Enhanced Ground Proximity Warning System (EGPWS): Definition, Operation, Modes, Warnings

Enhanced Ground Proximity Warning System (EGPWS) is a Terrain Awareness and Warning System (TAWS) that continuously compares GPS position and flight path against an onboard digital terrain database, applying predictive algorithms to keep the aircraft clear of the ground.

Should the aircraft descend below the clearance floor, the system issues a too low, terrain warning; simultaneously, terrain display and obstacle alerting heighten situational awareness while further auditory and visual warnings can be triggered by the seven modes:

Mode 1 - Excessive Descent Rate Mode 2 - Excessive Terrain Closure Rate Mode 3 - Altitude Loss After Takeoff Mode 4 - Unsafe Terrain Clearance Mode 5 - Descent Below Glideslope Mode 6 - Advisory Callouts Mode 7 - Reactive Windshear

Expert behind this article

Jim Goodrich

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

What is the definition of EGPWS in aviation?

EGPWS is the Enhanced Ground Proximity Warning System, which provides flight crews with accurate information about terrain and obstacles, reducing the risk of controlled flight into terrain, and it is fitted to a large number of commuter and airline aircraft.

The United States Federal Aviation Administration defines EGPWS as a type of terrain awareness warning system. Enhanced Ground Proximity Warning System is a vital safety technology designed to prevent controlled flight into terrain. The system predicts and warns flight crews of potential conflicts with obstacles or terrain and advises the flight crew of a potential conflict with the terrain or obstacle. Enhanced Ground Proximity Warning System generates visual and audio caution or warning alerts when a conflict is detected and provides flight crews with timely, accurate information about terrain and obstacles.

Enhanced Ground Proximity Warning System uses various aircraft inputs and an internal database to predict potential conflicts with obstacles or terrain. The internal database consists of terrain data and obstacle data of varying degrees of resolution, creating a virtual map of the area around the airplane. Conflicts are recognized when terrain violates specific computed envelope boundaries. Enhanced Ground Proximity Warning System improves terrain awareness and extends the warning area almost to the runway threshold, providing a warning in advance of steeply rising ground.

What are the functions of EGPWS?

The functions of EGPWS are explained below.

• EGPWS performs enhanced functions including terrain look-ahead alerting
• EGPWS output functions are the result of the configuration state read each time the EGPWS is powered on
• EGPWS offers alerts and warnings for terrain clearance
• EGPWS system warns flight crews of potential conflicts with obstacles or terrain
• EGPWS obstacles alerting function utilizes obstacle database
• EGPWS provides integration in flight management system (FMS)
• EGPWS offers SmartRunway function
• EGPWS provides SmartRunway and SmartLanding systems
• EGPWS provides runway picker capability
• EGPWS uses aircraft inputs
• EGPWS works with other avionics manufacturers to develop modules
• EGPWS aircraft inputs include geographic position
• EGPWS crew receive aural alert and visual warning from multicolor image

From these inputs it performs enhanced functions, including terrain look-ahead alerting that warns flight crews of potential conflicts with terrain or obstacles. Terrain clearance offers alerts and warnings, while the obstacles alerting function, utilizing an obstacle database, provides the crew with an aural alert and a visual warning from multicolor image.

Is EGPWS mandatory?

EGPWS is mandated by aviation authorities for defined fleets. Since 1974 the FAA has made GPWS mandatory on Part 121 air transport aircraft, and since 1992 the FAA has made GPWS mandatory on Part 135 aircraft bearing 10 passengers or more. By March of 2000 the FAA declared that all US-registered aircraft with six or more passengers must be equipped with an EGPWS, this rule now reads that turbine-powered fixed-wings configured for six or more passenger seats must have a Class B TAWS under Part 91, while Part 121 and Part 135 regulations also mandate that older GPWS be replaced with a TSO-C151-approved TAWS. ICAO Annex 6 Part 1 Chapter 6 and Part 2 Chapter 6 echo the fixed-wing requirement, and ICAO Annex 6 Part 3 Chapter 4 Paragraph 4.4.4 requires helicopters to be equipped with GPWS as well.

