GPWS vs EGPWS: Ground Proximity Warning Systems, Modes and Differences

A Ground Proximity Warning System (GPWS) is a safety system fitted to aircraft that alerts pilots when the aircraft is in immediate danger of flying into terrain or an obstacle. GPWS uses radar altimeter data, vertical speed, and aircraft configuration parameters to detect proximity to the ground and trigger warning thresholds, issuing voice and visual alerts such as “SINK RATE,” “TERRAIN,” and “PULL UP” when unsafe conditions are detected. It does not use GPS or a terrain database. The enhanced version, EGPWS, integrates GPS positioning with a global terrain and obstacle database, providing forward-looking predictive warnings before a terrain conflict develops.
GPWS was introduced in the 1970s following a series of Controlled Flight Into Terrain (CFIT) accidents in which airworthy aircraft under pilot control flew into terrain without prior warning. Canadian engineer Donald Bateman is credited with developing the first functional system. According to Airbus safety data on aircraft generation performance, CFIT accident rates have decreased by approximately 87% in generation 3 aircraft compared with generation 2 aircraft, associated with the introduction of glass cockpits, Flight Management Systems (FMS), and Terrain Awareness and Warning Systems (TAWS).

GPWS Warning Modes: What Each Alert Means

Basic GPWS operates across seven defined modes. Each mode monitors a specific flight parameter and triggers a specific aural warning when a safety boundary is crossed.

Mode	Trigger Condition	Aural Warning	Also Known As
Mode 1	Excessive descent rate at low altitude	“SINK RATE” / “PULL UP”	Sink Rate Warning
Mode 2	Excessive terrain closure rate (rising ground)	“TERRAIN TERRAIN” / “PULL UP”	Terrain Warning
Mode 3	Altitude loss after takeoff or go-around	“DON’T SINK”	Don’t Sink Warning
Mode 4	Unsafe terrain clearance (gear/flaps not configured)	“TOO LOW GEAR” / “TOO LOW TERRAIN”	Terrain Clearance Warning
Mode 5	Excessive deviation below ILS glideslope	“GLIDESLOPE”	Glideslope Warning
Mode 6	Altitude callouts during descent and landing	Altitude announcements	Altitude Callouts
Mode 7	Windshear detection below 1,500 ft	“WINDSHEAR”	Windshear Warning (EGPWS)

Sink Rate Warning: What It Means and When It Triggers

A sink rate warning is a Mode 1 GPWS alert that triggers when the aircraft’s rate of descent is too high for its current altitude above the ground. The system compares vertical speed against radio altitude and activates when the descent rate would put the aircraft at risk of ground impact before the pilot can recover.

The warning sequence works in two stages. The outer boundary triggers the voice alert ‘SINK RATE, SINK RATE.’ If the descent rate is not corrected and the aircraft crosses the inner boundary, the warning escalates to ‘WHOOP WHOOP PULL UP,’ repeated continuously until the pilot initiates a climb. A red GPWS warning light also illuminates on the glareshield.
Common scenarios that produce a sink rate warning include a steep final approach, a hard landing attempt in strong headwind conditions, and descent in a windshear event. The warning is most frequently heard during approach and landing phases. The correct crew response is to immediately reduce descent rate or initiate a go-around.

EGPWS vs GPWS: Key Differences

EGPWS (Enhanced Ground Proximity Warning System), also called TAWS (Terrain Awareness and Warning System), is the modern evolution of the original GPWS. The critical limitation of basic GPWS was its blind spot: it could only look directly below the aircraft using a radar altimeter. If the aircraft was flying toward a steep cliff or sudden rise in terrain ahead, GPWS would not detect the threat until it was too late to avoid.

EGPWS solves this by adding a worldwide digital terrain and obstacle database combined with GPS positioning. The system continuously compares the aircraft’s current position and projected flight path against the terrain database, providing look-ahead warnings up to minutes before a conflict instead of seconds.

Feature	GPWS	EGPWS / TAWS
Detection method	Radar altimeter only (looks down)	Radar altimeter + GPS + terrain database
Warning type	Reactive (threat is already below)	Predictive (threat is ahead on flight path)
Terrain display	None	Color-coded terrain map on navigation display
Blind spot	Cannot detect steep terrain ahead	Look-ahead function eliminates blind spot
Windshear detection	Not available	Mode 7 adds windshear detection
Terrain Clearance Floor	Limited protection	TCF protects even in landing configuration
Introduced	1970s	1996 (Honeywell EGPWS)
Regulatory standard	Basic requirement	Required for most turbine aircraft with 6+ seats

How GPWS and EGPWS Prevent CFIT

Controlled Flight Into Terrain (CFIT) occurs when an airworthy aircraft under full pilot control is unintentionally flown into the ground, water, or an obstacle. The crew is typically unaware of the hazard until it is too late. CFIT was one of the leading causes of aviation fatalities before GPWS became mandatory.

GPWS prevents CFIT through four mechanisms. First, it generates timely alerts early enough for the crew to take corrective action. Second, EGPWS adds a forward-looking terrain awareness display showing a color-coded map of terrain relative to the aircraft’s altitude. Third, mandatory training means pilots respond to GPWS/EGPWS alerts with immediate, practiced escape maneuvers. Fourth, the Terrain Clearance Floor (TCF) function in EGPWS protects against premature descent even when the aircraft is in landing configuration, a scenario where basic GPWS would remain silent.

The impact has been significant. Prior to mandatory GPWS installation, large commercial aircraft experienced approximately 3.5 fatal CFIT accidents per year. Following widespread EGPWS adoption, CFIT accident rates in commercial aviation dropped by 89% from 1999 to 2019.

