Terrain Awareness and Warning System (TAWS) is the generic name for equipment that alerts the flight crew in a timely manner of terrain proximity ahead of the aircraft in order to avoid Controlled Flight Into Terrain (CFIT). Class A equipment provides every function of Class B, adds a terrain awareness display to the aircraft's display system, and contains a fully autonomous Ground Proximity Warning System (GPWS), while Class B may provide a terrain awareness display that shows either the surrounding terrain or obstacles relative to the airplane, or both. The FAA's TSO-151c codifies these distinctions, requiring Class A installations to supply both the display and the autonomous GPWS, whereas Class B units are permitted to offer the display as an option rather than a mandate.
What is the classification of TAWS?
The classification of TAWS is given below.
- Class A TAWS
- Class B TAWS
- Class C Taws
What is Class A TAWS?
Class A TAWS is an equipment that is installed on all but the smallest commercial transport aircraft and combines forward-looking, classic and display functions. The equipment provides Forward Looking Terrain Avoidance (FLTA) and Terrain Clearance Floor (TCF) modes that issue early aural and visual warnings of impending terrain and obstacle impact by comparing predicted flight path against a worldwide terrain database. It uses radio altimeter and barometric altitude information to perform classic GPWS modes 1 to 6, delivering alerts for excessive descent rate, altitude loss after take-off, inadvertent descent below glideslope, excessive bank angle, excessive closure rate to terrain, reduced required terrain clearance and other imminent ground-contact situations.
Voice callouts like ‘Five Hundred’ mark 500 ft (152.4 m) above terrain or nearest runway elevation, and attention alerts are produced from temperature-compensated GPS altitudes. The system places an airplane symbol on a digital terrain map, applies terrain-display algorithms and drives color-coded, high-resolution terrain and obstacle information to an MFD, EFIS or other compatible indicator, thereby offering pilots a visual plan view of the surrounding world with terrain and runway data. Class A equipment supports steep-approach functionality and some installations add windshear, premature descent and smart bank-angle alerts, meeting or exceeding Class B approval while providing appropriate caution and warning visual/aural signals.
What are the requirements for Class B TAWS?
Class B TAWS requirements include that it must provide indications of imminent contact with the ground for excessive rates of descent, negative climb rate or altitude loss after takeoff, and a voice callout ‘Five Hundred’. Class B TAWS is required for turbine-powered airplanes operated under part 91 with six or more passenger seats. Class B TAWS equipment includes a Forward Looking Terrain Avoidance (FLTA) function, a Premature Descent Alert (PDA) function, and Basic GPWS functions. Turbine powered airplanes having an MCTOM of less than 5700 kg must be equipped with a TAWS that meets the requirements for Class B equipment. Class B TAWS equipment must be capable of driving a terrain display function if a display is installed.
No person is allowed to operate a turbine-powered airplane configured with 6 to 9 passenger seats after March 29, 2005, unless that airplane is equipped with an approved terrain awareness and warning system that meets as a minimum the requirements for Class B equipment in Technical Standard Order TSO-C151. The system must include Forward-Looking Terrain Avoidance, Premature Descent Alert and basic GPWS functions, and shall be capable of driving an optional terrain display.
What is Class C TAWS?
Class C TAWS is voluntary equipment aimed at general-aviation aircraft that are not required to be equipped with a TAWS. The FAA developed Class C to make voluntary TAWS usage easier for small aircraft; it is recommended but not required for smaller general-aviation airplanes. Class C TAWS provides basic terrain-awareness features similar to Class B but is optimized for light aircraft with fewer than six seats. The equipment provides appropriate visual and aural discrete signals for both caution and warning alerts, and it issues a single aural call-out of ‘Five Hundred’.
What is EGPWS?
