Ground Proximity Warning System (GPWS) and Terrain Awareness and Warning System (TAWS)


The GPWS is an example of a TAWS. The FAA (Federal Aviation Administration) uses the term TAWS to cover all systems that exist or will exist to help prevent controlled flight in to terrain (CFIT). At this current time there is the GPWS and the Enhanced GPWS.
These systems can give audio and visual warnings to the pilots to indicate when they are approaching a situation that will lead to them flying into the ground water or other obstacles on the surface. They are integrated with glass cockpits and other systems on more advanced aircraft with the possibility to provide warning information with Synthetic vision systems (SVS).
However, like other systems uses to prevent accidents they still have their own limitations.


The TAWS was a response to the growing concern about aircraft flying into terrain. So on December 24th 1974, the FAA issued a rule that large aircraft must be fitted with a device to give an aural and visual warning when aircraft was below 2500ft1. These initial devices were simply a radar altimeter that could tell what height the aircraft was above the ground. These early systems however, suffered from a few limitations which meant that the systems quickly evolved. The most noticeable limitation was the fact the radar altimeter could not look ahead and there for could not pick up steep terrain2.


(Image embedded from [Universal Electronics])

The GPWS uses a number of instruments to tries predict the future and indicate whether the pilot is a situation that may cause an accident. There are several situations that the GPWS will issue an alarm, each situation has been given a mode number and may given a different alarm depending on what is happening.

The list of modes:

Mode 1: Excessive descent rate
Mode 2: Excessive terrain closure rate
Mode 3: Altitude loss after takeoff or go-around
Mode 4: Unsafe terrain clearance when not in Landing Configuration
Mode 5: Excessive deviation from ILS glide slope
Mode 6: Descent below the selected minimum radio altitude
Mode 7: Windshear condition encountered

The GPWS gathers information from the instruments and uses computer calculations to determine what the aircraft is doing and its relation with the ground. Some modes depending on the severity of the situation will give an aural alert or an aural warning.

Appropriate TAWS response procedures to each mode are determined after careful study of aircraft type performance capability. They are clearly defined by so that in case of a Warning, they can be followed without hesitation as soon as a triggered. Operators normally define different response procedures based upon memory drills for a Warning (sometimes called a Hard Warning) and an immediate review in the case of an Alert (sometimes called a Soft Warning)2.

Difference in Terms

As mentioned above the GPWS is an example of a TAWS. Terrain Awareness Warning Systems is a term often used to indicate an Enhanced GPWS, however, this is incorrect. TAWS refers to all systems that help prevent CFIT.


There are still however, limitations to theses systems that require the pilots to use their experience and situation awareness to determine if the warning is real. For example, if the instruments are giving false alerts or if the pilot is intentionally in that situation, the pilot must then ignore the warnings that are given to them.

Also with the introduction of glass cockpit into aviation is can create a situation where the pilots either learn to rely on this system to prevent CFIT or they learn incorrect responses to the alert. Either way this can lead to pilots who find that they cause accidents that the TAWS is trying to prevent.

Future Uses

The TAWS’ have done really well to help reduce the number of CFIT in commercial aviation. There are however, possibilities to also prevent disasters happening by intentional flight into terrain. An example would be terrorism. It is possible that the systems could be built to be integrated with other advanced systems to only allow aircraft to fly certain routes thus preventing anyone from flying new over populated areas3.

1. Brown D. (December 2009). FAA History Lesson — December 29 Retrieved From
2. SKYbrary (13th September 2011). GPWS/TAWS Retreived From
3. (n.d.). The Challenge of Security. Retrieved From pxx199-200

Want to know more?

Human Factors and TWAS
Wiki -
TAWS in Action
YouTube Clip of the TAWS
This is the AC that describes the Definition of TAWS and what is required for a system to be class as one$FILE/ATTOVW46/AC23-18.pdf
Explains the EGPWS
Warning System

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