Ground Proximity Warning System (GPWS)

The Ground Proximity Warning System (GPWS) is a safety system that was developed to alert and warn pilots if their aircraft is in close danger to ground terrain or an obstacle and the potential for an accident is about to occur.

This system allows crews to navigate around terrain much closer and to conduct precision approaches to lower minimums. This system is crucial on the flight deck and greatly enhances safety by increasing situational awareness.

In airline operations and commercial flights, there are regulations and legal procedures that are mandatory for pilots to follow whenever the ground proximity warning system is signaled. When there is a GPWS alert, both pilots must respond promptly and act immediately according to the procedures in place.

The GPWS is defined by the United States Federal Aviation Administration as a definite type of Terrain Awareness Warning System (TAWS). There are other advanced warning systems introduced known as the Enhanced Ground Proximity Warning Systems (EGPWS).

Ground Proximity Warning System History

The main objective of this system is to prevent Controlled Flight into Terrain (CFIT). A controlled flight into terrain accident is a kind of accident that involves a perfectly functioning airplane that is under the control of a very highly qualified flight crew and is flown into terrain, like an obstacle or water without the awareness of any member of the flight crew. In the 1960s, as jet aircraft became more common, the aircraft moved faster and everything taking place on the flight deck needed to move quicker also. As a result, more CFIT accidents took place.

During the 1970s, some research was carried out on CFIT accidents and it was discovered that a large number of those accidents would have been prevented if there was a warning provided to the pilots and that began the development of a GPWS system. These findings gave rise to a prerequisite passed by the FAA that requires turbine-powered airplanes registered in the U.S. with about six or more passenger seats excluding the pilot and copilot seats, to be installed with FAA-approved GPWS systems with the focus of reducing CFIT accidents. All new aircraft that are manufactured after March 2002 are also mandated to have the proper GPWS systems installed. Additionally, the International Civil Aviation Organization (ICAO) specifically mandates the carriage of GPWS for certain aircraft weight categories.

How the Ground Proximity Warning System Works

GPWS works by relying on an aircraft’s radio altimeter. The radio altimeter determines an aircraft’s altitude by bouncing a radar signal off of the ground and measuring the time it takes for the signal to return. This data is fed into the GPWS system which is responsible for keeping track of the signal and calculating signal trends. When an aircraft’s proximity to the ground reaches an altitude limit programmed into the GPWS system, the crew receives aural and visual alerts on the flight deck.

The GPWS system has been improved over time and has reduced the number of aircraft accidents over the years. Newer enhanced systems also use information from Inertial Reference Systems (IRS) and Global Positioning Satellites (GPS). Enhanced systems can superimpose an aircraft’s position and fight path on flight deck monitors. This can be combined with the data from radio altimeters, IRS and or GPS systems to give pilots a detailed view of the aircraft’s position relative to the terrain.

GPWS has become a standard system on airliners, business jets and even many general aviation aircraft. The lower rate of CFIT accidents can be greatly attributed to GPWS and the enhancements made to this system over the last four decades.

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