How does EGPWS work?

Class A EGPWS continuously ingests aircraft inputs that include geographic position, altitude and attitude. GPS systems precisely track its location while radar altimeter gauges aircraft height above ground. These data are fused with information from navaids, the FMS and air data sensors to determine the aircraft position both longitudinally and vertically.

Once position is fixed, the system compares the trajectory to its on-board terrain database that contains highly accurate topographical information. Look-Ahead monitors the aircraft glide path against the digital map, providing both horizontal and vertical look-ahead. When the predicted flight path will penetrate the safety envelope, warning lights illuminate and audible alerts are issued, giving the crew time to correct the flight path before ground contact becomes imminent.

The Enhanced Ground Proximity Warning System combines information from GPS, radio altimeter and cockpit avionics to build an elaborate representation of the ground ahead. Inside the module a digital terrain database is held, the unit incessantly analyses our height above that inner map and projects the result as a color-coded picture on the main display. Amber regions mark rising land, red regions mark imminent earth, and the chart gives scarlet sections only when headroom is about to vanish. Because the device is not an alarmist gadget but an ever-present consultant, the optical picture permitted me to foresee the ground's cross-sectional shape long before any aural alert. Thus, EGPWS makes the aircraft respond not just to a direct threat, it provides continual support, so the crew can keep the evening trip across hilly terrain inside safe boundaries.

Jim Goodrich

Pilot, Airplane Broker and Founder of Tsunami Air

What are the components of EGPWS?

EGPWS requires ILS or Glideslope Receiver, Radio Altimeter, Air Data, AHRS/IRS/VG/DG, GPS, Landing gear, Landing flaps, Weather Radar Indicator or EFIS or dedicated terrain display for full activation. The Radio Altimeter provides absolute altitude above ground level (AGL) and is an essential airborne electronic device within the system. Aircraft inputs include geographic position, attitude, altitude and airspeed, supplied by IRS, AHRS and Air Data sensors that give both corrected and uncorrected barometric altitude.

The EGPWS Configuration Module tells the system the type of aircraft it resides in and its interface, Level 3 EGPWS Configuration lists detected hardware, software, databases and program-pin inputs. A discrete Mode 6 Low Volume Select and an Autopilot Engaged discrete are also included.

The differences between EGPWS and GPWS are given in the table below.

GPWS uses radio altimeter readings to provide only reactive alerts, so it warns when the aircraft is already close to terrain. Because GPWS lacks predictive capability, it cannot see rising ground ahead, EGPWS, however, stores a global terrain database and compares present position with projected flight path, delivering earlier warnings that give pilots more time to react and thereby preventing CFIT-Controlled Flight Into Terrain.

Ground Proximity Warning System functionality was restricted, the system could view merely terrain beneath us and could not look at a piece of ground forward. Consequently, it stayed quiet until the advanced period, issuing PULL UP order as last-ditch alert. Enhanced GPWS, by contrast, employs a combination of global terrain information and GPS application, offering prognostic capacity. This transformation to aircraft outfitted with Enhanced GPWS was revealing: guidance device now delivers premature CAUTION TERRAIN warning, permitting flight crew to evaluate environment and establish gradual ascent comfortably before any present danger was.

What are the EGPWS operating modes?

The EGPWS operating modes are given in the table below.

Enhanced Ground Proximity Warning System divides protection into seven basic modes and two enhanced functions. Mode 1 is Excessive Descent Rate, it is desensitized above the glide-slope beam to eliminate unwanted alerts. Mode 2 is Excessive Closure to Terrain and exists in two forms: 2A and 2B. Mode 2A is active during climb-out, cruise, and initial approach; its envelope is lowered to reduce nuisance alerts. Mode 2B provides a desensitized alerting envelope to permit normal landing approach maneuvers close to terrain, it causes EGPWS caution lights to illuminate and produces the TERRAIN TERRAIN aural alert.

