Ground Proximity Warning System (GPWS) is a ground-proximity warning system that generates advisory alerts and mandatory response warnings to the flight crew in respect of their proximity to terrain. The system employs sophisticated sensors and algorithms to monitor the aircraft's proximity to the ground. It uses radar altimeter data and terrain database information to detect terrain hazards and alert pilots if the aircraft is in immediate danger of flying into terrain or an obstacle. Recognized by the United States Federal Aviation Administration (FAA) as a type of terrain awareness and warning system (TAWS), GPWS issues an alert when the aircraft is below 500 feet above the terrain or nearest runway elevation during a non-precision approach (Mode 6).
What is the meaning of the ground proximity warning system (GPWS)?
The Ground Proximity Warning System (GPWS) provides pilots with advanced warnings of potential collisions with the ground or other obstacles during flight, and it is designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle.
Ground Proximity Warning System is an advanced safety system in aircraft that represents critical safety features installed in aircraft, GPWS is a defined type of Terrain Awareness Warning System. The system uses various aircraft inputs and an internal database to predict and warn flight crews of potential conflicts with obstacles or terrain, Mode 2 calculates future flight path of the aircraft with respect to terrain data and recognizes presence of obstacles. In general aviation this equipment compares real time position input with an internal terrain database so pilots can react and fly away safely when advisory alerts addressed to flight crew announce risk. For military use Enhanced Ground Proximity Warning System (EGPWS) generates warning alerts such as Pull Up, Terrain Ahead, Obstacle Ahead, reducing risk of controlled flight into terrain even when aircraft is very close to ground. Fighter jet installations rely on the same principle: real time position input is compared with an internal terrain database, mandatory response warnings addressed to flight crew demand immediate manoeuvre.
During landing, SmartLanding systems developed to help flight crews avoid potential runway excursions, while SmartRunway was developed to help flight crews avoid potential runway incursions, Standard Operating Procedures (SOPs) will state the actions required and whether there is any element of discretion in the response depending on whether it is an Alert or a Warning. Location accuracy is achieved because equipment has a real time position input which is compared with an internal terrain database, allowing crews to fly safely at low level. Helicopter operations benefit equally: Enhanced Ground Proximity Warning System (EGPWS) provides flight crews with timely, accurate information about terrain and obstacles in the area, significantly improving safety by increasing awareness of the situation.
What is the function of a GPWS?
A Ground Proximity Warning System monitors hazardous flight-path conditions. Its basic functions include excessive rates of descent and excessive closure rate to terrain. The system uses a radar altimeter to measure height above ground, when the aircraft drops too fast, the radar altimeter warns the pilot. If a dangerous profile is detected, GPWS provides visual warnings and voice callouts so the flight crew can act. The principle is continuous comparison of radio altitude with a model of safe flight, when the model is violated the system issues an alert.
The differences between GPWS and TAWS are given in the table below.
| GPWS | TAWS |
|---|---|
| Mode 2B looks down at the ground directly below the aircraft. | Relies on a terrain database and cannot be tricked by terrain anomalies |
| Does not provide solid and explicit l terrain images. | Provides more solid and explicit images of the terrain compared to basic GPWS |
| Gives last minute warnings only when danger is already approaching | Improves on existing GPWS systems by providing earlier aural and visual warnings |
| Provides warnings like TOO LOW TERRAIN or PULL UP | Provides the flight crew much earlier aural and visual warning of impending terrain |
| Only observe height below the aircraft. | Provides forward looking capability, effectively looks ahead for obstacles and terrain |
| Gives limited time for corrective action | Gives more time for the flight crew to make smoother and gradual corrective action |
The basic difference is that TAWS relies on a terrain database and cannot be tricked by terrain anomalies. GPWS Mode 2B simply looks down at the ground directly below the aircraft, whereas TAWS effectively looks ahead for obstacles and terrain. TAWS improves on existing GPWS systems by providing the flight crew much earlier aural and visual warning of impending terrain and by providing continued operation in the landing configuration. TAWS also provides more solid and explicit images of terrain and gives more time for smoother and gradual corrective action.
GPWS was the common application at the day, it observed height immediately below the aircraft and put out warnings like TOO LOW, TERRAIN or PULL UP only when a dangerous scene was already emerging, delivering a last-minute message of the hazards of the piece of ground. TAWS incorporates GPWS functionality yet is improved with forward-looking terrain information, it exhibits a graphical appearance of menacing hazards forward and permits the crew to design a securer course long before any alert chimed, offering a prognostic tier of security instead of a merely responsive mesh.
