Enhanced Ground Proximity Warning System

Information compiled by: Ben Koop


A ground proximity warning system (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle. The United States Federal Aviation Administration defines GPWS as a type of terrain awareness warning system (TAWS). More advanced systems, introduced in 1996, are known as enhanced ground proximity warning systems (EGPWS), although sometimes confusingly called terrain awareness warning systems.[1]

An EGPWS is simply a GPWS that uses GPS (and possibly DME) and Barometric information to allow the EGPWSystem to sense forward obstacles and windshear. The EGPWS takes many other inputs into account as well when deciding whether or not to give the crew a terrain warning. It also provides a terrain map referenced to the aircraft's surroundings. This is the main advantage of EGPWS over GPWS.

The following diagram shows the difference in inputs and outputs when comparing the first GPW Systems and the now current EGPW Systems.
image created by: Ben Koop, 2011


Basic Principles

A EGPWS (TWAS) works by using digital elevation data and aircraft instrument values to predict if a likely future position of the aircraft intersects with the ground.[2] This system is used to prevent controlled flight into terrain (CFIT) accidents and monitor controlled flight to terrain (CFTT) by giving an aural and visual warning when terrain or another obstacle posses an imediate threat to the aircraft.

The EPGWS has 7 modes of opertion that work in conjuntion to protect the aircraft from a CFIT and/or a hazardous CFTT condition.

Mode 1: Excessive Descent Rate Alert/Warning
Mode 1 relies on the Rad Alt system to send accurate altitude information to the EGPWS to sense sink rates.
Click here to watch a video on MODE 1.

Mode 2: Excessive Closure Rate of Terrain
Mode 2 uses a combination of the on board flight management system (FMS) Database, GPS and Rad Alt information to sense forward terrain.
Click here to watch a video on MODE 2.

Mode 3: Altitude Loss After Takeoff
Mode 3 monitors the aircraft configuration (landing gear & flaps) to determine when the aircraft has successfully taken off or is on a missed approach. EGPWS monitors the aircraft's positive rate of climb to make sure that the pilot is not letting the aircraft sink.
Mode 3 also sets up a minimum safe altitude which must be reached after takeoff in order for no warnings to occur. This altitude is also part of the minimum terrain clearance altitude tiers used for mode 4 warnings.
Click here to watch a video on MODE 3.

Mode 4: Unsafe Terrain Clearance
Mode 4 builds tiered minimum safe terrain clearance levels based on the aircraft's configuration, speed and proximity to the ground. These tiers provide the system with a reference for whether or not the aircraft is in danger of a CFIT or a CFTT incident or whether it is simply landing under normal conditions.
Click here to watch a video on MODE 4.
Mode 5: Excessive Glideslope Deviation
Mode 5 sense the aircraft's position in relation to the center line of the received glideslope information. Depending on how far off the aircraft is relative to that line it issues a soft or hard "GLIDESLOPE" warning.
Click here to watch a video on MODE 5.

Mode 6: Advisory Callouts
Mode 6 provides advisory callouts to the aircraft crew. These callouts are specific to each aircraft and are setup (programmed) for the operations and flight conditions that that specific aircraft will undergo. These callouts include, but are not limited to: altitudes, minimums, and bank angles. The point at which these parameters necessitate a warning depends on the configuration. For example, a bank angle callout with happen sooner on takeoff than during normal flight.
Click here to watch a video on MODE 6.
Mode 7: Windshear Detection
Mode 7 compiles information from the aircraft's air data computer (ADC) and the different barometric sources on board the aircraft to determine if the aircraft is in a windshear condition, also know as a micro burst.
Two alerting envelopes provide either a Windshear Caution (light) alert or a Windshear Warning (aural)
alert depending on the aircraft's proximity to, and involvement with, the micro burst event.

Click here to watch a video on MODE 7.

There are two more modes of operation which relate more to the visual terrain map database.

