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Terrain avoidance and warning system equipment, and TAWS classes

TAWS - Terrain avoidance and warning system

In aviation, a terrain awareness and warning system (TAWS) is generally an onboard system aimed at preventing unintentional impacts with the ground, termed “controlled flight into terrain” accidents, or CFIT.

The specific systems currently in use are (GPWS) the ground proximity warning system, and (EGPWS) the enhanced ground proximity warning system. (FAA) The U.S. Federal Aviation Administration introduced the generic term TAWS to encompass all terrain-avoidance systems that meet the relevant FAA standards, which include GPWS, EGPWS, and any future system that might replace them.

As of 2007, 5% of the world’s commercial airlines still lacked a TAWS. A study by the International Air Transport Association examined 51 accidents and incidents and found that pilots did not adequately respond to a TAWS warning in 47% of cases.

Several factors can still place aircraft at risk for CFIT accidents: older TAWS systems, deactivation of the EGPWS system, or ignoring TAWS warnings when an airport is not in the TAWS database.

How Does Terrain Awareness and Warning System Works?

A modern TAWS works by using digital elevation data and airplane instrumental values to predict if a likely future position of the aircraft intersects with the ground.

The flight crew is thus provided with “earlier aural and visual warning of impending terrain, forward-looking capability, and continued operation in the landing configuration.

The difference between TAWS and GPWS and EGPWS

The difference between an early GPWS system and a TAWS or EGPWS system is the addition of (FLTA) the forward-looking terrain awareness function, which uses a database to alert the pilot to hazardous terrain or obstructions that are ahead of the aircraft. The TAWS/EGPWS units also have the premature descent alert (PDA) mode.

Before getting too far, there is another player to introduce: terrain awareness systems (TAS). On this piece of equipment, which is not certified, is a set of maps so you can basically look at a topographical map and see the terrain in your area.

Now that could basically be called a terrain awareness system. While the system will make the pilot aware of the changing terrain by varying the colors, it has no way of actively alerting the pilot of the rising elevation around him.

Basically, it’s just telling the pilot there’s a potential problem based on its understanding of position and GPS altitude. While it has no FLTA capabilities, it’s certainly better than nothing.

An awareness system can break the chain of events that lead to CFIT if you are paying attention to it. A warning system will tell you if you don’t change your flight path you’re in big trouble.

When you start to look at the distinctions between a terrain awareness system, a generic terrain system, and EGPWS what you will get into is really the fidelity of the database and the sophistication of the warning algorithms.

The fidelity of the detailed knowledge of the ground and the knowledge of the obstacles and how the aircraft understands its position are relative to that information.

Classes of TAWS equipment

Terrain avoidance and warning system equipment is classified as:

  1. Class A systems which are required for all but the smallest commercial air transport aircraft,
  2. Class B systems are required by larger General Aviation (GA) aircraft and are recommended for smaller commercial or GA aircraft.

Class A TAWS

Class A TAWS equipment must provide indications of imminent contact with the ground for the following conditions:

  • Excessive Rates of Descent
  • Excessive Closure Rate to Terrain
  • Negative Climb Rate or Altitude Loss After Take-off
  • Flight Into Terrain When Not in Landing Configuration
  • Excessive downward deviation from an Instrument Landing System (ILS) glideslope, Localizer Performance and Vertical Guidance (LPV), or Global Navigation Satellite System (GNSS) Landing System (GLS) glide path.
  • Voice callout “Five Hundred” when the airplane descends to 500 feet above the terrain or nearest runway elevation.

Class B TAWS

Class B TAWS equipment must provide indications of imminent contact with the ground during the following aircraft operations:

  • Excessive Rates of Descent
  • Negative Climb Rate or Altitude Loss After Takeoff
  • A voice callout “Five Hundred” when the airplane descends to 500 feet above the nearest runway elevation.

the purpose of a Terrain avoidance and warning system (TAWS)

Terrain avoidance and warning system (TAWS) equipment provides the following functions:

A Forward-Looking Terrain Avoidance (FLTA) function.

The FLTA function looks ahead of the aircraft along and below its lateral and vertical flight path and provides suitable alerts if a potential controlled Flight into Terrain (CFIT) threat exists.

A Premature Descent Alert (PDA) function.

The DA function of the TAWS uses the aircraft’s current position and flight path information as determined from a suitable navigation source and airport database to determine if the aircraft is hazardously below the normal (typically 3 degrees) approach path for the nearest runway as defined by the alerting algorithm.

An appropriate visual and aural discrete signal for both caution and warning alerts.

Note: Class A TAWS equipment must provide terrain information to be presented on a display system.

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