Aerodynamic Warnings — Page 443, Lesson 543

Let's start with the regulatory requirement, because that's the foundation everything else sits on. For EASA-registered Commercial Air Transport aircraft, an altitude alerting system is mandatory. The rule applies if the aircraft is a turboprop weighing more than 5700 kilograms, or has more than 9 passenger seats — or if it's a turbojet aircraft, in which case the weight and seat criteria don't matter, it's required regardless. Now, what must that system actually do? Two things. First, it must alert the crew when you're approaching the preselected altitude. Second, it must alert the crew by at least an aural signal — that's a sound, not just a light — when you deviate above or below that preselected altitude. So approaching gets you a warning, and deviating away from it gets you an aural warning. Now let's move to the overspeed warning system. Its purpose is to alert the flight crew if the airspeed exceeds the VMO / MMO limits. Let me define those terms. VMO is the maximum operating speed in terms of indicated airspeed. MMO is the maximum operating speed in terms of Mach number. Both are calculated by the air data computer, the ADC. Here's how it operates. When an overspeed situation occurs in an aircraft with electronic instrumentation, three things happen simultaneously. The system sounds the siren or horn. It illuminates the red master WARNING lights. And it displays the message OVERSPEED on the EICAS upper display in red. EICAS is the Engine Indication and Crew Alerting System. Now, an important detail: the warning continues while the overspeed situation exists, and it cannot be cancelled by depressing the red master WARNING light switch. So you can't silence it just by pressing the button — it stays until the overspeed condition is resolved. Where does the system get its input? From the air data computers, the ADCs, via the flight warning system. And you can test it on the ground before flight by pressing a test switch, which would then sound the siren or horn as appropriate for that aircraft. Here's a critical failure consideration. In case of system failure, the pilot would receive no warning if VMO or MMO is exceeded. So the system failing means you lose that protection entirely — you'd have no alert when you exceed those limits. Now let's look at the displays. The maximum allowable speed is shown on the airspeed indicator by means of a barber's pole on a conventional instrument. That's a red-and-white striped pointer that indicates the speed limit. On an EFIS display — that's the Electronic Flight Instrument System — it appears on the airspeed tape on the primary flight display or EADI, the Electronic Attitude Director Indicator. Here's the key behavior of the barber's pole. It indicates the VMO up until the MMO becomes limiting. Then the barber's pole moves anti-clockwise to indicate the maximum allowable speed. Why does it move? Because as altitude increases when climbing at a constant indicated airspeed, the MMO, when expressed as an indicated airspeed, will decrease. So at lower altitudes, VMO is the limiting factor. But as you climb, the Mach limit expressed as indicated airspeed drops, and eventually MMO becomes the constraint — that's when the barber's pole moves anti-clockwise to show the lower maximum allowable speed. Let me show you the conventional instrument with the VMO pointer. And here's the overspeed warning on the PFD. So to tie it together: the altitude alerting system is a regulatory requirement with specific triggers, and the overspeed warning system protects you against exceeding VMO or MMO, with a barber's pole that shifts between those two limits as altitude changes.