Mode 3 is Altitude Loss After Take-off, it sounds if the aircraft descends during initial climb or go-around and activates the GPWS light up to about 700 ft (213.36 m) radio altitude. Mode 4 is Unsafe Terrain Clearance and has three sub-modes: 4A, 4B, and 4C. Mode 4A is active during cruise and approach with gear up. Mode 4B is active during cruise. Mode 4C is active during the take-off phase with gear or flaps not in landing configuration, it issues the TOO LOW TERRAIN aural alert. Mode 5 is Excessive Deviation Below Glideslope. Mode 6 is Advisory Callouts, providing altitude and bank-angle protection. Mode 7 is Windshear Alerting, issuing caution and warning reactive windshear alerts in severe conditions. Enhanced functions add Terrain Look-ahead Alerting and Terrain Clearance Floor, while Modes 1 to 5 remain unaffected.

What are all EGPWS warnings?

All EGPWS warnings are listed below.

• EGPWS cautions with the aural message TERRAIN AHEAD, TERRAIN AHEAD when caution ribbon is intruded
• EGPWS generates aural message TERRAIN, TERRAIN if aircraft penetrates the Mode 2A caution envelope
• EGPWS warning lights illuminate and the aural warning message PULL UP repeats continuously until the warning envelope is exited
• EGPWS warning lights illuminate simultaneously with master WARNING light illumination when a critical warning is active
• EGPWS warning lights the aural message TERRAIN, TERRAIN, PULL UP
• EGPWS activates the aural message OBSTACLE, OBSTACLE, PULL UP
• EGPWS issues alert Obstacle, Obstacle, Pull Up repeated continuously while conflict is within warning area
• EGPWS warning ribbon intrusion typically occurs 30 seconds prior to terrain/obstacle conflict
• EGPWS caution lights will illuminate if aircraft penetrates the Mode 2A caution envelope
• EGPWS aural warning PULL UP repeats continuously while conflict is within the warning area
• EGPWS mode 7 is flight into windshear conditions

Look Ahead Terrain, Obstacle, and PULL UP Hard Warnings are generated when the aircraft penetrates the caution ribbon, typically 30 seconds prior to terrain or obstacle conflict. Mode 1 Excessive Descent Rate (SINK RATE) protects from excessive descent rate by issuing SINK RATE caution callouts that escalate to a hard warning if the rate persists. Mode 2 Excessive Terrain Closure has two sub modes: Mode 2A caution ribbon intrusion with the aural message TERRAIN, TERRAIN, and Mode 2B for flaps in landing position while still providing the caution light illumination and the same caution statement. Mode 3 Altitude Loss after Take off or Go around monitors altitude loss after take off or go around, giving successive cautions if height is not regained.

Mode 4 Too Low Gear / Too Low Flaps / Unsafe Terrain Clearance is a family of three alerts (4A, 4B, 4C) that calls attention to unsafe terrain clearance based on configuration and speed. Typical aural statements are TOO LOW GEAR in Mode 4A and TOO LOW FLAPS when landing flaps are not set. Mode 4B alerts operate when landing gear is not down or flaps are not in landing position. Mode 5 Bank Angle and SINK RATE below Glide Path issues the caution BANK ANGLE and advisories if angle or glide path deviations below safe thresholds persist. Mode 6 Miscellaneous Callouts and Advisories manages advisory situational information callouts such as ONE THOUSAND and 100 ABOVE during approach, plus altitude, decision, and deceleration advisories. Mode 7 Windshear Override detects flight into windshear conditions. When actual windshear is sensed, look ahead terrain alerts are inhibited and a distinctive warning is broadcast. Pilots use FLAP Override or similar overrides to allow manually selected interference, yet the system still triggers hard warning messages if the caution envelope is penetrated. Ground proximity time critical warnings are accompanied by master WARNING light illumination and voice aural alerts, giving the flight crew clear information to aggressively position throttles for maximum rated thrust and continue climbing until the warning is eliminated.

