GPWS (Ground Proximity Warning System)

Aircraft · 5 min read · Aug 22, 2022

You are probably familiar with these phrases: “Terrain! Terrain!”, “Pull Up! Pull Up!”, “Too Low – Gear!”, “Too Low – Flaps!”, and “Sink Rate!” Simply put, these are warning calls that help in aviation safety. However, there is more background story to their development. Let’s have a look.

Note that, two flight rules aid the flight crew, particularly the pilots, to fly the aircraft – instrument flight rules, IFR, and visual flight rules, VFR. By definition, with VFR, the pilot flies within visual meteorological conditions and as such, nearly all flights are limited from dawn to dusk making it ideal for short-haul flights and low altitude flights, for example scud running.

However, with IFR, the flight crew rely on aircraft instruments for navigation aid because the meteorological or weather conditions are not clear enough to allow for a clearer sight of the flight path. Nonetheless, this makes it possible to even fly at night and have long-haul flights. As such, the aircraft can reach high altitudes up to 30,000 feet.

This answers the question, how do pilots avoid colliding with obstacles or the ground, particularly when flying under unclear weather conditions?

Following previous studies in the early years of the onset of aviation that most airworthy accidents collided with mountains and other steep terrains for no evident reason, a basic system, known as a GPWS, was developed. The system detects obstacles and provided visual warnings and aural warnings to the flight crew against colliding into the ground.

Let’s have a look at how the GPWS works, its shortcomings, and how it was redesigned into the enhanced ground proximity warning system, EGPWS, and terrain awareness warning system, TAWS. But first, what is the GPWS?

An airplane cockpit with visible GPWS screens illuminated.

What is the ground proximity warning system, GPWS?

The basic ground proximity warning system, GPWS, a product of the Bendix Corporation, actuates ground-collision avoidance. It was developed against the backdrop of controlled flight into terrain, CFIT, accidents. CFIT accidents are those that a properly functioning airplane under the airmanship of qualified pilots and other flight crew ram into terrain, ground, water bodies, or other obstacles without evident awareness from the flight crew.

So, how does the GPWS work?

Working principle of the GPWS

The system has a radio altimeter, which measures the aircraft’s height from the immediate obstacle, ground, or water body, below the aircraft then sends visual warnings or sometimes aural warnings, depending on the proximity to the obstacle, at preset altitudes. Through redevelopments, the GPWS had many inputs from many other sensors of the aircraft to detect other dangerous conditions.

Equally, the GPWS is programmed with predefined flying configurations known as modes, which rely on inputs such as the aircraft’s speed to make a visual or aural warning. Some of these modes include:

1.      Unsafe terrain clearance: “Terrain! Terrain!” “Whoop! Whoop! Pull Up!”

2.      Excessive diving speed: “Sink rate!”

3.      Wind shear protection: “Wind shear!”

4.      Excessive deviation from the glide path provided by the instrument landing system: “Glideslope!”

5.      Loss of altitude after takeoff in initial climb: “Don’t sink!”

6.      Excessive terrain closure rate: “Terrain! Pull Up!”

However, the GPWS had some limitations as discussed.

Limitations of the ground proximity warning system

Foremost, the radio altimeter equipped with the GPWS cannot predict future terrain features. That is, it can only calculate the terrain clearance for those obstacles directly below the aircraft.

Consequently, in the sudden change of terrain such as steep terrain, the pilot is not accorded adequate time to evade it considering that it was not initially captured by the GPWS. As such, the aircraft closure rate is not detected in time by the GPWS for the flight crew to have a better terrain awareness and subsequently, an aircraft safety mitigation strategy.

Secondly, the relatively low sophistication of the GPWS makes it output undesirable warnings, particularly when the aircraft’s landing gear system is actuated in the down configuration.

The GPWS is unable to differentiate a landing from a too low terrain clearance or low-attitude flight in the region of a landing, with the landing gear down, such that unless the mode is preset to a landing configuration, its warning system would be actuated, suggestive of low terrain clearance.

Notably, these two limitations of the GPWS led to its development into an EGPWS.

Airplane cockpit at night with illuminated instrument screens, including GPWS.

The Enhanced GPWS or TAWS

Your guess is as good as mine. It is rather obvious that the enhanced GPWS helped to fix the limitations of the basic or traditional GPWS. The EGPWS, which is generically referred to as the terrain awareness warning system, TAWS, offers a clearer view of the flight path such that it can predict imminent obstacles and other hazards.

Having made a market entry in 1996, the EGPWS provides warnings to the flight crew with a broader field of view almost the stretch of a runway threshold. It is equipped with a forward-looking terrain awareness, FLTA, system that alerts the flight crew of any hazardous obstacle or terrain ahead of them and Terrain Clearance Floor (TCF) function.

In doing this, it is integrated with the global positioning system, GPS, which provides an accurate aircraft navigational data such as position, ground track, and speed based on a database of digital terrains and obstacles. These data are automatically captured and together with the aircraft’s altitude, they are cross-referenced with the information in the database, stored within a computer in the aircraft.

Ultimately, there is a match of the prevalent flight conditions and profile with that of the database, rendering quality and detailed images of the imminent terrain on a dedicated monitor or a weather radar screen.

Other key differences between GPWS and EGPWS/TAWS

Unlike the basic GPWS, the TAWS provides more solid and explicit images of the terrain, enabling the flight crew to have a more in-depth terrain awareness. Additionally, some TAWS utilize the flight plan as set in the flight management system, FMS, analyze it and then plot all the potential hazards along the route. In this way, the pilots and the copilot can make better judgments on how to evade colliding with such obstacles or terrain.

Further, whereas the warning system of the basic GPWS was limited to generic lights and beep sounds, most of the TAWS monitors have uniquely coded lights and audio warnings that “come alive” depending on the situation or terrain. These are summarized below.

  • Green/black display: For those below the aircraft’s altitude.
  • Yellow display: For those near the aircraft’s altitude.
  • Red display: For those that are way above the aircraft’s altitude.
  • Aural and visual warnings: When the aircraft is about a minute from ramming into the ground.
A pilot controlling an airplane and referring to a GPWS.

Regulations around the GPWS and EGPWS/TAWS

To reduce the rapidly increasing CFIT accidents, the Federal Aviation Administration of the USA, in 1974, initially mandated that all Part 121 and some Part 135 certificate holders have aircraft equipped with the GPWS. This requirement however suffered a setback with the limitations of the GPWS and was further revised in 1978, 1992, 2001, and 2007.

Fast forward, with the new technologies of the EGPWS and broadly the TAWS, in 2017, the Federal Aviation Administration mandated that all turbine-powered aircraft, both helicopters and fixed-wing, carrying at least 6 passengers, be equipped with modern TAWS.

Similarly, in Europe, all commercial air transport aircraft with a minimum take-off weight of 5700 kg or a passenger-carrying capacity of at least 9, are mandated to install Class A and Class B TAWS for turbine and piston-powered engines respectively. Note, TAWS is broadly divided into Class A and Class B with the degree of sophistication increasing in B than in A. As such, Class B TAWS are often used in larger airplanes than those in Class A. Class C is an optional configuration.


The inception of the GPWS, with further redesign and improvements into EGPWS/TAWS, has undoubtedly improved terrain awareness for the flight crew. Ultimately, this has reduced the number of CFIT accidents by making the rather dangerous situations predictable and allowing for the adoption of evasive strategies ahead of time.

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Jet pilot @NASA

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