Let me walk you through it — Page 443, Lesson 541

We're starting a new chapter now — Chapter 33, "Aerodynamic Warnings." This is where the aircraft's warning systems protect you from getting into aerodynamic trouble. Let me walk you through it. The chapter opens with the Flight Warning System. Its job is to alert the crew if there are deviations from certain aerodynamic parameters — specifically altitude, airspeed, and angle of attack. So the system provides three alerts and warnings: the Altitude Alerting System, the Overspeed Warning, and the Stall Warning. Those are the three pillars of this chapter. Let's start with the Altitude Alerting System. Its function is to warn the pilots that the aircraft is approaching or deviating from the altitude selected on the autopilot control panel. It does this in certain height bands above and below the selected altitude. Now, the height bands — these differ by manufacturer. For Boeing aircraft, the bands are typically 300 feet to 900 feet. For Airbus aircraft, they're 250 feet to 750 feet. So the system only operates within those windows around your selected altitude. Let me walk you through the operation on a Boeing 747-400, which is Figure 33.1. When you're approaching a selected altitude — say you're climbing to 10,000 feet — at 900 feet prior to that selected altitude, a white box will be displayed around the current altitude display on the Primary Flight Display, the PFD. That white box appears together with a momentary audible alert. Then, at 300 feet prior to the selected altitude, the white box disappears. So you get that visual cue early, then it clears as you get closer. Now, what happens if you deviate from the selected altitude? Let's say you overshoot it or drift away. At 300 feet from the selected altitude, several things happen simultaneously. The master caution lights illuminate. A continuous caution beeper sounds. The EICAS caution message "ALTITUDE ALERT" is displayed. And the current altitude box changes to amber. So you get visual, audible, and text warnings all at once. Here's an important operational detail: the audible alert and the master caution light can both be cancelled by pressing the light. And the warning ceases under two conditions — either when you return to within 300 feet of the selected altitude, or when the aircraft exceeds 900 feet from the selected altitude. So the warning stops once you're back in the safe band, or once you've drifted far enough that the system gives up on that alert. Let me also mention the block diagram for the Boeing 767, which is Figure 33.2. When the aircraft approaches the selected altitude, the advisory light on each electric altimeter illuminates. That's a softer, advisory-level indication. But if the aircraft deviates by more than 300 feet from the selected altitude, the system generates a level B warning — that's a caution. It consists of a level B message on the EICAS display, an alert tone from the speakers, and illumination of the master caution amber light plus the ALT ALERT light. So you have a distinction here: approaching is advisory, deviating is a caution. Finally, there's the system inhibition. Altitude alerting is inhibited in flight whenever glide slope is captured, or when landing flaps are selected with the gear down. So during an approach, once you're locked onto the glide slope, or once you've configured for landing, the system stops alerting — because you're intentionally deviating from the selected altitude to land. That's the altitude alerting system. Next we'll move into the overspeed warning.