Ground Proximity Warning System and Enhanced GPWS (EGPWS) differ mainly in data sources and timing of alerts. GPWS relies solely on radio altimeter readings and gives primarily reactive aural and visual alerts only when an immediate danger is detected. EGPWS represents a significant advancement by integrating terrain mapping data, GPS inputs, and advanced algorithms; it provides predictive terrain warnings, GPS-based location tracking, and windshear alerts. EGPWS adds the ability to compare aircraft position to a worldwide terrain database and a runway database that includes most public-use and military runways of 3,500 feet or longer, giving pilots greater situational awareness and reaction time. What is the primary difference between GPWS and EGPWS?
The primary difference between GPWS and EGPWS is explained in the table below.
| Feature | GPWS | EGPWS |
|---|---|---|
| FLTA function | Not included | Included (assesses threat of collision with obstacles on flight course) |
| Use of Radar | Uses radar to detect terrain | Does not use radar to detect terrain |
| Use of GPS | Solely radio altimeter | Combination of GPS and terrain database |
| Terrain Database | No | Yes, extensive and updated |
| Predictive Warnings | No | Yes, includes predictive terrain warnings |
| Terrain Display | No | Yes, adds terrain display |
| Internal Database | No | Includes internal database with information about obstacles around major aerodromes |
| Virtual Map | No | Uses terrain database to create virtual map |
| Aircraft Inputs | Limited | Uses aircraft inputs including ground speed and vertical speed |
| Reaction Time | Limited | GPS-based location tracking allows pilots greater reaction time |
The main difference between GPWS and EGPWS is the addition of the Forward-Looking Terrain Avoidance function. While the original GPWS issues alerts only when the aircraft is already descending toward rising terrain sensed by the radio altimeter, EGPWS continuously compares the aircraft's GPS-based position to an internal digital terrain database and forecasts potential conflicts before the aircraft begins to drop.
What are the operating modes of GPWS and EGPWS?
The operating modes of GPWS and EGPWS are listed below.
GPWS operating modes include: Mode 1 : Excessive Descent Rate Mode 2 : Excessive Terrain Closure Rate Mode 3: Altitude Loss After Takeoff Mode 4 : Unsafe Terrain Clearance Mode 5 : Excessive Deviation Below Glideslope
EGPWS operating modes include: Mode 1 : Excessive Descent Rate Mode 2A : Terrain Closure (cruise and approach configurations) Mode 3 : DON’T SINK (Altitude Loss After Takeoff) Mode 4 : Unsafe Terrain Clearance Mode 5:Excessive Deviation Below Glideslope Mode 6 : Altitude Callouts, Minimums, and Bank Angle Alerts Mode 7 : Windshear Alerting
Mode 3 warnings for altitude loss after takeoff are accompanied by the aural DON'T SINK warning. It provides Mode 6 advisory callouts, including altitude callouts, minimums, and flight envelope limits, as well as bank angle protection alerts for overbank or excessive roll. Input sets are as simple as GPS position and pressure altitude. When configured, the auto pop-up function will select the Terrain display automatically, and aural PULL UP warnings trigger the appropriate crew procedure.
What are the advantages of GPWS and EGPWS?
The original Ground Proximity Warning System supplies timely warnings of excessive descent rate, insufficient terrain clearance, descent below glideslope, inadvertent descent shortly after takeoff, and excessive bank angle, allowing flight crews to regain altitude quickly and thereby reducing the incidence and risk of CFIT accidents. Enhanced GPWS preserves all those benefits and expands them with earlier warnings, improved accuracy, and broader terrain and obstacle awareness that operates in all flight phases. By continuously monitoring GPS-based position and using various aircraft inputs together with an internal database, the system predicts potential conflicts a full minute or more than four miles away, giving pilots ample reaction time even during non-precision approaches when the aircraft is fully configured for landing. Honeywell's implementation further enhances safety by providing flight crews with clear, timely information about terrain, obstacles, and water, while options such as the KGP 560 GA-EGPWS exceed FAA specifications and offer the most advanced protection against CFIT.
In addition, SmartRunway and SmartLanding functions can be blended with the same core equipment, helping flight crews avoid runway incursions and excursions without imposing extra cost for civil operations that routinely remain well clear of terrain.
What are the limitations of GPWS and EGPWS?
Standard GPWS is limited because it only looks down: the radio altimeter measures distance from the ground directly below the aircraft, so if steeply rising terrain is ahead or to the side, it will not be detected until it enters the radio altimeter sensing radius During high-speed descents or rapid altitude changes this leaves warning time down to fifteen seconds or less, and because the system is ineffective in predicting terrain ahead it gives no advance notice of sudden terrain, leading to nuisance warnings at airports ringed by challenging terrain.
EGPWS removes the old limitation by looking ahead: its forward-looking terrain-alerting function assesses the threat of collision with obstacles on the flight course and extends the warning area almost to the runway threshold. Yet EGPWS still has weaknesses. Some capabilities are limited in GPS-restricted or unreliable environments, the updated database includes less-accurate relief data away from major aerodromes, and man-made obstacles are not fully represented. If EGPWS drops out of service, the old limitation of GPWS returns.
Expert behind this article
Jim Goodrich
Jim Goodrich is a pilot, aviation expert and founder of Tsunami Air.