Aerodynamic Warnings — Page 443, Lesson 543

I want to walk you through the Aerodynamic Warnings chapter. This is the start of a new topic, so let's build it from the ground up. First, the regulatory requirement. For EASA registered Commercial Air Transport aircraft, an altitude alerting system is mandatory. The rule is specific: you need it if the aircraft is a turboprop weighing more than 5700 kg, OR if it has more than 9 passenger seats, OR if it's a turbojet aircraft. So a turbojet always needs it, regardless of size. Now, what must this altitude alerting system actually do? Two things. First, it must alert the crew when you're approaching the preselected altitude — that's the altitude you've dialled in as your target. Second, it must alert the crew by at least an aural signal — that means a sound, not just a light — when you deviate above or below that preselected altitude. So the system watches both the approach and the departure from your chosen level. Let me show you what that looks like on a Boeing 747-400. This figure shows the altitude alert when approaching a selected altitude. You can see how the system presents the alert to the crew as you close in on your target level. Now let's move to the overspeed warning system. This is a separate system, and its purpose is to alert the flight crew if the airspeed exceeds the VMO / MMO limits. Let me define those terms carefully. VMO is the maximum operating speed expressed in terms of indicated airspeed — that's the speed limit in knots. MMO is the maximum operating speed expressed in terms of Mach number — that's the limit relative to the speed of sound. Both limits are calculated by the air data computer, which we call the ADC. Here's the key point about how these two limits interact. At lower altitudes, VMO is the limiting factor. But as you climb higher, the air gets thinner, and the Mach number becomes the constraint. So the system switches from VMO to MMO as the limiting value, depending on your altitude. The air data computer calculates which one applies. Now, what happens when an overspeed situation occurs in an aircraft with electronic instrumentation? Three things happen simultaneously. First, the system sounds the siren or horn. Second, it illuminates the red master WARNING lights. Third, it displays the message OVERSPEED on the EICAS upper display in red. EICAS stands for Engine Indication and Crew Alerting System — that's the electronic display that shows you engine parameters and system warnings. Here's an important operational 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 just silence it and ignore it — the system keeps telling you until you correct the speed. Where does the system get its input? From the air data computers, via the flight warning system. The ADCs feed the speed data through the flight warning system, which then triggers the alerts. There's also a ground test capability. Before flight, you can press a test switch, and the system will sound the siren or horn as appropriate for that aircraft. So you can verify the warning works before you take off. Now, a critical failure consideration. In case of system failure, the pilot would receive no warning if VMO or MMO is exceeded. That's a serious point — if the system fails, you have no overspeed alert. This is why the ground test matters, and why you as a pilot must be aware of the speed limits even without the warning. Let me show you the displays. This figure shows the conventional ASI with the VMO pointer. On a conventional instrument — that's the traditional round dial airspeed indicator — the maximum allowable speed is shown by means of a barber's pole. That's the red-and-white striped pointer that indicates your speed limit. On an EFIS display — that's the Electronic Flight Instrument System — the same information appears on the airspeed tape on the primary flight display, or PFD, which is also called the EADI, the Electronic Attitude Director Indicator. Here's how the barber's pole works. It indicates the VMO up until the MMO becomes limiting. Then the barber's pole moves anti-clockwise to indicate the maximum allowable speed. So as you climb, the pole sweeps around the dial to show you the new, lower speed limit. 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. Let me unpack that. You're climbing at a constant indicated airspeed — say 300 knots indicated. But the Mach number is increasing as you climb, because the speed of sound decreases with altitude. So at some point, your Mach number reaches MMO, even though your indicated airspeed hasn't changed. When expressed as an indicated airspeed, that MMO limit corresponds to a lower and lower value as you climb. That's why the barber's pole moves anti-clockwise — it's tracking the decreasing speed limit. So to summarise the whole picture: you have the altitude alerting system for level changes, and the overspeed warning system for speed limits. The overspeed system uses the barber's pole on conventional instruments or the airspeed tape on EFIS to show you the limit, and it warns you with the siren, the red master WARNING lights, and the OVERSPEED message on EICAS when you exceed it. And remember — that warning cannot be cancelled, and if the system fails, you get no warning at all.