Aerodynamic Warnings — Page 443, Lesson 546

I want to walk you through the stall warning system now. This is the first of the aerodynamic warnings, and it's a system you'll rely on every single flight, so let's build it up properly from the ground. The purpose of the stall warning system is simple to state: it warns the pilot of an impending stall. But the precise wording matters. It does this when the aircraft approaches the stalling angle of attack for the current speed and configuration of the aircraft. So it's not just about one fixed angle — the warning is tied to the angle of attack that would stall the aircraft given the speed and the configuration it's in right now. Let's look at the simplest form of system first, because it teaches you the principle. This is the type adopted in several small aircraft. It consists of a hinged-vane-type sensor mounted in the leading edge of a wing, so the vane protrudes into the airstream. In normal level flight, the airstream keeps the vane aligned with the relative airflow. Now, if the aircraft's attitude changes so that the angle of attack increases — and remember, angle of attack is often abbreviated AoA — then by definition the airflow meets the leading edge at an increasing angle, and that deflects the vane. When the vane reaches the angle at which the warning unit has been preset, it activates a switch that completes a circuit to an aural warning unit in the cockpit. So you get a sound in the cockpit. That's the whole principle: a mechanical vane, a preset angle, a switch, and a warning. Now, in larger aircraft, the stall warning and prevention systems are designed to perform a more active function. That's where you get the 'stick-shaker' or the 'stick-push or nudger' type. These don't just tell you — they physically act on the controls. Let me give you the regulatory margin, because this is a number you need to know cold. The margin between the stall and the stall warning is 5 knots or 5% of the CAS, whichever is the greater. CAS is calibrated airspeed. So the warning must come at least 5 knots before the stall, or at least 5% of your calibrated airspeed before the stall — and you take whichever of those two values is bigger. The warning itself can be tactile, aural, or visual, or a combination of these signals. Tactile means something you feel, aural means something you hear, visual means something you see. Most aircraft have the warning provided by stick-shakers, which vibrate the control column and also produce a rattling noise. So you get both the feel and the sound from the same device. In fly-by-wire systems, the warning is different: it consists of a cricket sound — like the insect — plus a synthetic voice saying STALL, plus the red master WARNING light illuminating. So in a fly-by-wire aircraft you get three simultaneous cues: the cricket sound, the spoken word STALL, and the red master warning light. There's a critical requirement about how long the warning must last. The stall warning must continue until the angle of attack is reduced to approximately that at which the stall warning is initiated. In other words, it doesn't just flash once and stop — it stays on until you've recovered, until the angle of attack comes back down to roughly the point where the warning first triggered. Now let's talk about how the system actually operates. The stall warning module processes the signals from the various inputs to produce the appropriate stall warning output signals. So there's a central module, and it takes in information from several sources. The inputs are: angle of attack, flap and slat positions, landing gear weight-on position, and airspeed. Let me unpack each of those. The angle of attack sensors are usually located on either side of the front fuselage. So they're not in the wing like the small-aircraft vane — they're on the fuselage sides, forward. Sensing relays denote the positions of the flaps and slats. Now here's the clever bit: since the pitch attitude of the aircraft is also changed by the extension of flaps or slats, the angle of attack signal has to be modified when these are extended. Because extending flaps changes the airflow and the attitude, so the raw angle of attack reading needs adjusting to stay accurate. During take-off, when the nosewheel lifts off, microswitches operate to make the stall warning system active. So the system is armed by the weight coming off the nosewheel. And the airspeed is usually derived from the ADC — that's the air data computer. Finally, the output signals from the system can be applied to several devices: a stick-shaker motor, an angle of attack indicator aural warning, a synthetic voice warning, and the red master WARNING light. So the module takes all those inputs, processes them, and drives whichever of those outputs the aircraft is fitted with. That's the complete stall warning system — the function, the simple vane version, the active stick-shaker and stick-push versions, the regulatory margin, the warning forms, the inputs, and the outputs.