Stalling — Page 153, Lesson 183

Let's talk about stall warning. This is the certification requirement that sits between the stall itself and the pilot's ability to avoid it. The whole point is simple: we've established a stall speed for each configuration, but that's not enough. There must be a clear, distinctive warning that comes early enough that the pilot can actually prevent the stall from happening. Let me walk you through the regulatory requirements, because they're very precise. First, the warning must be clear and distinctive to the pilot in both straight and turning flight, and it must have sufficient margin to prevent inadvertent stalling with the flaps and landing gear in any normal position. So the warning has to work whether you're wings level or banked, and it has to cover every normal flap and gear setting. Now, how is the warning furnished? It can come from two sources. Either the inherent aerodynamic qualities of the aeroplane — that's the natural buffet you feel as the airflow separates — or by a device that gives clearly distinguishable indications under expected flight conditions. That's your stall warning horn, stick shaker, or similar. But here's a critical limitation: a visual stall warning device that requires the attention of the crew within the cockpit is not acceptable by itself. Why? Because in the cockpit, your eyes are busy. The warning must be something you feel or hear, not something you have to look at. If a device is used, it must provide a warning in each of the aeroplane configurations prescribed, at the speed prescribed. Now let's get into the numbers, because this is where the precision matters. We have a speed called VSW — that's the stall warning speed. When the speed is reduced at rates not exceeding 1 knot per second, stall warning must begin, in each normal configuration, at a speed VSW that exceeds the speed at which the stall is identified by not less than 5 knots or 5% CAS, whichever is the greater. Let me unpack that. CAS is calibrated airspeed. So the warning must come at least 5 knots before the stall, or 5% of the calibrated airspeed before the stall — whichever gives the bigger margin. And once initiated, the stall warning must continue until the angle of attack is reduced to approximately that at which the warning began. So it doesn't just blip and stop; it stays on until you unload the wing back to the angle where it started. There's a second, additional requirement. When the speed is reduced at rates not exceeding one knot per second, in straight flight with engines idling and a specific CG position, VSW in each normal configuration must exceed VSR by not less than 3 knots or 3% CAS, whichever is greater. Now, VSR is the reference stall speed — that's the speed at which the stall is identified per the Stall Recognition criteria. So we have two margins: 5 knots or 5% CAS above the stall identification speed, and additionally 3 knots or 3% CAS above VSR. The 5% margin is the primary one; the 3% is an extra floor tied to the reference stall speed. Then there's the stall warning margin requirement — this is about giving the pilot enough time to react. The margin must be sufficient to allow the pilot to prevent stalling when recovery is initiated not less than one second after the onset of stall warning. And this is tested in a demanding scenario: slow-down turns with at least 1.5g load factor normal to the flight path, and airspeed deceleration rates of at least 2 knots per second. So you're turning, pulling 1.5g, decelerating at 2 knots per second — and the pilot gets one second after the warning to start recovery and must still avoid the stall. This is with the flaps and landing gear in any normal position, with the aeroplane trimmed for straight flight at a speed of 1.3VSR, and with the power or thrust necessary to maintain level flight at 1.3VSR. So the trim and power are set for a specific reference condition, and the warning margin has to be big enough that even in that aggressive turn, one second of reaction time is enough. Finally, there's the abnormal configuration requirement. Stall warning must also be provided in each abnormal configuration of the high lift devices that is likely to be used in flight following system failures. That includes all configurations covered by Flight Manual procedures. So if a flap or slat system fails and you end up in an asymmetric or partial configuration that the Flight Manual says you can fly, the stall warning has to work there too. It's not just about the normal settings — the certification covers the degraded ones as well. So the whole picture is this: stall warning is a layered safety net. It has to be clear and distinctive, it has to come early enough — at least 5 knots or 5% CAS before the stall, and at least 3 knots or 3% CAS above VSR — it has to persist until you reduce angle of attack, it has to give you enough margin to recover even with a one-second delay in a 1.5g turn, and it has to cover abnormal flap configurations after failures. That's the full regulatory envelope for stall warning.