Enhanced Ground Proximity Warning System (EGPWS) is a terrain awareness warning system introduced to reduce risk of controlled flight into terrain. EGPWS technology emerged from earlier Ground Proximity Warning Systems. Honeywell developed the Enhanced Ground Proximity Warning System. In everyday usage EGPWS have effectively become interchangeable with TAWS. Mark XXI EGPWS Mark XXII EGPWS are examples. All modern transport-category aircraft are equipped with EGPWS.
EGPWS improves terrain awareness and warning times by introducing the Terrain Display and the Terrain Data Base Look Ahead protection. EGPWS incorporates data from terrain databases, generates predictive warnings, and provides alerts like Terrain Ahead, Pull Up, Obstacle Ahead. EGPWS provides the flight crew with sufficient information and alerting to detect a potentially hazardous terrain situation and gives pilots timely information to take corrective action.
Basic GPWS is a Ground Proximity Warning System designed to alert pilots if the aircraft is in immediate danger of flying into ground or obstacle. EGPWS is an advanced version of basic GPWS and of the basic Terrain Proximity Warning mode.
AlliedSignal developed a smaller and less expensive version of EGPWS for general aviation and private aircraft. ACSS later released an integrated solution combining traffic awareness and terrain awareness functions called Traffic and Terrain Collision Avoidance System.
What is the difference between terrain proximity and TAWS?
Terrain proximity warning is the basic warning component of TAWS. Ground proximity warning system (GPWS) is the specific system currently in use that provides this. TAWS uses databases and onboard sensors to calculate position relative to surrounding terrain and provides a forward-looking capability. It adds a terrain awareness display that shows obstacles relative to the airplane and depicts them in bright yellow and red relative to the current altitude. These improvements provide earlier aural and earlier visual warnings, giving pilots more time to make corrections.
Terrain proximity warning is the foundation, whereas TAWS builds upon it to prevent unintentional impacts with the ground and to indicate potentially hazardous proximity to terrain well in advance.
How many modes are there for EGPWS/TAWS?
The basic GPWS modes are modes 1-5 (and mode 6 on A300/A310). Mode 1 is high rate of descent. Mode 2 is high rate of closure with the ground. Mode 3 is loss of altitude after take off. Mode 4 is proximity to the ground when not in the landing configuration. Mode 5 is descent below the Instrument Landing System (ILS) glideslope. Mode 6 is excessive bank angle (available on A300/A310 aircraft only).
What is a helicopter terrain awareness and warning system (HTAWS)?
HTAWS compares aircraft position to known terrain or obstacles and contains an obstacle and terrain database. HTAWS automation monitors terrain or object situations and provides audio-visual caution or warning information. TAWS and HTAWS equipment computes the precise three-dimensional position and velocity of aircraft, but HTAWS alone does not prevent controlled flight into terrain.
HTAWS is a computer-based alerting system that provides the flight crew with both aural and visual alerts when the rotorcraft is in potentially hazardous proximity to terrain or obstacles. By constantly comparing the helicopter's position, derived from its global positioning system, with an on-board navigation and terrain database, the unit generates cautions and warnings designed to prevent a Controlled Flight Into Terrain (CFIT) accident and to increase overall terrain and obstacle awareness.
Honeywell markets its HTAWS as the Enhanced Ground Proximity Warning System (EGPWS) and the Mark XXI and Mark XXII versions were the first to receive FAA TSO-C194 approval and are optimised for the unique flight characteristics of helicopters, using higher-resolution terrain data and a comprehensive aviation obstacle data set. The system places an aircraft symbol on a terrain map, applies look-ahead algorithms, and supports over 7,000 heliports without requiring the pilot to create user waypoints. A reduced-protection mode is available for low-level operations, allowing minimal alerting while the safety net continues to guard against CFIT. Good planning, execution and decision-making, reinforced by HTAWS vigilance, reduces accidents attributable to CFIT and loss-of-control-in-flight events.
What is the difference between TAWS and HTAWS?
TAWS is an umbrella label for any terrain-awareness box that meets the minimum performance standards of TSO-C151 whereas HTAWS is TAWS hardware whose software algorithms have been tuned for the low-altitude, low-speed, hovering and off-airport airspace that helicopters routinely inhabit.
