Ground Proximity Warning System — Page 461, Lesson 557

We're now into the heart of the Ground Proximity Warning System, and I want to walk you through the specific modes that protect you in the critical phases of flight. We've already covered Mode 1 and Mode 2, so let's build on that foundation. Let's start with Mode 3, which is the Altitude Loss after Take-off or Go-around. This mode is your protection against that dangerous situation right after you've committed to climbing. Imagine you've just taken off, or you're in a go-around, and you start to sink back toward the ground. Mode 3 is watching for exactly that. It provides an alert if a descent is made during the initial climb or go-around. The aural alert here is a voice message of "DON'T SINK", and it's repeated every 1.5 seconds until the flight condition is corrected. So the system keeps nagging you until you stop sinking and start climbing again. Now, the activation parameters are precise. Mode 3 is effective between 50 and 700 feet radio altitude. Radio altitude, remember, is your height above the actual terrain below you, measured by the radio altimeter. It generates the alert when the accumulated barometric loss equals approximately 10 percent of the existing barometric altitude. So it's not just a fixed sink rate; it's a proportional loss. If you lose about 10% of your barometric altitude, the system decides you're in trouble. There's a critical arming condition here. Mode 3 does not arm during the descent until below 200 feet radio altitude. So if you're descending from high up, the system won't arm until you get down to 200 feet. But there's a note that clarifies this: Mode 3 arms when the aeroplane descends below 200 feet in the landing configuration. So the arming is tied to your configuration and your altitude. The visual alert for Mode 3 is "PULL UP". So you get the aural "DON'T SINK" and the visual "PULL UP" warning. Now let's move to Mode 4A, which is Unsafe Terrain Clearance with Landing Gear Not Down. This is your protection against landing with the gear up. The terrain clearance mode, with gear retracted, is armed after take-off upon climbing through 700 feet radio altitude. So once you're above 700 feet with the gear up, this mode is active and watching. Here's where it gets interesting, because the alert depends on your speed. When this envelope is penetrated at less than Mach 0.35, the aural alert "TOO LOW GEAR" is sounded. That makes sense—if you're slow and low with the gear up, the system tells you to get the gear down. But when the envelope is penetrated at more than Mach 0.35, the aural alert changes to "TOO LOW TERRAIN", and the upper boundary of the envelope is increased to 1000 feet radio altitude. So at higher speeds, the system expands its protection zone upward. The applicable voice message is repeated each 1.5 seconds until the flight condition has been corrected. The visual alert for Mode 4A is again "PULL UP". Now let's look at Mode 4B, which is Unsafe Terrain Clearance with Flaps Not in Landing Configuration. This mode provides an alert when the gear is down and the flaps are not in the landing position. So you've got the gear down, but you haven't configured the flaps for landing—that's the scenario this mode watches. Again, the alert depends on speed. If the envelope is penetrated at less than Mach 0.28 with the flaps not in the landing position, the aural alert "TOO LOW FLAPS" is sounded. But when the envelope is penetrated at more than Mach 0.28, the aural alert changes to "TOO LOW TERRAIN", and the upper boundary of the envelope is increased to 1000 feet radio altitude. The applicable voice message is repeated each 1.5 seconds until the flight condition has been corrected. There are two important operational notes here. First, the "TOO LOW GEAR" alert takes priority over the "TOO LOW FLAPS". So if both conditions exist, the system prioritizes the gear warning. Second, the too low flaps alert and associated too low terrain alert are inhibited with the flap inhibit switch in the FLAP OVRD position. So if you're flying with the flap inhibit switch in the FLAP OVRD position, those specific alerts are silenced. The visual alert for Mode 4B is again "PULL UP". Now, I want to draw your attention to the Mach number conversion table that accompanies Mode 4B. This is crucial for understanding how the system interprets your speed. The table shows the MACH - IAS CONVERSION at various altitudes. For example, at Mach 0.28, the indicated airspeed is 185 knots at sea level, 169 knots at 5000 feet, 180 knots at 8000 feet, and 154 knots at 10000 feet. At Mach 0.45, the indicated airspeed is 298 knots at sea level, 272 knots at 5000 feet, 258 knots at 8000 feet, and 249 knots at 10000 feet. This is important because the system uses Mach number, not indicated airspeed, to determine which alert to sound. So you need to understand how Mach converts to IAS at your current altitude to anticipate which warning you might get. Let me also show you the visual representation of these modes. shows Mode 2, and shows Mode 3. These figures illustrate the envelopes and the alert conditions graphically. So to summarize what we've covered: Mode 3 protects you from altitude loss after take-off or go-around with the "DON'T SINK" alert. Mode 4A protects you from unsafe terrain clearance with the gear not down, giving you "TOO LOW GEAR" or "TOO LOW TERRAIN" depending on your Mach number. And Mode 4B protects you from unsafe terrain clearance with flaps not in landing configuration, giving you "TOO LOW FLAPS" or "TOO LOW TERRAIN" depending on your Mach number. Each mode has its own arming conditions, alert thresholds, and speed-dependent behavior. Understanding these distinctions is essential for recognizing what the system is telling you and responding appropriately in the cockpit.