First, let's set the scene — Page 461, Lesson 563

I want to walk you through the limitation of the basic GPWS Mode 2 and how Enhanced GPWS, or EGPWS, overcomes it with the Terrain Threat Display. First, let's set the scene. Basic GPWS Mode 2 is the warning that fires when the aircraft is closing too fast on terrain. But here's the catch — that warning depends entirely on the steepness of the terrain. If you're flying toward a sheer cliff, that vertical face doesn't generate a Mode 2 warning at all, because the closure rate with the ground isn't building up the way it does over a slope. And any warning that comes later, from rising ground beyond the cliff, will be delayed until the aircraft is actually over that ground. So you could be flying straight at a cliff face and get no timely warning. That's the fundamental weakness. Now, Enhanced GPWS solves this with the Terrain Threat Display. Let me break down what that system actually uses. It relies on essentially an electronic map of the world — that map gives you ground elevation for everywhere. Then it takes information from the aircraft's navigational system, whether that's INS, GPS, or any combination of the two. INS is the Inertial Navigation System, and GPS is the Global Positioning System. So you have the terrain database on one side, and the aircraft's position on the other. Given the location of the aircraft, its course, and its height — and that height can come either from the ADC, the Air Data Computer, or it can be derived from GPS — the system can create a display. That display shows the locations of terrain that could threaten the safety of the aircraft. So instead of waiting for the closure rate to build up, EGPWS can warn of approaching high terrain even when that terrain is not close enough to trigger a Mode 2 warning. Now, the timing is critical. This terrain threat display and warning will be initiated in sufficient time to comfortably avoid any threat of flight into terrain. The numbers to remember: approximately 40 to 60 seconds for an alert, and 20 to 30 seconds for a warning. So you get an early heads-up, then a more urgent warning closer in. Let me walk you through how the display actually looks and behaves. The terrain is shown in shades of green, yellow, and red. And importantly, the display indicates terrain not only below the aircraft but also ahead of its flight path. So you can see what's coming. Here's the sequence of events. At a certain time before predicted impact, the warning will issue a voice message. That message is either "Caution Terrain" or "Terrain Ahead." If the system database contains obstacle data — meaning man-made obstacles like towers or masts — the alert may instead be "Obstacle Ahead." At this stage, the threatening terrain will turn solid yellow on the display. Now, if the situation is allowed to deteriorate, if you keep closing with the high ground, the second message sounds. That's the classic "Whoop Whoop Pull Up." And at that point, the most threatening terrain will turn solid red. This happens at sufficient spacing to avoid impact with the terrain, but the key difference is this — this time, it's using more positive control movements. So the first alert gives you time for a comfortable, measured response, and the second warning demands a firmer, more immediate action. Let me tie it all together. The core idea is that basic Mode 2 is reactive — it depends on terrain steepness and closure rate. EGPWS with the Terrain Threat Display is proactive — it looks at the electronic terrain map, your position, course, and height, and it shows you the threat ahead in green, yellow, and red, with escalating voice warnings from "Caution Terrain" to "Whoop Whoop Pull Up." That's how the limitation of the sheer cliff is overcome.