Ground Proximity Warning System — Page 451, Lesson 554

Let’s start with the big picture, because GPWS is one of those systems where the purpose defines everything else. I want you to think of it as a safety net that sits between the aircraft and the ground. 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 one thing 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 those accidents which could result from crew errors or distraction, malfunction or misinterpretation of navigational equipment, or inappropriate ATC instructions. So it’s a defence against human and equipment error — 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 is the brain. It will also indicate a computer failure and any failures of the six input signals. So the system is self-monitoring — it tells you when it’s not healthy. The system operates between 50 feet and 2450 feet actual height above the surface, and it automatically selects the correct mode of operation. That’s a key point: you don’t select the mode — the system does it for you, based on the phase of flight and the inputs it’s receiving. Now, before we go any further, I need to give you two definitions that are absolutely critical, because they’re used throughout the whole system and they mean very specific things. An ALERT is a caution generated by the EGPWS equipment. A WARNING is a command generated by the EGPWS equipment. So an alert is a caution — it’s telling you to be aware. A warning is a command — it’s telling you to act. That distinction matters, because the aural messages we’ll see are split into alerts and warnings. Now, there’s a classification of warnings and alerts that you need to know, because it affects how you interpret what you hear. There are three types. 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, and the warning is real. Nuisance — the equipment provides an alert in accordance with its technical specification, but the pilot is flying an accepted safe procedure. So the system is working correctly, but it’s giving you an alert even though you’re doing something perfectly safe. That’s a nuisance alert. False — a fault or failure in the system causes the equipment to provide a warning that is not in accordance with its technical specification. So the system is malfunctioning, and the warning is not valid. Now, let’s move into the specific modes. The excerpt gives us Mode 1 and Mode 2 in detail. Mode 1 is the excessive descent rate mode. Look at Figure 34.3. The aural alert is “SINK RATE, SINK RATE”, and the aural warning is “WHOOP WHOOP PULL UP”. The visual warning is PULL UP. The graph shows barometric descent rate in feet per minute on the horizontal axis, from 0 up to 6000, and radio altitude on the vertical axis, from 0 up to 2500 feet. The envelope — the shaded area — is where the warning triggers. So if you’re descending at a high rate at a low altitude, you get the SINK RATE alert first, and if you continue into the warning region, you get WHOOP WHOOP PULL UP. Now Mode 2 is the excessive terrain closure rate mode. This one is more complex. Mode 2 monitors Mach number, radio altitude rate of change, barometric altitude, and aircraft configuration. So it’s looking at how fast you’re closing on the terrain, and it takes into account your speed and your configuration. 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”. After leaving the PULL UP area, the repeating TERRAIN message will again be heard while in the terrain portion of the envelope. So you get TERRAIN, TERRAIN first, then WHOOP WHOOP PULL UP, and if you’re still in the terrain portion but out of the PULL UP area, you get the repeating TERRAIN again. Here’s an important detail: if both boundaries are penetrated while in the landing configuration, only the repeating TERRAIN aural alert will occur. So in the landing configuration — gear down, flaps down — you don’t get the WHOOP WHOOP PULL UP; you just get the repeating TERRAIN. And the terrain message is repeated each 1.5 seconds. Now, there’s a Mach number effect. As Mach number increases from 0.35 to 0.45 with gear up, the highest radio altitude at which the Mode 2 alert warning will occur is increased to 2450 feet. So at higher Mach numbers, the envelope extends higher. 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 suppress that higher portion of the envelope. Let me summarise Mode 2 for you. The aural alert is “TERRAIN, TERRAIN”, the aural warning is “WHOOP WHOOP PULL UP”, and the visual warning is PULL UP. So, to tie it together: GPWS is a self-monitoring system that operates between 50 and 2450 feet, takes inputs, processes them in the central processing unit, and produces alerts and warnings. Alerts are cautions, warnings are commands. And the warnings can be genuine, nuisance, or false. Mode 1 handles sink rate, Mode 2 handles terrain closure rate, and each has its own specific aural and visual outputs. That’s the foundation. When you’re ready, we can move on to the next mode.