Because helicopters operate so close to obstacles, their HTAWS units issue Class A alerts based on excessive closure rate to terrain with far tighter thresholds than fixed-wing TAWS, and they add vertical speed/landing site deceleration modes that a pure TAWS lacks.
To keep the crew from jerking the cyclic every few seconds, designers give HTAWS additional map, radar-altimeter and flight-mode logic so that Class B alerts similar but with wider tolerances are automatically suppressed during deliberate contour flying. This selective damping is not present in standard TAWS, where every terrain is treated as inadvertent.
Does TAWS use GPS?
Yes, TAWS relies on GPS technology. The TAWS computer receives position information from a GPS receiver and compares that position with the internal terrain or obstacle database to generate alerts. This GPS input supplies the horizontal position and altitude the system needs to calculate predictive warnings and to guarantee the flightcrew take appropriate action.
To do this, Class A and Class B equipment must use an approved GPS capable of detecting a positional error that exceeds the appropriate alarm limit for the phase of flight. When this limit is exceeded the GPS-computed position is deemed unsuitable, the functions that require GPS are automatically disabled, and an annunciation is provided to the flightcrew. Once the signal is re-established and the aircraft is within the database coverage area, the aural message TAWS AVAILABLE is generated.
Because the TAWS position source is independent of the FMS, deselecting GPS on the FMS will not affect the TAWS position source. The system can pull aircraft position, speed and direction data from GPS, from WAAS-GPS sensors, or from integrated GPS, INS and radar-altimeter combinations. Radio altimeter and barometric altitude remain available as backups to GPS.
When TAWS is fitted it must therefore be connected to a dedicated GNSS-position source that continually supplies the computer with validated GPS data and meets the error-detection requirements of the installation.
What is the TAWS test?
The TAWS test is a structured sequence of ground checks that confirms every sensor and data path feeding the Terrain Awareness and Warning System. It begins by verifying barometric altitude, barometric altitude rate and radio altitude, then confirms that height above terrain is correctly derived from radio altitude or from the terrain database. Navigation source input, GPS position, gear and flap discretes are examined next, followed by a review of the airport database and the corresponding terrain elevation file to guarantee database validity. Electromagnetic interference and compatibility, electrical transient effects, self-test routines and identified failure modes are exercised, while hardware status is read and recorded. Location and operation of TAWS controls, displays, visual alerts and aural warnings are inspected, and one test run in an aircraft simulator or with computer-generated test equipment accepted by the manufacturer is required to determine proper installation.
What is TAWS system failure?
TAWS system failure is any condition that removes the protection promised by the Terrain Awareness and Warning System. It is declared when the yellow TAWS FAIL caution appears or when the aural TAWS SYSTEM FAILURE warning sounds. Both warnings tell the crew that the computer can no longer guarantee terrain clearance data.
Why does TAWS fail? A TAWS failure starts with the TAWS power-up self-test: if that test fails, the system refuses to enter operational mode and immediately sets the TAWS FAIL caution. Inside the box, a hardware status issue or a GPS status issue will have the same effect. Once the logic sees either fault, it labels the event a TAWS system failure. Database validity problems or an error in either terrain or obstacle database corrupt the solution path, prompting the same warning. The system presents TAWS NOT AVAILABLE or TAWS N/A on the display and silences all forward-looking protection. From a certification standpoint, TSO C151a treats any loss of terrain warning functions, as well as the presentation of hazardously misleading information on the terrain display, as a major failure condition.
When is TAWS not available?
TAWS is not available when the 3-D GPS navigation solution becomes degraded or the aircraft is out of the database coverage area. The flight deck produces the aural message TAWS NOT AVAILABLE / TAWS N/A. If the airport is not included in the airport database, TAWS is not available for that runway, so the system must be inhibited before approach to prevent unwanted alerts.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
