Let's pick up right where the classic GPWS leaves off — Page 461, Lesson 563

Let's pick up right where the classic GPWS leaves off. I want to walk you through the limitation we just hit — the sheer cliff problem — and then show you exactly how Enhanced GPWS, or EGPWS, solves it. So, the core limitation of a basic Mode 2 GPWS is that its warning is dependent upon the steepness of the terrain. Think about what that means physically. The system looks at the radio altimeter and the rate of closure with the ground below. If you're flying toward a sheer cliff, the ground directly beneath you isn't rising yet — it's flat until you're right at the edge. So the closure rate stays low, and the system sees no threat. A sheer cliff ahead will not generate a Mode 2 warning. And here's the kicker — any subsequent warning due to rising ground beyond that cliff will be delayed until the aircraft is actually over that ground. By then, you're already committed. That's the fundamental blind spot of the basic system. Now, how do we overcome it? Enhanced GPWS, with its Terrain Threat Display. The heart of this is essentially an electronic map of the world — a database that gives you ground elevation for virtually any location on the planet. But a map alone isn't enough. The system also needs to know where you are. So it takes information from the aircraft's navigational system — and I want you to note the flexibility here — that can be INS, GPS, or any combination of the two. INS is the Inertial Navigation System, GPS is the Global Positioning System. Either way, the system now knows your position. But position alone still isn't enough. To build a threat picture, it needs your course and your height. And here's another important detail — that height can come from either of two sources. It can come from the ADC, the Air Data Computer, which derives altitude from static pressure. Or it can be derived from GPS itself. So, given the location of the aircraft, its course, and its height, a display can be created. And what does that display show? It shows the locations of terrain that could threaten the safety of the aircraft — not just what's below you, but what's ahead of you along your flight path. Now here's the crucial operational advantage. Using this system, EGPWS can warn of approaching high terrain even when that terrain is not in close enough proximity to initiate a Mode 2 warning. Remember the cliff — the basic system couldn't see it until it was too late. EGPWS sees it from miles away because it's comparing your position and track against the terrain database, not just measuring closure rate with the ground below. And the timing is what makes it genuinely useful. This terrain threat display and warning will be initiated in sufficient time to comfortably avoid any threat of flight into terrain. Let me give you the specific numbers, because these matter. You get approximately 40 to 60 seconds for an alert, and 20 to 30 seconds for a warning. That's the difference between "something's out there" and "you need to act now." Now let's talk about how the display actually communicates this to you. The terrain is shown in shades of green, yellow, and red. And I want to be precise about what the display indicates — it shows terrain not only below the aircraft but also ahead of its flight path. So you're seeing a picture of the world in front of you, not just under you. The warning sequence is staged, and I want you to understand the escalation. At a certain time before predicted impact, the warning will issue a "Caution Terrain" or "Terrain Ahead" message. Now, there's a conditional here — if the system database has obstacle data, the alert may instead be "Obstacle Ahead." So the message depends on what's in the database. At this first stage, the threatening terrain will turn solid yellow. That's your cue that something is there, but you have time. If the situation is allowed to deteriorate — if you keep closing with the high ground — the second message sounds. And this is the one you all know from every aviation movie: "Whoop Whoop Pull Up." At this point, the most threatening terrain will turn solid red. And here's the key operational point — this will happen at sufficient spacing to avoid impact with the terrain, but this time it's demanding more positive control movements. The first alert is a heads-up. The second is a command to act decisively. So let me tie the whole picture together for you. Basic GPWS is reactive — it measures closure with what's below. EGPWS is predictive — it compares your position, course, and height against a worldwide terrain database and gives you a visual picture in green, yellow, and red, with a two-stage audio warning. The first stage, "Caution Terrain" or "Terrain Ahead," with solid yellow, gives you 40 to 60 seconds. The second stage, "Whoop Whoop Pull Up," with solid red, gives you 20 to 30 seconds and demands positive control. That's how we turned the sheer cliff from a blind spot into a visible, avoidable threat.