Terrain Awareness and Warning Systems

your aircraft. The TCAS system may have its own display or be incorporated into the weather radar display (Figure 14.12).

Some TCAS systems are coupled into special vertical speed indicators, incorporating a row of red and green lights around the perimeter. In the event of a traffic advisory, the flight crew pitches the aircraft in such a manner that the vertical speed indicator points toward the green lights.

It’s important to remember that TCAS is unable to detect any aircraft that do not have operating transponders. So, while a TCAS system is a significant aid in avoiding midair collisions, it cannot replace the see-and-avoid concept of air traffic separation you’ve been using for years. Think of it as an extra set of eyes.

The ground proximity warning system (GPWS) is designed to warn flight crews of excessive descent or closure rate with terrain. A GPWS typically consists of an electronic control unit, warning light, and audio warning located in the cockpit. The control unit monitors the aircraft’s radar altimeter when the aircraft is within 2,500 feet of the ground. When an excessive descent or closure rate with terrain is sensed, the warning light illuminates, and a synthesized voice calls out one of several warnings, depending upon circumstances.

GPWS units can be programmed to provide a variety of warnings. Among them (and their voice warnings): inadequate rate of climb after takeoff (“Don’t sink!”); excessive descent rate and closure with the ground (“Pull up!”); and closure with rising terrain (“Terrain!”). Improper aircraft configuration for landing may also be covered by monitoring gear and flap positions. On EMB-145s, for example, if the aircraft is not properly configured for landing, there are GPWS warnings at 500 feet above ground (“Too low gear!”) and at 200 feet (“Too low flaps!”).

In a further effort to reduce CFIT accidents (controlled flight into terrain), the enhanced ground proximity warning system (EGPWS) is a new advanced system specifically designed for “glass cockpit” aircraft. EGPWS operates by comparing surface terrain data contained in an onboard computer database with the aircraft’s exact position as determined by the navigation computer. EGPWS systems display threatening terrain on the EFIS primary flight display’s map mode, giving the flight crew a visual indication and therefore situational awareness of terrain around them (see Figure 14.13).

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Green = -2000 ft. below to -500 ft. below aircraft Yellow = -500 ft. below to +2000 ft. above aircraft Red = +2000 ft. or more above aircraft Clear = terrain more than 2000 ft. below aircraft

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Pilot’s Navigation Display (terrain mode)

(Exact depiction and altitudes vary by manufacturer and flight configuration.)

Note: terrain displayed in altitude difference from aircraft’s current altitude.

FIGURE 14.13 | Enhanced ground proximity warning (EGPWS) terrain display.