Ground Proximity Warning System — Page 474, Lesson 564

Let's pick up with the Ground Proximity Warning System, and I want to start with something that's a real safety trap for pilots. We've been talking about the terrain display — the picture that shows you the ground ahead. But here's the critical catch: the accuracy of that display is linked to the accuracy of the navigational equipment. If you have a poor navigation fix, or a malfunctioning navigation system, the display will be dangerously inaccurate. And here's the misuse I want to hammer home: some pilots have been found using the threat display to "thread" their way through high terrain — weaving between peaks. That is a gross misuse of the system, and it's strongly advised against. The display is a warning tool, not a terrain-following instrument. Now, how does the terrain display get turned on? It can be selected by the pilot, or it may be automatically activated whenever the terrain becomes a threat. And where does it live? The threat display may 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 map view with your position at the center. Take a look at Figure 34.12 — that's the terrain display itself. Now, let's move to the "look ahead" warning system, Figure 34.13. The key number here is the warning alert at 1.1 NM — one point one nautical miles. That's the distance ahead at which the system will alert you to terrain. The idea is that the system looks ahead of your flight path, not just below you, and gives you that warning margin. Next, we have the Terrain Clearance Floor. This is a specific function, and I want you to understand exactly what it does. It alerts the crew to possible premature descent for non-precision approaches, regardless of aircraft configuration. So whether your gear is down, flaps are set, or not — it doesn't care. It uses the present aircraft position with respect to the runway. Right now, the EGPWS database is separate from the FMS, and therefore not subject to regular updates. That's a limitation — the terrain data isn't being refreshed as often as the flight management system data. This is likely to change in the future, especially when the Runway Awareness and Advisory System — that's RAAS — is in general use. RAAS is designed to enhance situational awareness on the ground and reduce the likelihood of runway incursions — that's when an aircraft or vehicle gets onto a runway without permission. Figure 34.14 shows the terrain clearance floor. Now, let's talk about Integrity Testing. The GPWS has built-in test equipment — that's BITE. BITE allows all the system's functions and its visual and audible warnings to be tested prior to flight. The preflight BITE is inhibited in flight — you can't run the full test once you're airborne. The test is initiated by 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. There is a short confidence check possible while airborne, but that is not a full BITE check — it's just a quick sanity check. Finally, the Inhibition of EGPWS Modes. This is a serious regulatory point. The EGPWS must not be de-activated — for example, by pulling the circuit breaker — except for approved procedures. And the 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. So there's a hard rule here. When might inhibition 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 — that's an approach using just the localizer for lateral guidance, without the glide slope for vertical guidance. Second, inhibition may also be required when the gear or flap position inputs are known to be non-standard — meaning the system is getting signals that don't match the actual configuration, so you'd inhibit to avoid nuisance warnings. So to tie it together: the terrain display is only as good as your navigation, the look-ahead gives you that 1.1 NM warning, the terrain clearance floor protects you on non-precision approaches, BITE keeps the system honest before flight, and inhibition is tightly controlled by the Operations Manual.