Ground Proximity Warning System — Page 461, Lesson 557

We're moving into the heart of the Ground Proximity Warning System now, and I want to walk you through the individual modes. We've already covered Mode 1 and Mode 2, so let's pick up with Mode 3. Mode 3 is the Altitude Loss after Take-off or Go-around mode. Its job is to catch you if you start descending during the initial climb or during a go-around. The aural alert is a voice message of "DON'T SINK", and it repeats every 1.5 seconds until the flight condition is corrected. Now, the operating envelope here is precise. Mode 3 is effective between 50 and 700 feet radio altitude. It generates the alert when the accumulated barometric loss equals approximately 10 percent of the existing barometric altitude. So if you're climbing out and you lose altitude, the system is comparing that loss against your current barometric altitude, and when the loss hits about ten percent of that value, you get the warning. There's a critical arming detail here. Mode 3 does not arm during the descent until you're below 200 feet radio altitude. And there's a note in the figure: Mode 3 arms when the aeroplane descends below 200 feet in the landing configuration. The visual warning that accompanies the "DON'T SINK" aural is "PULL UP". Now let's move to Mode 4A — Unsafe Terrain Clearance with Landing Gear Not Down. This is the terrain clearance mode with the gear retracted. It arms after take-off upon climbing through 700 feet radio altitude. Here's where the Mach number comes into play. When the envelope is penetrated at less than Mach 0.35, the aural alert "TOO LOW GEAR" is sounded. 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. The applicable voice message repeats each 1.5 seconds until the flight condition has been corrected. The visual warning is again "PULL UP". Now Mode 4B — 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. 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. When the envelope is penetrated at more than Mach 0.28, the aural alert "TOO LOW TERRAIN" is sounded, and again the upper boundary of the envelope is increased to 1000 feet radio altitude. The applicable voice message repeats each 1.5 seconds until the flight condition has been corrected. There's a priority and an inhibit logic here that you need to know. The "TOO LOW GEAR" alert takes priority over the "TOO LOW FLAPS" alert. And the too-low-flaps alert, along with its associated too-low-terrain alert, are inhibited with the flap inhibit switch in the FLAP OVRD position. Now, look at Figure 34.7 for Mode 4B. It shows the envelope plotted against Mach number on the horizontal axis and radio altitude on the vertical axis, going from 0 to 0.7 Mach and 0 to 1000 feet. There's a Mach-IAS conversion table there. At sea level, Mach 0.28 corresponds to 185 knots IAS, and Mach 0.45 corresponds to 298 knots. At 5000 feet, Mach 0.28 is 169 knots and Mach 0.45 is 272 knots. At 8000 feet, Mach 0.28 is 180 knots and Mach 0.45 is 258 knots. At 10000 feet, Mach 0.28 is 154 knots and Mach 0.45 is 249 knots. The figure labels the "TOO LOW FLAP" region and the "TOO LOW TERRAIN" region, showing how the envelope expands at higher Mach. So the key takeaway across these modes: the system is continuously comparing your radio altitude, your descent rate, your gear and flap configuration, and your Mach number, and it's giving you distinct aural warnings depending on which envelope you've penetrated.