Enhanced Terrain Clearance Floor (ETCF) Mode
This mode uses a world wide terrain database with varying degrees of resolution and an airport database containing all hard-surface runways at least 3500 feet long, which have a published instrument procedure. [3] These databases are used to create tiers of safe altitude clearance levels. When ETCF is used in conjunction with GPS and INS systems it can virtually eliminate CFIT accidents.

Click here to watch a video on ETCF.

Peaks Mode
Peaks mode displays terrain at varying levels, in multiple colors, in order to give the flight crew and accurate picture of the surrounding terrain.
Click here to watch a video on PEAKS MODE.

-all mode information [3],[4]-


There are no frequences for EGPWSsystems as a whole. The frequencies that pertain to EGPWS are the ones related to the other systems that EGPWS uses to gather information. Theses include, but are not limited to: Rad Alt, GPS, and DME. The frequencies for these systems are as follows:

Rad Alt: 4.2 GHz - 4.4 GHz
GPS: 1.2 GHz - 1.6 GHz (L Band)
DME: 978 MHz - 1213 MHz

Ground Equipment

The ground equipment that works in conjunction with EGPWS is all related to the individual systems that EGPWS relies on for information. Systems like DME have ground stations as well as GPS, which also has space stations. These stations transmit their respective information via radio waves, which are then picked up by the aircraft and feed to the EGPWS processor to be analyzed.

Aircraft Equipment
Although the EGPWSystem is complicated, it's hardware integration into the aircraft is fairly simple. The EGPWS on board the aircraft is generally only one box with many inputs and few outputs. This system relies on the other various navigation systems to calculate and collect information for the EGPWS so that is can assess if the plane is in danger.
EGPWS Processor Computer
The inputs for the EGPWS can include, but are not limited to:
Airspeed, Barometric Altitude, Angle of attack, Rad Alt, GPS, DME, Glideslope deviation, Landing gear, Flaps, INS/IRS, Air Data Computer, and the Flight Management Computer (Terrain database).[3]
The are no inputs for the pilots to be concerned with except for activating the other systems that feed the EGPWS computer.
There are only 3 basic outputs from the EGPWS, an audio warning, a visual warning and a terrain map.The audio warnings are usually sent to the cockpit speakers but in some cases may also be routed to the pilot and co-pilots headsets. The visual warnings are displayed on annunciators mounted to the instrument panel like this one:
The terrain map can be displayed in a variety of locations. It can been seen on the weather radar display, on a multi-function display (MFD), on an electronic flight instrument system (EFIS), or on its own dedicated display seen here:


Self Test

A self test can be performed simply by powering up the aircraft, making sure that the associated systems are running and then pressing the test button on the annunciator panel. A simple self test for a EGPWSystem might look like this:

EGPWS - Self Test from Ben Koop on Vimeo.

Diagnostic Test
Diagnostic testing of the EGPWS system can usually be done with a laptop and software provided by the manufacturer. In order to do this the maintenance personal will usually have to locate the EGPWS Processor Computer and plug the laptop directly into a service port on the front face in order to program, update or diagnose the unit.

Operational Test

Due to the nature of EGPWS a full operational test is only possible during flight. The aircraft will have to be put into different fligh conditions in order to test each mode of the EGPWS. This test is usually not necessary. A simply self test is adequate enough.to test the "operation" of the system.


[1] Wikipedia, “Ground Proximity Warning System,”: Accessed on 2011-12-14, http://en.wikipedia.org/wiki/Ground_proximity_warning_system.

[2] Wikipedia, “Terrain Awareness and Warning System,”: Accessed on 2011-12-14,

[3] Jeppesen Sanderson Training Products, Avionics Fundamentals. "Chapter 18 - Ground Proximity Warning System (GPWS),": Englewood, Colorado; Library of Congress Cataloging-in-publishing.

[4] Weatherby, Lisa. Power Point Presentation. "Terrain Awareness Systems,": Accessed on 2011-12-14.

For more detailed information please see:
Honeywell, “Enhanced Ground Proximity Warning Systems,”: Accessed on 2011-12-15,