Stalling — Page 186, Lesson 218

I want to walk you through the stalling chapter, and we're going to finish it with a critical topic: the effects of ice and frost on the wing. This is where the theory turns into a real safety warning, so let's take it carefully. We've already learned that CLMAX is the maximum lift coefficient, the peak of the lift curve, and that stalling happens when the airflow separates from the wing. Now, here's the key point I want you to grasp: while the reduction in CLMAX due to frost formation is not usually as great as that due to ice formation, it is usually unexpected. Why unexpected? Because you might think that you need a large change in the aerodynamic shape, like a big chunk of ice, to reduce CLMAX. But that's not the whole story. The kinetic energy of the boundary layer is an important factor influencing separation of the airflow, and this energy is reduced by an increase in surface roughness. So even frost, which is a roughness issue more than a shape issue, robs the boundary layer of energy, and that makes the airflow separate earlier. Let me show you this with a figure. Here we have the lift curve, CL versus angle of attack. The basic smooth wing has the highest CLMAX. The wing with frost has a lower CLMAX, and the wing with ice has the lowest. Notice the labels: leading edge ice formation, upper surface frost, basic smooth wing, wing with frost, wing with ice. The general effects of ice and frost formation on CLMAX are typified by these illustrations. Now, here's a practical problem: the increase in stall speed due to ice formation is not easy to quantify, because the accumulation and shape of the ice formation is impossible to predict. So we can't give you a precise number for every situation. But the rule is absolute: even a little ice is too much. Ice or frost must never be allowed to remain on any aerodynamic surfaces in flight, nor must ice, frost, snow, or other contamination be allowed to remain on the aircraft immediately before flight. Let me summarize the physical effect in one sentence: ice, frost, and snow change the aerofoil section, decrease the stall angle, and increase the stall speed. So the wing stalls earlier and at a higher speed. Now, let's move to the warning to the pilot of icing-induced stalls. This is a serious safety bulletin. There have been recent cases involving loss of control in icing conditions due to undetected stalling at speeds significantly above the normal stalling speed, accompanied by violent roll oscillations. Control of an aeroplane can be lost as a result of an icing-induced stall, and the onset can be so insidious as to be difficult to detect. Insidious means it creeps up on you quietly, without obvious warning. Here's the advice on recognition and recovery, point by point. First, loss of performance in icing conditions may indicate a serious build-up of airframe icing, even if you can't see it. This causes a gradual loss of lift and a significant increase in drag. Second, this build-up of ice can cause the aeroplane to stall at approximately 30% above the normal stall speed. So if your normal stall speed is 100 knots, you could stall at 130 knots. Third, the longitudinal characteristics of an icing-induced wing-stall can be so gentle that the pilot may not be aware that it has occurred. The nose might not drop dramatically. Fourth, and this is crucial: the stall warning system installed on the aeroplane may not alert the pilot to the insidious icing-induced wing-stall. Why? Because the angle of attack will be below that required to trigger the switch. So you cannot rely on the stall warning. However, airframe buffet may assist in identifying the onset of wing-stall. Fifth, the first clue may be a roll control problem. This can appear as a gradually increasing roll oscillation or a violent wing drop. Sixth, a combination of rolling oscillation and onset of high drag can cause the aeroplane to enter a high rate of descent unless prompt recovery action is taken. And seventh, if a roll control problem develops in icing conditions, the pilot should suspect that the aeroplane has entered an icing-induced wing-stall and should take immediate stall recovery action. So the takeaway is: in icing, don't trust the stall warning, watch for roll problems and buffet, and act immediately. That's the end of the stalling chapter.