Right, let's pick this up — Page 186, Lesson 220

Right, let's pick this up. We're in the middle of the stall chapter, and we've just covered the wing stall in icing. Now I want to walk you through what happens when ice contaminates the tailplane — the stabilizer — because that's a different and very dangerous failure mode. First, the key fact: the tailplane is itself an aerofoil. But because it is thinner than the wing, it is likely to experience icing before the wing does. So ice will form on the tailplane first. And the effect is the same as for the wing — the stall will occur at a lower angle of attack. So a tailplane that's contaminated with ice will stall sooner than a clean one. Now here's the critical part about the tailplane's normal working condition. The tailplane is normally operating at a negative angle of attack, and it's producing a down load — that is, it's pushing the tail down to keep the nose up in normal flight. So if the tailplane stalls and that down load is lost, the nose of the aircraft will drop, and you will lose longitudinal control. That's the pitch control. So a stabilizer stall isn't just a loss of lift — it's a loss of pitch authority, and the nose drops. Now, what can precipitate — that is, trigger — a stall of an ice-contaminated tailplane? The answer is extension of the wing flaps. Here's the mechanism. When you lower the flaps, you increase the downwash — the air being deflected downward behind the wing. That increased downwash increases the negative angle of attack of the tailplane. If the tailplane already has ice contamination, that extra negative angle of attack could be sufficient to cause it to stall. So the recovery procedure in this situation is to retract the flaps again, which reduces the downwash and unloads the tailplane. Now let me connect this back to the wing-icing situation we started with, because the book gives us the pilot action there. If you're in icing conditions and you suspect the aeroplane has entered an icing-induced wing-stall, you should take immediate stall recovery action — and that means decrease the angle of attack. The de-icing system should also be activated. Now, if the aeroplane is fitted with an anti-icing system, that should have been activated prior to entry into icing conditions, in accordance with the Flight Manual or Operations Manual procedures and recommendations. If the anti-icing system has not been in use, then it should be immediately activated. And you should also give consideration to leaving the icing conditions by adjusting track and/or altitude if possible. The book adds a footnote here: "insidious" — meaning advancing imperceptibly, without warning. That's why icing is so dangerous — it builds up without you noticing until the stall happens. Now let's move to a different environmental factor: the effect of heavy rain on stall speed. The book breaks this into several components, and I want to take them one at a time. First, weight. Heavy rain will form a film of water on the aircraft and increase its weight slightly — maybe as much as 1 to 2 percent. That in itself will increase stall speed. So there's a small weight penalty. Second, the aerodynamic effect. The film of water will distort the aerofoil, roughen the surface, and alter the airflow pattern on the whole aircraft. The result is that CLMAX — that's the maximum lift coefficient, the peak lift the wing can generate — will decrease, causing stall speed to increase. So the wing can't produce as much lift before it stalls. Third, drag. The film of water will increase interference drag, profile drag, and form drag. The book gives specific numbers here. In light rain, drag may increase by 5 percent. In moderate rain, by 20 percent. And in heavy rain, by up to 30 percent. This obviously increases the thrust required to maintain flight. Fourth, impact. This is an additional consideration, and note that it does not affect stall speed — but it's still important. The impact of heavy rain on the aircraft means momentum will be lost and airspeed will decrease, requiring increased thrust. At the same time, heavy rain will also be driving the aircraft downwards. So you have a double effect: the rain is slowing you down and pushing you down, and you need more thrust to compensate. So to summarize the heavy rain picture: weight goes up slightly, CLMAX goes down, drag goes up significantly with rain intensity, and the impact of the rain itself robs you of momentum and pushes you down. All of that, except the impact effect, feeds into a higher stall speed. Let me make sure you've got the distinction between the two icing scenarios clear. For the wing, you recover by decreasing angle of attack and activating de-icing or anti-icing. For the tailplane, the trigger is often flap extension, and the recovery is to retract the flaps. Both are about managing angle of attack, but the tailplane case is specifically about the downwash from the flaps. Now, I want to show you the figure that goes with this section — it illustrates the tailplane stall situation. That's the stabilizer stall due to ice, and the heavy rain effects on stall speed