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.
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.
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.
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.
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.
TAWS A and TAWS B
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:
- Excessive rate of descent
- Excessive closure rate to terrain
- Altitude loss after takeoff
- Negative climb rate
- Flight into terrain when not in landing configuration
- Excessive downward deviation from glideslope
- 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:
- Excessive rate of descent
- Excessive closure rate to terrain (per Advisory Circular (AC) 23-18, to 500 feet above terrain)
- Negative climb rate or altitude loss after takeoff
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.