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Terrain Systems (Part One)

in Information Systems

Terrain systems provide information about significant terrain along your route of flight. Terrain systems were designed to help reduce controlled flight into terrain (CFIT) accidents. Remember, however, that use of these terrain proximity information systems for primary terrain avoidance is prohibited. The terrain proximity map is intended only to enhance situational awareness; it remains the pilot’s responsibility to ensure terrain avoidance at all times.

Safe flight practices include pilot knowledgeability of the maximum elevation figures (MEF), published in blue for each grid square on sectional charts, and planning flight altitudes above those elevations. Despite all efforts by the charting agency to be current, there will always be obstructions in place before the documentation arrives for charting. Therefore, the competent pilot always allows for sufficient clearance for unknown towers and buildings. Experienced pilots have learned that many aircraft cannot outclimb certain mountainous slopes. You should always fly down (descend) into a valley or canyon, rather than attempting to fly up the valley and become trapped in a box canyon too narrow for a turn and too steep to climb over. One regularly overlooked factor is the loss of power generally associated with the higher elevations at which canyons and steep slopes are often found.

Early Systems

Various terrain avoidance systems have been certificated and used in the past. One early system was termed Ground Proximity Warning System (GPWS, often pronounced “GipWhiz”). One major shortcoming of the system was a lack of predictive terrain warnings. Most early systems simply used a radar altimeter as the sensor. The radar altimeter simply indicated the altitude of the aircraft above the ground immediately below the airframe. The subsequently developed enhanced GPWS (EGPWS or eGPWS) used GPS location data combined with a worldwide terrain database to predict that a canyon wall was just ahead and a climb should be started. The older GPWS had no indication of a very close hazard. However, the system did prevent numerous gear-up landings and offered warnings when terrain presented a slope to much higher terrain.

Terrain Display

The most basic type (not necessarily certified) of terrain system is the terrain display. A terrain display uses the MFD to plot the position of the aircraft against a pictorial presentation of surrounding terrain. A terrain display usually relies on a GPS location signal to compare the position and altitude of the aircraft against the terrain found in an internal topographical database. Figure 5-10 shows the position of the aircraft and surrounding terrain displayed on an MFD.

Figure 5-10. Terrain depicted on an MFD.

Figure 5-10. Terrain depicted on an MFD. [click image to enlarge]

Terrain displays use a simple color-coding convention to portray the difference between the present altitude of the aircraft and the height of the surrounding terrain. Terrain more than 1,000 feet below the aircraft is coded black. Terrain less than 1,000 feet but more than 100 feet below the aircraft is coded yellow. Terrain less than 100 feet below the aircraft is coded red. Man-made obstacles (e.g., radio towers, power lines, buildings) generally do not appear in a topographical database.

Monitoring Surrounding Terrain During Departure and Arrival

Terrain displays are especially useful during departure and arrival phases of flight. For example, the aircraft shown in Figure 5-10 has departed Denver and is heading for a waypoint situated in high terrain in the Rocky Mountains. A pilot with good situational awareness has many concerns during the departure. For instance, is climb performance meeting expectations? The terrain display reduces the need for you to perform mental calculations by verifying that the depiction of the terrain that lies ahead of the aircraft is steadily changing from red to yellow to black. If the terrain depiction remains red as the aircraft approaches, you know there is a problem. Similarly, if the aircraft has been assigned a heading and altitude vector by air traffic control, the terrain display provides a simple way of monitoring the safety of these directives. If the equipment is not certified as meeting TAWS requirements (see Title 14 of the Code of Federal Regulations (14 CFR) part 91, section 91.223), the accuracy can be in doubt.

Evaluating a Direct-To Routing

One of the risks involved in proceeding directly to a waypoint is that you may be yet unaware of any significant terrain between the present position and the waypoint. A terrain display offers a convenient way of seeing clearly what lies between here and there as reported and documented in the database. Always consult the MEF values along the path of flight to ensure terrain and obstruction clearance.


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