What are EGPWS callouts during landing?

EGPWS callouts during landing are generated by Mode 6 advisory function and by specific clearance protections. Mode 6 announces height above field elevation: FIVE HUNDRED ABOVE once during each approach when the aircraft flies below 500 ft (152.4 m) above the landing field, the same 500 Above Field callout is chosen to say either FIVE HUNDRED or FIVE HUNDRED ABOVE . Further selectable altitude callouts include ONE THOUSAND at 1 000 ft AGL (304.8 m AGL), APPROACHING MINIMUMS , and MINIMUMS MINIMUMS at minimums altitude.

If the aircraft is too low with gear or flaps not in landing configuration, Mode 4 issues TOO LOW TERRAIN , TOO LOW GEAR , or TOO LOW FLAPS depending on airspeed. When terrain or obstacle conflict intrudes the warning ribbon, the system enunciates TERRAIN, TERRAIN, PULL UP or OBSTACLE, OBSTACLE, PULL UP .

What triggers the EGPWS sink rate alert?

Mode 1 provides alerts for excessive descent rate. Penetration of the outer boundary activates the EGPWS caution lights and SINKRATE, SINKRATE alert enunciation, additional messages occur for each 20 % additional penetration of that boundary until the aircraft exits the outer boundary. If the excessive descent continues and the inner boundary is penetrated, the audio message changes to PULL UP and the red PULL UP master warning lights illuminate. The alert curve is biased upwards by half of the aircraft's descent rate whenever sink exceeds 1000 ft/min (304.8 m/min), and the boundary remains fixed when Aural Declutter is disabled. Mode 1 is triggered by a noisy barometric altitude-rate signal below 100 ft (30.48 m) or by high final-approach speeds that demand a higher descent rate to maintain the glide-path.

Mode 7 provides reactive low-level windshear detection according to TSO-C117a. The system continuously compares inertial and air-mass-derived vertical and horizontal acceleration data, when the computed energy loss exceeds the threshold for the current altitude and aircraft configuration, it announces WINDSHEAR and illuminates the red PULL UP lights, directing the crew to execute the windshear escape maneuver.

What do EGPWS color codes mean?

Each specific color and density represents terrain (and obstacles) below, at, or above the aircraft's altitude relative to the calculated true height above sea level, the reference altitude for the color-coding logic. GREEN indicates safe terrain/obstacle clearance: low-density green marks terrain 500-1 000 ft (152.4-304.8 m) below the aircraft, high-density green marks 1 000-2 000 ft (304.8-609.6 m) below, and BLACK is used for terrain more than 2 000 ft (609.6 m) below the aircraft, signalling no significant threat. YELLOW is the caution band: low-density yellow shows terrain 0-152 m below the aircraft, while high-density yellow indicates terrain 0-305 m above the aircraft, the appearance of any yellow is accompanied by the aural caution terrain and the illuminated amber Caution annunciator lamp, giving the crew roughly 60 s prior to predicted impact (0-152 m, 0-305 m). RED is the warning band: high-density red depicts terrain 2 000 ft (609.6 m) or more above the aircraft, signalling immediate danger within 30 s of impact, the display changes to red, the red Warning annunciator lamp illuminates, and the aural terrain, terrain, pull up is enunciated. MAGENTA is reserved for areas where no terrain data are available, while CYAN, when supported, indicates sea level. Thus, the encoding is: green (safe below), black (no threat, far below), yellow (caution-near or above), red (warning-above and imminent), magenta (data unavailable), and cyan (sea level reference).

What does the EGPWS inhibitor do?

