record it. Recognizing that CFIT accidents are once again on the rise, regulators issue an InFO to inform operators about risks associated with distraction and complacency brought about by routine use of the TAWS terrain inhibit feature. Review of training programs ensures procedures for use of terrain warning system inhibit switches are adequately addressed, because early warning gives pilots sufficient time and information to make adjustments. A hard warning indicates that the aircraft is in a dangerous situation and immediate action is required.

When is it permissible to inhibit TAWS in aviation?

It is permissible to inhibit TAWS during emergencies. Emergency procedures allow pilots to prevent unwanted aural alerting during ditching or other off-airport landings by selecting the INHIBIT Softkey on the TAWS Page. When approaching an airport absent from the terrain database, the inhibit switch suppresses nuisance alerts. The inhibition condition is then automatically cleared upon landing so that TAWS is enabled at the start of the next flight.

During radar-vectored flight or after a ‘direct-to’ clearance, terrain clearance becomes the controller's responsibility, and the crew temporarily inhibits the system. Proposed logic limits this inhibition to a short, fixed period like five minutes, after which the inhibit is removed.

Under day VFR conditions, terrain warnings are regarded as cautionary rather than mandatory, and NTSB Safety Recommendation A-18-014 notes that continuous nuisance alerts distract crews operating under Part 135 VFR in mountainous terrain. In such cases, the pilot's decision to inhibit the system is recognised, provided the inhibit duration is restricted and crews demonstrate knowledge of TAWS limitations.

What is the difference between GPWS and TAWS?

Ground Proximity Warning System (GPWS) is the original terrain-avoidance system introduced in the 1970s to combat the high incidence of controlled flight into terrain (CFIT) accidents. It alerts pilots if their aircraft is in immediate danger of flying into the ground or an obstacle, using radio altimeters to monitor aircraft height above terrain and to determine the rate of closure with the ground. Alerts are issued when unsafe combinations of radio altitude, barometric altitude, airspeed, glide-slope deviation and airplane configuration are detected, typically producing the aural warning ‘whoop whoop, PULL UP’. Because basic GPWS lacks a terrain database and any forward-looking capability, it cannot generate sufficient warning when approaching sharply rising terrain. The alert occurs at a time of high workload and nearly always surprises the flight crew.

Terrain Awareness and Warning System (TAWS) is the regulatory umbrella term defined by the FAA. GPWS is a type of TAWS, and Enhanced GPWS (EGPWS) is an advanced type of TAWS. EGPWS overcomes the shortcomings of basic GPWS by including an internal worldwide terrain and obstacle database and by matching aircraft GPS position against this database. It provides predictive terrain warnings, look-ahead terrain alerts, windshear alerts, and a comprehensive view of the terrain. The Terrain Clearance Floor (TCF) function introduced in EGPWS gives GPWS protection even in the landing configuration, and the system can anticipate potential conflicts long before they pose an immediate threat. Thus, while basic GPWS reacts to imminent impact, EGPWS - functioning as a modern TAWS - provides earlier aural and visual warning of terrain that lies ahead at all stages of flight.

Ground Proximity Warning System supplied no early visuals of the surrounding terrain and left me to depend exclusively on instruments and maps. Terrain Awareness and Warning System offered better situational consciousness. The monitor presented a clean, color-coded representation of the terrain, so the warning was provided in advance allowing better decision-making.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the FAA TAWS requirements in aviation?

The FAA TAWS requirements in aviation are outlined below.

• The FAA TAWS requirements specify that Class A TAWS equipment must provide indications for excessive rates of descent and excessive closure rate to terrain.
• FAA TAWS requirements dictate that Part 121 operators must have an approved terrain awareness and warning system including a terrain awareness display meeting Class A equipment standards of TSO-C151a.
• FAA TAWS requirements necessitate that the applicant provide evidence that the TAWS meets the minimum operational performance standards (MOPS) for Terrain Awareness and Warning Systems (TAWS) Airborne Equipment (RTCA DO-367).
• The FAA TAWS requirements specify that Class A TAWS equipment must provide indications for excessive downward deviation from an ILS glideslope.
• FAA TAWS requirements specify that the terrain awareness and warning system must meet the standards for Class B equipment in Technical Standard Order (TSO)-C151a.
• The FAA TAWS requirements require that turbine-powered U.S.-registered airplanes with ten or more passenger seats must have an approved terrain awareness and warning system meeting Class A equipment standards.
• FAA TAWS requirements mandate that the Airplane Flight Manual contain procedures for using the terrain awareness and warning system and proper flight crew reactions to warnings.
• FAA TAWS requirements specify that Class A TAWS equipment must provide indications for negative climb rate or altitude loss after takeoff.
• The FAA TAWS requirements stipulate that TAWS software must meet Level C of RTCA DO-178B or an acceptable alternative.
• FAA TAWS requirements specify that Class B TAWS equipment must incorporate either an integral GPS or interface with an independent approved GPS for horizontal position information.
• The FAA TAWS requirements specify that Class A TAWS equipment must provide indications for various conditions including excessive closure rate to terrain.
• The FAA TAWS requirements specify that Class B TAWS equipment must provide aural alert SINKRATE for excessive descent rate.
• FAA TAWS requirements specify that TAWS alerting thresholds for Reduced Terrain Clearance for Class A and Class B systems are 700 feet under TSO C151c and earlier versions.
• FAA TAWS requirements mandate a terrain awareness display for part 135 operations conducted with airplanes configured with ten or more passenger seats.
• The FAA TAWS requirements specify that Class B TAWS equipment must include a minimum ground clearance plane function.
• FAA TAWS requirements mandate the installation of a TAWS (the EGPWS) providing initial alerts approximately 60 seconds before potential impact.
• The FAA TAWS requirements specify that Class C TAWS equipment shall meet all requirements of Class B TAWS.
• FAA TAWS requirements specify that Class B TAWS equipment must not require an interface to a radio altimeter.
• FAA TAWS requirements specify that Class B TAWS equipment does not require a display but must meet provisions if one is installed.
• The FAA TAWS requirements recognize that TAWS software should be developed at least to Level C as defined in RTCA DO-178B.