Is TAWS the same as EGPWS?
TAWS and EGPWS are not identical, yet the difference is mostly a matter of branding and scope. The term terrain awareness and warning system (TAWS) is the generic classification created by the FAA to describe any equipment that meets the terrain-avoidance rules. Enhanced Ground Proximity Warning System (EGPWS) is a Honeywell trademark and denotes a specific family of TAWS-compliant computers. Thus every EGPWS unit is a TAWS, but not every TAWS is an EGPWS, other suppliers sell functionally similar boxes under the trade names T3CAS, HTAWS, TAWS, T2CAS, etc.
Mark VI, VII, VIII, XXI and XXII EGPWS computers are all certified as Class A TAWS, while HTAWS variants serve rotorcraft and T3CAS integrates transponder, traffic and terrain into one rack. Because the FAA elected to regulate the function rather than endorse one brand, the documents speak of TAWS performance, yet the airborne hardware often carries the Honeywell EGPWS label. In daily usage the two names have become interchangeable, but legally the distinction remains: TAWS is the requirement, EGPWS is one company's product that satisfies it.
How does GPWS work?
The GPWS computer receives data from the radio altimeter and from the barometric altimeter, keeps track of the readings and calculates trends, it also receives data from flap sensors and from the air-data computer while real-time GPS position is compared with an internal terrain database. When the distance from the ground becomes unsafe for the current configuration, the system provides visual warnings on the primary flight display and aural warnings through the cockpit speakers. Mode 2 predicts the future flight path of the aircraft with respect to the terrain data and recognises the presence of obstacles, stopping the crew from flying into rising ground, this is how GPWS prevents CFIT. Mode 5 signals glideslope deviation during approach, Mode 6 signals descent below the selected minimum radio altitude, and Mode 7 detects wind shear, so every phase from cruise to touchdown is monitored. During approach the callouts two-thousand-five-hundred , one-thousand , five-hundred are generated, the 2 500 ft (762 m) call being based on radio altitude while the 1 000 ft (305 m) and 500 ft (152 m) callouts are based on barometric altitude above landing-field elevation, airlines pick and choose callout altitudes by configuration strapping so some aircraft add FIVE and HUNDRED calls. The operating range extends from 2 500 ft (762 m) above ground level down to touchdown, and certification requirements oblige all large transport aeroplanes to be equipped with this Terrain Avoided and Warning System.
What are the GPWS warning modes?
GPWS warning modes include Mode 1 - Excessive Descent Rate, announced as SINK RATE , Mode 2 - Excessive Terrain Closure Rate, subdivided into Mode 2a with flaps not in landing position and Mode 2b with flaps in landing position, both issuing TERRAIN followed by whoop whoop, PULL UP, PULL UP. Mode 3 - Altitude Loss after Take-off or Go-around, calling DON'T SINK , Mode 4 - Unsafe Terrain Clearance when not in landing configuration, split into Mode 4a: Too Low - TERRAIN when gear and flaps are up, and Mode 4b when either gear or flaps remain retracted.
Mode 5 - Below Glide-slope Deviation, sounding GLIDESLOPE and inhibiting the 500-foot call-out unless the Glide-slope Inhibit Switch is pressed, Mode 6 - Below Selected Minimum Radio Altitude plus bank-angle protection, issuing BANK ANGLE above 35-45 roll and altitude call-outs, and Mode 7 - Windshear Alerting, declaring WINDSHEAR .
EGPWS enlarges these basic modes with look-ahead predictive alerting: Terrain Awareness Display and Terrain Clearance Floor compare real-time position against an internal terrain database to generate early visual and audio cautions CAUTION TERRAIN or hard warnings TERRAIN AHEAD, PULL UP and OBSTACLE, PULL UP , distinct from the reactive Modes 1-4. Class A installations must furnish both the classic seven modes and this forward-looking TAD/TCF suite plus windshear, Class B units offer a reduced set yet still provide the same phraseology hierarchy: caution alerts are amber aural/visual, mandatory warnings repeat red PULL UP until the envelope is exited.
What are the components of GPWS?