The Terrain Inhibit switch, when engaged by the pilot, silences every visual and aural warning that the EGPWS produces. Audio Inhibit and the Terrain Inhibit switch therefore disable all EGPWS audio outputs, while the Terrain Inhibit switch additionally extinguishes the corresponding visual alerts. Because the switch inhibits TAD and TCF alerting and display, obstacles and peaks are also removed from view, the terrain picture itself, however, remains operational so the crew can still consult the background map. An external annunciator lamp illuminates and the cockpit shows the message Warnings Inhibited whenever the switch is engaged, confirming that the inhibition applies to warnings associated with the EGPWS.

All alerts generated by the Terrain Awareness and Warning System-modes 1-6 caution and warning calls, TAD/TCF terrain cautions, obstacle and peak symbols-can be muted. Higher-priority systems such as stall warning or windshear automatically override EGPWS audio outputs, and cockpit switches including the Ground-proximity G/S INHB switch can cancel the glide-slope alert below 1 000 ft (304.8 m) radio altitude. Manual inhibition is recommended within 15 NM of an airport or runway that is not in the database, and is compulsory for ditching or off-airport landings, to prevent nuisance or unwanted warnings while still allowing the aircraft to operate.

What is EGPWS recovery?

EGPWS recovery is a function of the Honeywell Enhanced Ground Proximity Warning System that warns flight crews of potential conflicts with obstacles or terrain, thereby preventing controlled flight into terrain, and it provides timely, accurate information about terrain and obstacles in the area.

EGPWS recovery is the immediate and correct response a flight crew must perform when the Honeywell Enhanced Ground Proximity Warning System generates a Pull Up warning. The system uses various aircraft inputs and an internal database to predict and warn flight crews of potential conflicts with obstacles or terrain, when the calculation shows that continued flight path will place a fully functional aircraft in controlled flight into terrain, the Pull Up audio and visual hard warning is issued as a vital safety net. Recovery then requires the crew to disregard any previous clearance, apply full power, rotate to the demanded pitch attitude, and climb until the terrain or obstacle ahead alerting ceases and the safe terrain clearance floor is restored. Because CFIT incidents happen when pilots typically remain unaware of the danger until it becomes too late to recover, the EGPWS recovery maneuver must be executed without hesitation, the Polish Air Force Tupolev Tu-154M on 10 April 2010 ignored TAWS Pull Up warning and continued descent off track and into the ground, demonstrating that failure to accomplish the recovery ends with the aircraft colliding with ground or water. Thus, EGPWS recovery is the disciplined, trained action that converts the warning into improved aviation safety.

What is EGPWS terrain?

EGPWS terrain is terrain information provided by the Honeywell Enhanced Ground Proximity Warning System, which predicts potential conflicts with obstacles or terrain and warns flight crews of these conflicts, thereby reducing the risk of controlled flight into terrain.

Honeywell Enhanced Ground Proximity Warning System employs a worldwide terrain database of varying degrees of resolution that contains highly accurate topographical information, with higher-resolution grids for airport areas. By continuously comparing aircraft position, flight path angle, track, and speed against this digital elevation model, Terrain Look-ahead Alerting predicts potential conflicts with terrain and warns flight crews while there is still time to climb.

Terrain Clearance Floor (TCF) is one of the enhanced functions that works hand-in-hand with the terrain database. Around every runway 3 500 ft (1 067 m) or longer stored in its runway database, the system builds a terrain clearance floor; as soon as an aircraft descends below this invisible barrier while still outside the normal approach profile, an immediate aural and visual alert is generated. Together, Terrain Look-ahead Alerting and TCF ensure that the aircraft is never operated closer to surrounding terrain than normally occurs, providing both predictive forward-looking protection and a discrete safety buffer around each runway.

What is the height range monitored by EGPWS?

The height range monitored by EGPWS is determined by a radar altimeter, which measures aircraft height above ground. The system provides a terrain clearance floor that varies between 200 (60.96 meters) and 600 feet (182.88 meters) radio altitude based on barometric descent rate. The height of the floor of the Mode 2B envelope varies between 200 feet (60.96 meters) and 600 feet (182.88 meters) radio altitude based on barometric descent rate.

