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Terrain Systems (Part Two)

in Information Systems

Terrain Awareness and Warning Systems

A terrain awareness and warning system (TAWS) offers you all of the features of a terrain display along with a sophisticated warning system that alerts you to potential threats posed by surrounding terrain. A terrain awareness and warning system uses the aircraft’s GPS navigation signal and altimetry systems to compare the position and trajectory of the aircraft against a more detailed terrain and obstacle database. This database attempts to detail every obstruction that could pose a threat to an aircraft in flight.


There are presently two classes of certified terrain awareness and warning systems that differ in the capabilities they provide to the pilot: TAWS A and TAWS B.

A TAWS A system provides indications for the following potentially hazardous situations:

  1. Excessive rate of descent
  2. Excessive closure rate to terrain
  3. Altitude loss after takeoff
  4. Negative climb rate
  5. Flight into terrain when not in landing configuration
  6. Excessive downward deviation from glideslope
  7. Premature descent 8. Terrain along future portions of the intended flight route

A TAWS B system provides indications of imminent contact with the ground in three potentially hazardous situations:

  1. Excessive rate of descent
  2. Excessive closure rate to terrain (per Advisory Circular (AC) 23-18, to 500 feet above terrain)
  3. Negative climb rate or altitude loss after takeoff

TAWS Alerts

Aural alerts issued by a terrain awareness and warning system warn you about specific situations that present a terrain collision hazard. Using a predictive “look ahead” function based on the aircraft’s ground speed, the terrain system alerts you to upcoming terrain. At a closure time of approximately 1 minute, a “Caution! Terrain!” alert is issued. This alert changes to the more serious “Terrain! Terrain!” alert when the closure time reaches 30 seconds. In some areas of the world, this terrain warning may very well be too late, depending on the performance of the aircraft. You need to determine the equipment’s criteria and note if the unit makes allowances for lower power output of the powerplant(s) at higher elevations, resulting in lower climb rates than may be programmed into the unit for that aircraft.

A second type of aural alert warns about excessive descent rates sensed by the system (“Sink Rate!”) or inadvertent loss of altitude after takeoff (“Don’t Sink!”).

The introduction of terrain awareness and warning systems has sharply reduced the number of CFIT accidents. Despite this significant leap forward in safety, incidents and accidents involving terrain still happen. In the modern TAWS-equipped cockpit, some of these incidents have been related to pilot reaction to TAWS alerts. TAWS sometimes gives nuisance alerts that desensitize the pilot to TAWS alerts, which can result in the pilot’s decision to ignore a valid alert deemed unnecessary by the pilot. Most TAWS systems contain software logic that attempts to recognize and remain silent in situations in which proximity to terrain is normal. This logic is partly based on the aircraft’s distance from the runway of intended landing. For example, flying at an altitude of 200 feet AGL when 3,500 feet away from the runway is reasonable, but flying at an altitude of 200 feet AGL when 5 miles from the runway is not reasonable. TAWS’ logic attempts to silence itself in normal situations, and to sound in abnormal situations.

Risk: Silencing TAWS Alerts

Despite efforts to minimize nuisance alerts, they still occur occassionally. For this reason, most TAWS systems offer a terrain inhibit switch that allows you to silence TAWS alerts. There have been cases in which pilots have used the inhibit switch or ignored TAWS alerts, thinking they were nuisance alerts, when in fact the alerts were valid indications of a dangerous situation. For this reason, you should train yourself to respond to TAWS alerts just as you would to any other sort of emergency. Always, if in any doubt, set “Full Power and Climb” at VX or VY, depending on the equipment manual and AFM/POH. The practice of simply ignoring or disabling TAWS alerts based on pilot intuition has not proved to be a safe one. Your manufacturer’s reference manual and aircraft flight manual supplement will prescribe specific procedures for responding to TAWS alerts.

The only current, fully certified systems, known as TAWS, are certified under Technical Standards Order (TSO)-C151. TAWS equipment is required for turbine-powered airplanes having six or more passenger seats and manufactured after certain dates (see 14 CFR part 91, section 91.223). TAWS is now an affordable option in many advanced avionics due to decreased cost and increased capabilities of computer circuits and components. All aircraft would be safer with TAWS and crews trained to use the technology.

Risk: Flying in Close Proximity to Terrain

A display that plainly shows your position with respect to surrounding terrain is sometimes cited as the most reassuring system available in the advanced avionics cockpit. The same display can also invite the unwary pilot to attempt risky maneuvers. Suppose that, on a VFR flight to an airport located in hilly terrain, you encounter a layer of fog at 1,100 feet. In an aircraft with no terrain system, you would not consider proceeding to the airport because you have a personal minimum of 1,500 feet. With a ceiling of less than 1,500 feet, you deem the situation simply too risky. With the surrounding terrain neatly displayed in front of you, however, you may feel more confident and be tempted to give it a try. However, a wise pilot remembers that, unless the equipment is TAWS certified, accuracy is suspect. Even with TAWS certification, the information presented is no better than the database accuracy. Consult the equipment handbook or manual to determine the accuracy of the database in that area.

CFIT accidents are still occurring despite the advent of advanced avionics. What has happened here? Psychologist Gerald J. S. Wilde coined the phrase risk homeostasis to refer to a tendency for humans to seek target levels of risk. Our hill-flying scenario illustrates the concept. After pondering the perceived risks, you decide that having the terrain display gives you the same level of perceived risk with a 1,000-foot ceiling as you felt you had at 1,500 feet without the terrain display. You see no need to “give away” this new margin of perceived safety. Rather, you decide to use it to your advantage. Equipped with the terrain display, your new minimum ceiling becomes 1,000 feet, and you continue on your way to the airport.

Wilde does not support the idea of using technology to seek target levels of risk. Rather, he argues that safety measures such as seat belt laws and anti-lock brakes have not resulted in drastic reductions in highway fatalities in part because, in response to the added sense of safety provided by these measures, drivers have emboldened their driving behavior to maintain existing levels of risk.

Another issue is the lack of training in the new equipment and its uses. The functions of TAWS and basically how it works have been previously described, yet there is no training program outside the military that teaches anyone to fly based on the TAWS display. It requires much precision flight training to learn the timing and skills to fly from a display depicting a myriad of data and converting that data into close and low terrain flight directions. All advanced avionics are designed to help the pilot avoid a hazard, not enable the pilot to get closer to it. TAWS is not a terrain flight following system.


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