EGPWS Terrain Display: Color Coding Explained

When EGPWS is active, pilots see a color-coded terrain overlay on the navigation display that gives immediate visual awareness of surrounding terrain relative to the aircraft’s altitude:

• Black or blank: Terrain more than 2,000 ft below the aircraft. No threat.
• Green: Terrain 1,000 to 2,000 ft below. Low concern.
• Yellow: Terrain within 1,000 ft below or rising to aircraft altitude. Caution.
• Red: Terrain at or above aircraft altitude. Immediate warning.
• Magenta: Terrain significantly above aircraft altitude. Extreme hazard.

EGPWS aural warnings in look-ahead mode always include the word ‘AHEAD,’ for example ‘TERRAIN AHEAD PULL UP’ or ‘OBSTACLE AHEAD PULL UP,’ which distinguishes them from reactive basic GPWS alerts.

TAWS and Regulatory Requirements

In the United States, FAA regulations (14 CFR 91.223) require an approved TAWS on all turbine-powered aircraft configured for six or more passenger seats. For Part 135 charter operations, jets with 10 or more seats must carry Class A TAWS (with terrain display), while aircraft with 6 to 9 seats require at least Class B TAWS. ICAO Annex 6 defines Standards and Recommended Practices for Terrain Awareness and Warning Systems (TAWS), with applicability based on aircraft type, weight, and passenger seating configuration. Part I applies to Commercial Air Transport operations, while Part II applies to International General Aviation.
Operators must ensure terrain databases are kept current. Database coverage must match planned routes. If a planned destination is not in the TAWS database, the database must be updated before the flight.

GPWS and EGPWS in Business Aviation Operations

For business aviation operators, GPWS and EGPWS compliance is part of every flight support plan. Just Aviation coordinates end-to-end trip support at airports worldwide, including flight permit management, ground handling, and pre-departure documentation verification.

Related services: Trip Planning | Ground Handling | Flight Permits | Aviation Fuel

Operational Support for Flight Safety Systems

GPWS and EGPWS are key onboard safety systems that enhance terrain awareness and help prevent Controlled Flight Into Terrain (CFIT) through timely and predictive alerts during flight operations. Just Aviation supports operators with operational coordination and flight support services where system awareness and safety-focused planning are part of broader flight operations.

Need operational support or flight coordination assistance?
Contact the operations team at [email protected] for assistance with operational coordination and flight support requirements.

Frequently Asked Questions

What is GPWS in aviation?
GPWS stands for Ground Proximity Warning System. It is an onboard safety system that alerts pilots when the aircraft is at risk of flying into the ground or an obstacle. It uses radar altimeters and flight parameter sensors to monitor the aircraft’s proximity to terrain and issues voice alerts such as ‘SINK RATE,’ ‘TERRAIN,’ and ‘PULL UP’ when thresholds are exceeded. GPWS became mandatory for large commercial aircraft in the United States in 1974.

What is the difference between GPWS and EGPWS?
Basic GPWS is a reactive system that looks directly below the aircraft using a radar altimeter. EGPWS adds a global terrain and obstacle database combined with GPS positioning, giving it the ability to look ahead along the flight path and provide earlier, predictive warnings. EGPWS also displays a color-coded terrain map in the cockpit. EGPWS effectively eliminates the blind spot that basic GPWS has for steep rising terrain ahead.

What does a sink rate warning mean?
A sink rate warning (GPWS Mode 1) means the aircraft’s rate of descent is too high for its current altitude above the ground. The system issues ‘SINK RATE, SINK RATE’ as the first alert. If uncorrected, it escalates to ‘WHOOP WHOOP PULL UP.’ The correct response is to immediately reduce the descent rate or initiate a go-around. It is the most frequently heard GPWS alert during the approach phase.

What is TAWS in aviation?
TAWS stands for Terrain Awareness and Warning System. The FAA uses TAWS as the official regulatory term for the class of systems that includes both GPWS and EGPWS. TAWS equipment is classified as Class A (required for larger aircraft, including terrain display) or Class B (for smaller aircraft, basic terrain warnings without display).

How many modes does GPWS have?
Basic GPWS has five core modes covering excessive descent rate (Mode 1), terrain closure rate (Mode 2), altitude loss after takeoff (Mode 3), unsafe terrain clearance (Mode 4), and glideslope deviation (Mode 5). EGPWS adds Mode 6 for altitude callouts and Mode 7 for windshear detection, giving a total of seven modes.

Which aircraft are required to have GPWS or EGPWS?
In the US, all turbine-powered aircraft configured for six or more passenger seats are required to carry an approved TAWS under 14 CFR 91.223. ICAO mandates it on all commercial aircraft over 5,700 kg or authorized to carry more than nine passengers. Most modern business jets above the light single-engine class are equipped with EGPWS or an equivalent TAWS.

Does EGPWS work reliably in remote or mountainous regions?
EGPWS performs well in most regions where its terrain database has coverage. In very remote areas or regions with limited database granularity, some reduction in effectiveness is possible. Operators flying to such regions should verify database coverage in advance, combine EGPWS use with pre-flight chart analysis, and apply conservative altitude margins over uncharted terrain.

Sources:
https://www.ecfr.gov/current/title-14/chapter-I/subchapter-F/part-91/subpart-C/section-91.223
https://nbaa.org/aircraft-operations/communications-navigation-surveillance-cns/terrain-awareness-and-warning-systems-taws-ground-proximity-warning-systems-gpws/
https://www.icao.int/sites/default/files/APAC/RASG/SafetyTools/03-Advisory-Circular-Guidance-for-Operators-on-Training-Programme-on-the-use-of-Ground-Proximity-Warning-System-GPWS.pdf
https://en.wikipedia.org/wiki/Ground_proximity_warning_system
https://accidentstats.airbus.com/accident-rates-by-category-generation/