EGPWS detects terrain within 500 feet (250 feet with gear down) of aircraft altitude. The mode rearms when climbing above 1000 feet (304.8 meters) radio altitude or descending below 30 feet (9.144 meters) radio altitude. Dotted amber indicates terrain 500 feet (152.4 meters) below to 2000 feet (609.6 meters) above airplane's current altitude, while terrain 500 to 1000 feet (152.4 to 304.8 meters) above airplane current altitude is shown with dotted red.

Bank angle advisories for turbofan (jet) are 40 to 55 degrees between 150 and 2450 feet AGL (45.72 and 746.76 meters), and 10 to 40 degrees between 30 and 150 feet AGL (9.14 and 45.72 meters). Bank angle advisories for turbofan (jet) are 10 degrees between 5 and 30 feet AGL ( 10 degrees between 1.52 and 9.14 meters AGL). Mode 4B aural alert occurs below 245 feet AGL (74.6 meters), with 150 feet (45.7 meters) as the altitude threshold if 150 Mode 4B is selected and 170 feet (51.8 meters) if 170 Mode 4B is selected.

What is the relationship between EGPWS and flap?

The EGPWS warning envelope illustrates the relationship between airspeed and flap position at which a warning is generated. EGPWS compares flap position to minimum and maximum recommended positions: flap position signal is applied to comparators 22 and 24 to decide if an alert is needed.

EGPWS generates a flaps warning for two conditions. When flap position is less than the minimum recommended flap position and airspeed is greater than 40 knots, EGPWS provides a first aural warning between 40 and 60 knots and a continuous FLAPS-FLAPS warning when airspeed exceeds 60 knots. When flap position is greater than the maximum recommended flap position and airspeed is greater than 40 knots, a continuous FLAPS warning is generated. The warning remains generated as long as airspeed exceeds 40 knots.

The system needs to know whether flaps are up or down, because flap position determines landing configuration: when the landing gear is down and landing flaps are deployed, GPWS expects the airplane to land and issues no related warning. Conversely, Mode 4 alerts for unsafe terrain clearance often occur when the landing flaps switch fails or when radio altimetry is incorrect, and excessive speed on final approach results in TOO LOW, FLAPS alerts. Aircraft inputs include attitude, flap override turboprop discrete increases allowable altitude loss, and severe windshear detection is provided when enabled.

What is the role of EGPWS in monitoring glide slope?

The Enhanced Ground Proximity Warning System (EGPWS) monitors the glide slope by continuously comparing the aircraft's position to the published glide path stored in its digital terrain database. It determines that the aircraft is safely capturing the glideslope from above the beam and, once established, Mode 5 automatically provides glide slope deviation alerts during instrument approaches. These warnings activate whenever the system detects descent below the glide slope, ensuring crews receive timely notice to correct their flight path and maintain the required vertical profile to touchdown.

How does the EGPWS perform a self-test?

Self-test is initiated on the ground by pressing the cockpit Self-Test switch for less than two seconds, rotating the outer knob to TST, or, if installed, actuating a remote-mounted TEST switch, all aircraft power and systems must be up and running. Level 1 - Go/No Go Test is performed first: it lights the red Warning and amber Not Available annunciator lamps, produces audio at 6 dB below warning volume, and shows the Test Pattern on the KMD 550/850 Terrain Page while it checks configuration, radio altitude, magnetic heading, and signal/status discretes such as decision height, landing gear, and flap position. When Level 1 completes, the lamps extinguish and the EGPWS begins acquiring a GPS position, its result determines whether deeper testing is allowed. Levels 2 through 6 are accessed only on the ground, each begun with the TEST CONT. softkey after the previous level passes, Levels 3-6 are inhibited in flight. A short press (<2 s) cancels any running test, pressing and holding the switch for 2-8 s performs a long cancel.