Ground Proximity Warning System — Page 474, Lesson 564

Let’s pick up with the Ground Proximity Warning System, and I want to start with a critical safety point about the terrain display. The accuracy of this display is directly linked to the accuracy of your navigational equipment. If you have a poor navigation fix, or a malfunctioning nav system, the display will be dangerously inaccurate. Now, here’s the serious part: some pilots have been found using this threat display to “thread” their way through high terrain. That is a gross misuse of the system, and I strongly advise against it. You must never use it as a primary terrain-avoidance tool. The terrain display can be selected by you, the pilot, or it may activate automatically whenever the terrain becomes a threat. It can be incorporated with the weather radar display, the navigational display, or it may have its own Plan Position Indicator—that’s the PPI, a radar-style screen showing a top-down view. Now, let’s look at the “Look ahead” warning system. This is a forward-looking function. The warning alert triggers at 1.1 nautical miles—that’s the NM you see in the figure. So the system is giving you a warning when terrain ahead is within 1.1 NM of your flight path. Next, we have the Terrain Clearance Floor. This function alerts the crew to possible premature descent for non-precision approaches, regardless of aircraft configuration. It uses the present aircraft position with respect to the runway. Here’s an important detail: currently, the EGPWS database is separate from the FMS—the Flight Management System—and therefore it is not subject to regular updates. That is likely to change in the future, especially when the Runway Awareness and Advisory System, or RAAS, is in general use. RAAS is designed to enhance situational awareness on the ground and reduce the likelihood of runway incursions. Now, integrity testing. The GPWS is provided with built-in test equipment, which we call BITE. This allows all its functions and visual and audible warnings to be tested prior to flight. The preflight BITE is inhibited in flight—so you can’t run the full test while airborne. The test is initiated by you, the pilot, pressing the test switch. During flight, the system is continuously monitored to confirm its serviceability, and any in-flight failure is automatically indicated on the flight deck. A short confidence check is possible while airborne, but that is not a full BITE check. Finally, inhibition of EGPWS modes. The EGPWS must not be de-activated—for example, by pulling the circuit breaker—except for approved procedures. Instructions on inhibition must include a statement that no person may de-activate the EGPWS except in accordance with the procedures stated in the Operations Manual. There are specific cases where inhibition may be desirable. First, inhibition of the glide slope mode may be desirable when a glide slope signal is present, but the aircraft is deliberately being flown without reference to it. For example, the pilot may have discontinued the ILS to land on a different runway, or is performing a localizer-only approach. Second, inhibition may also be required when the gear or flap position inputs are known to be non-standard. So, to tie it together: the terrain display is only as good as your nav data, the look-ahead warns at 1.1 NM, the terrain clearance floor guards against premature descent on non-precision approaches, BITE lets you test before flight, and inhibition is tightly controlled—only for approved procedures.