The Ground Proximity Warning System is built around a dedicated ground proximity warning computer that fuses data from the aircraft's own sensors. This computer receives radio altitude, corrected barometric altitude, altitude rate, and computed airspeed from the air data system, radio altitude is measured separately by the radar altimeter transmitter, receiver, and display unit. A vertical speed sensor adds another redundant altitude-rate channel, while an ILS glideslope receiver supplies deviation signals. Additional control/discretes inputs-landing gear switch, flap switch, autopilot engaged flag and aircraft configuration-allow the software to determine aircraft phase and select the correct algorithms.
Aural alerts are generated by the same computer and sent through flight deck speakers and the interphone, visual alerts are supplied either through discrete alert lamps and, when installed, on EFIS navigation displays or ADI/HSI units. Deactivation switches are provided in the cockpit so that the crew can silence nuisance calls. Original GPWS relied only on the air data system and the radio altimeter, with the advent of Modern Enhanced GPWS tools, GPS positional co-ordinates are combined with onboard inertial references (AHRS, IRS, VG, DG) and are compared in real time against an internal terrain database. A configuration module fitted inside the computer identifies the host aircraft type and tailors alert envelopes accordingly. The radar altimeter is therefore a primary, but not the only, altitude source, GPS supplements, but never replaces, that dedicated radio altimeter chain.
What is the GPWS maneuver?
The Ground Proximity Warning System (GPWS) generates advisory alerts and mandatory response warnings as a result of studies and recommendations from the U.S., which indicated that many accidents could have been avoided if GPWS had been used.
When the aural PULL UP warning occurs, the autopilot needs to be switched off manually, the pilot must then pull up to full back stick and maintain while applying maximum power and adjusting pitch attitude to around 20 degrees up. The procedure includes retracting speed brakes if deployed, but not retracting trailing edge flaps or raising landing gear immediately, the aircraft continues to climb at best angle of climb speed (Vx) until terrain clearance is assured. Standard Operating Procedures state these actions without discretion because PULL UP is a hard warning, and relying solely on visual cues is risky, especially in low visibility conditions. GPWS maneuver training therefore requires the pilot to demonstrate the knowledge and skills required to respond correctly to GPWS cautions and warnings, including the pull-up maneuver training that rehearses every step from autopilot disconnection to level-off at a safe altitude.
What does the GPWS light indicate?
The Ground Proximity Warning System (GPWS) light on the glareshield is the primary visual cue that distinguishes a caution from a warning. A steady amber BELOW G/S or G/S light indicates glideslope deviation or other cautions such as TOO LOW FLAPS, the red GPWS light, often combined with master WARNING light illumination, signals an immediate pull-up or terrain threat. When the outer warning envelope of Mode 2 is penetrated, the red GPWS light flashes, the master WARNING lights illuminate, and the voice aural TERRAIN TERRAIN or PULL UP sounds. A flashing GPWS light also accompanies the DON'T SINK alert.
Fault annunciation is amber: an amber FAULT light and the accompanying ECAM caution appear if TAD or TCF mode fails, and the amber INOP light comes on for computer malfunction or power loss.
What causes a GPWS failure?
A GPWS failure arises from two principal on-board faults: a short circuit within the GPWS wiring or a loss of the input data that the computer needs for terrain-logic calculations. When a short circuit develops, the GPWS unit either stops working entirely or emits erratic alerts. Loss of input data to theGPWS computer produces the same result-partial or total loss of functionality-because the system can no longer validate gear position, flap position or radio altitude.
A significant Flight Management System position error inhibits alerts, since the GPWS relies on FMS position to arm its modes. Nuisance warnings occur if a flap or gear position signal is prevented from reaching the GPWS unit, making the computer believe the aircraft is in a landing configuration when it is not.
What is the history of GPWS?
Don Bateman, a Canadian electrical engineer at Honeywell, conceived the GPWS in the late 1960s when controlled flight into terrain was the leading cause of fatal U.S. aircraft accidents, responding to the 1971 crash of Alaska Airlines Flight 1866 he refined a device that automatically warned pilots, and early test showed it provided sufficient warning for a small plane. The FAA published the first mandate in 1973, effective 1974, requiring GPWS on all Part 121 and certain Part 135 airplanes, ICAAO recommended installation worldwide in 1979, the requirement was extended to Part 135 operators with ten or more passenger seats in 1978. Honeywell engineers introduced Enhanced GPWS in 1996, the GPWS mandate by the FAA in 1974 and development of EGPWS in the 1990s made CFIT accidents involving commercial aircraft almost nonexistent, and Don Bateman's pioneering work saved countless lives worldwide.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.
