It operates within certain height bands above and below the selected altitude — Page 443, Lesson 541

I want to walk you through the start of Chapter 33, Aerodynamic Warnings. This is a new chapter, so let's set the scene. The whole idea here is that the aircraft has a Flight Warning System, and its job is to alert the crew if the aircraft deviates from certain aerodynamic parameters — specifically altitude, airspeed, and angle of attack. From those three parameters, the system produces three families of alerts: the Altitude Alerting System, the Overspeed Warning, and the Stall Warning. We'll get to overspeed and stall in a moment, but this excerpt focuses on the altitude alerting system, so let's dig into that. The function of the altitude alerting system is to warn the pilots that the aircraft is approaching, or deviating from, the altitude selected on the autopilot control panel. So you set a target altitude on the autopilot panel, and this system watches how close you are to it. It operates within certain height bands above and below the selected altitude. Now, those bands are aircraft-specific. For Boeing aircraft, the bands are typically 300 feet to 900 feet. For Airbus aircraft, they're typically 250 feet to 750 feet. So the system only starts caring once you're within those bands of the selected altitude. Let me walk you through the operation on a Boeing 747-400, which is shown in Figure 33.1. There are two scenarios: approaching the selected altitude, and deviating from it. First, approaching. When you're 900 feet prior to the selected altitude, a white box appears around the current altitude display on the Primary Flight Display, the PFD, and you get a momentary audible alert. Then, at 300 feet prior to the selected altitude, that white box disappears. So the white box is your visual cue that you're getting close, and it vanishes once you're within 300 feet. Now the second scenario: deviation from the selected altitude. If you drift 300 feet away from the selected altitude, four things happen simultaneously. The master caution lights illuminate. A continuous caution beeper sounds. The EICAS caution message "ALTITUDE ALERT" is displayed — EICAS is the Engine Indication and Crew Alerting System, the central warning display. And the current altitude box changes to amber. So the white box from the approach phase turns amber when you deviate. Here's an important detail: the audible alert and the master caution light can both be cancelled by pressing the light. So the pilot physically presses the master caution light to silence it. And the warning ceases under two conditions: either you return to within 300 feet of the selected altitude, or the aircraft exceeds 900 feet from the selected altitude. So if you drift far enough away, beyond 900 feet, the warning stops — you've clearly left the band. Now, there's a block diagram in Figure 33.2 for a Boeing 767 altitude alerting system, and it shows a slightly different implementation. When the aircraft approaches the selected altitude, the advisory light on each electric altimeter illuminates. So instead of a PFD box, here you get a light on the electric altimeters themselves. If the aircraft then deviates by more than 300 feet from the selected altitude, the system generates what's called a level B warning — that's a caution, not a full warning. That level B warning consists of three things: 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 can see the same 300-foot deviation threshold, but the presentation is a bit different on the 767. Finally, there's a system inhibition. Altitude alerting is inhibited in flight whenever the glide slope is captured, or when landing flaps are selected with the gear down. So during an approach, once you capture the glide slope, or once you configure for landing with flaps and gear down, the altitude alerting stops — because you're no longer in a phase where you want to be nagged about altitude deviations. Let me make sure you've got the key numbers straight: 900 feet and 300 feet for Boeing, 250 and 750 for Airbus. The white box at 900, disappearing at 300. The amber box and the four alerts at 300 feet deviation. The warning ceasing on return within 300 or exceeding 900. And the inhibition on glide slope capture or landing flaps with gear down. That's the altitude alerting system in full.