Ground Proximity Warning System — Page 451, Lesson 554

Let’s get into the Ground Proximity Warning System — GPWS. This is a safety net that sits between the aircraft and the ground, and I want you to understand it the way a professional pilot does, because one day your life and your passengers’ lives may depend on how you respond to it. First, the big picture. The aim of the system is to give you — the pilot — visual and audible warning signals when the aircraft’s proximity to the terrain poses a potential threat to its safety. Now, I want to be very clear about something right away: GPWS is not a foolproof means of preventing a collision with the earth’s surface. It enhances flight safety, and it can prevent accidents that could result from crew errors or distraction, malfunction or misinterpretation of navigational equipment, or inappropriate ATC instructions. So it’s a defence in depth, not a guarantee. Let’s look at the architecture. Figure 34.1 shows the three elements of a GPWS: inputs, outputs, and a central processing unit. The central processing unit — the CPU — is the brain. It does two important jobs beyond processing: it indicates a computer failure, and it indicates any failures of the six input signals. So the system is self-monitoring; if an input signal is lost or the computer itself fails, you get an indication. Now, the operating envelope. The system operates between 50 feet and 2450 feet actual height above the surface. That’s the band where it’s active. And within that band, it automatically selects the correct mode of operation. You don’t select modes — the system does it for you based on the situation. Before we go further, I need to lock in two definitions, because they are precise and they matter. An ALERT is a caution generated by the EGPWS equipment. A WARNING is a command generated by the EGPWS equipment. Notice the difference: an alert is a caution — it’s telling you to be aware. A warning is a command — it’s telling you to act, and act now. And you’ll see the EGPWS acronym — that’s Enhanced Ground Proximity Warning System, the enhanced version of GPWS. Now, there are three types of warnings or alerts, and you need to know how to classify them, because it affects how you trust the system. First, GENUINE. The equipment provides a warning in accordance with its technical specification. In other words, the system is doing exactly what it was designed to do — the warning is real and correct. Second, NUISANCE. The equipment provides an alert in accordance with its technical specification — so the system is working correctly — but the pilot is flying an accepted safe procedure. So the alert is technically valid, but you’re doing something safe, and the system is just being annoying. It’s a nuisance, not a malfunction. Third, FALSE. A fault or failure in the system causes the equipment to provide a warning that is not in accordance with its technical specification. Here the system is broken — it’s giving you a warning it shouldn’t be giving you. That’s a false warning. So: genuine is correct and real, nuisance is correct but unnecessary, false is incorrect due to a fault. Now let’s move to the specific system we’re studying — the Boeing 737 Mark II EGPWS. This is a real, specific installation. Figure 34.2 shows the first officer’s instrument panel, where you’d see the visual indications. Let’s look at MODE 1 first. This is the mode that deals with excessive sink rate. Look at Figure 34.3 — MODE 1. The aural alert is “SINK RATE, SINK RATE.” The aural warning is “WHOOP WHOOP PULL UP.” And the visual indication is the PULL UP light. Now look at the axes on that graph. The vertical axis is radio altitude in feet — from 0 up to 6000 feet. The horizontal axis is barometric descent rate in feet per minute — from 0 up to 5000, and I can see values like 70, 50, 500, 1000, 1500, 2000, 2500 marked. The envelope — the shaded area — defines where the warning triggers. If you’re descending at a rate that puts you inside that envelope at a given radio altitude, you get the alert and then the warning. Now, MODE 2 — Excessive Terrain Closure Rate. This is a different threat. Mode 1 is about how fast you’re descending. Mode 2 is about how fast the terrain is coming up at you — the closure rate with the ground. Mode 2 monitors four things: Mach number, radio altitude rate of change, barometric altitude, and aircraft configuration. So it’s watching your speed in Mach, how fast your radio altitude is changing, your barometric altitude, and whether you’re in a clean or configured state — gear up or down, flaps set, and so on. Mode 2 has two boundaries. Penetrating the first boundary causes an aural alert of “TERRAIN, TERRAIN,” followed by the repeated aural warning “WHOOP, WHOOP PULL UP.” So first you hear the alert, then the warning. Here’s a subtlety. After leaving the PULL UP area, the repeating TERRAIN message will again be heard while in the terrain portion of the envelope. So the sequence is: you enter the terrain portion, you hear TERRAIN, TERRAIN; you go deeper, you get WHOOP WHOOP PULL UP; you leave the PULL UP area but you’re still in the terrain portion — you hear the repeating TERRAIN message again. But there’s an important exception. If both boundaries are penetrated while in the landing configuration, only the repeating TERRAIN aural alert will occur. So if you’re configured for landing — gear down, flaps set — and you penetrate both boundaries, you do NOT get the WHOOP WHOOP PULL UP. You only get the repeating TERRAIN alert. And that terrain message is repeated each 1.5 seconds. So the message cycles every one and a half seconds. Now here’s a key performance characteristic. As Mach number increases from 0.35 to 0.45 with gear up, the highest radio altitude at which Mode 2 alert warning will occur is increased to 2450 feet. So at higher Mach, with the gear up, the envelope extends higher — up to 2450 feet radio altitude. But this higher portion of the envelope is inhibited with the flap override switch in the FLAP OVRD position. So if you put the flap override switch to FLAP OVRD, you inhibit that higher part of the envelope — the system won’t warn you at those higher altitudes in that condition. And for MODE 2, the aural alert is “TERRAIN, TERRAIN,” the aural warning is “WHOOP WHOOP PULL UP,” and the visual is PULL UP. So let me tie this together. You have a system that operates between 50 and 2450 feet above the surface. It takes inputs, processes them in a central unit that also monitors itself and its six inputs. It gives you alerts — cautions — and warnings — commands. Those can be genuine, nuisance, or false. And it operates in modes. Mode 1 catches excessive sink rate. Mode 2 catches excessive terrain closure rate, with two boundaries, a specific aural sequence, a landing-configuration exception, and a Mach-dependent envelope that can be inhibited by the flap override switch. That’s the foundation. When you hear “SINK RATE” or “TERRAIN, TERRAIN” or “WHOOP WHOOP PULL UP,” you’ll know exactly what the system is telling you and why.