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 one of those systems you'll rely on in the most critical phase of flight, so let's build it up properly from the ground. The purpose of the stall warning system is to warn the pilot of an impending stall. Now, what does "impending" mean here? It means the system triggers when the aircraft approaches the stalling angle of attack for the current speed and configuration of the aircraft. So it's not a fixed point — the warning is tied to the angle of attack, and that angle depends on how fast you're going and how the aircraft is configured at that moment. Let me give you the simplest form of system first, because it shows you the principle beautifully. 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 here's the key: if the aircraft's attitude changes so that the angle of attack — we call it AoA — increases, 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, which 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 sensor, a preset threshold, a switch, and an aural warning. In larger types of 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 warn you — they physically act on the controls. Now, the regulatory margin. 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 — that's calibrated airspeed — whichever is the greater. So the warning must come before you actually reach the stall, and that margin is either 5 knots or 5% of CAS, and you take whichever is bigger. The warning itself can be tactile, aural, or visual, or a combination of these signals. Most aircraft have warning provided by stick-shakers, which vibrate the control column and also produce a rattling noise. In fly-by-wire systems, the warning consists of a cricket sound — that insect sound — plus a synthetic voice saying "STALL," plus the red master WARNING light illuminating. And here's an important operational detail: the stall warning must continue until the angle of attack is reduced to approximately that at which the stall warning was initiated. So it doesn't just blip and stop — it stays on until you've recovered the angle of attack back down to roughly the trigger point. Now let's look at the operation of the system. The stall warning module processes the signals from the various inputs to produce the appropriate stall warning output signals. The system has these inputs: angle of attack, flap and slat positions, landing gear weight-on position, and airspeed. Let me unpack each one. The angle of attack sensors are usually located on either side of the front fuselage. Sensing relays denote the positions of the flaps and slats. Here's the subtlety: 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. So the system doesn't just read raw AoA — it corrects it based on flap and slat position. During take-off, when the nosewheel lifts off, microswitches operate to make the stall warning system active. That's the landing gear weight-on position input — the system knows the aircraft is airborne. 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 a stick-shaker motor, an angle of attack indicator aural warning, a synthetic voice warning, and the red master WARNING light. So you see, the module takes all those inputs — AoA, flap and slat position, weight-on-wheels, airspeed — processes them, and drives whichever output devices the aircraft is fitted with. Let me show you the layout of this system so you can see how the sensors and outputs hang together. That's the stall warning system in full. The key takeaway: it's an angle-of-attack-based warning, with a regulatory margin of 5 knots or 5% CAS, whichever is greater, and it must persist until you reduce the angle of attack back to roughly the trigger point.