We start with the key idea: the increase in surface roughness increases… — Page 239, Lesson 299

Let me walk you through what happens to the wing when frost or light ice forms on it — because the physics here is subtle and it catches a lot of pilots out. We start with the key idea: the increase in surface roughness increases skin-friction. Skin-friction is the drag caused by the air rubbing directly against the surface. When frost roughens the surface, that rubbing gets stronger. And here's the important consequence — that increased skin-friction reduces the kinetic energy of the boundary layer. The boundary layer is the thin sheet of air that clings to the wing surface. Kinetic energy is the energy of motion, so a rougher surface literally slows that clinging layer of air down. Now, what does a slower, lower-energy boundary layer do? It makes the airflow separate from the wing much earlier. So there will be an increase in drag — but I want you to note the magnitude. The drag increase from frost or light roughness will not compare with the considerable increase due to a severe ice formation. Severe ice is a much bigger drag problem. But frost has its own, more dangerous problem, and that's what we're about to look at. The reduction of boundary layer kinetic energy will cause incipient stalling of the wing. Incipient means just beginning — the very onset of the stall. What that means precisely is that separation will occur at angles of attack and lift coefficients lower than for the clean, smooth wing. So the wing starts to stall earlier than you expect, at a lower angle of attack and a lower lift coefficient. Now here's the trap. The reduction in CLMAX due to frost formation ordinarily is not as great as that due to ice formation. CLMAX is the maximum lift coefficient — the peak lift the wing can produce before it stalls. So frost doesn't cut CLMAX as much as ice does. But — and this is the critical point — it is usually unexpected. Why? Because it may be thought that large changes in the aerodynamic shape, such as those due to ice, are necessary to reduce CLMAX. People assume you need a big, obvious shape change to hurt the wing. Frost is thin, it doesn't look like it changes the shape much, so pilots don't expect it to matter. But it does, because 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. That's the whole chain — roughness robs the boundary layer of energy, and a low-energy boundary layer separates early, and early separation is what lowers CLMAX. So the effect of ice or frost on take-off and landing performance is of great importance. I want you to feel how strongly this is worded — the effects are so detrimental to the landing and take-off that no effort should be spared to keep the aircraft as free as possible from any accumulation of ice or frost. That's a professional standard, not a suggestion. Now let's trace the landing scenario, because this is where it gets genuinely dangerous. If any ice remains on the aircraft as the landing phase approaches, you must appreciate that the ice formation will have reduced CLMAX and incurred an increase in stall speed. Incurred — it has brought about, caused, an increase in stall speed. And here's the chain: if stall speed goes up, then the landing speed will be greater. You have to fly faster to stay above the higher stall speed. Now couple that with something else. When this effect is coupled with the possibility of poor braking action during the landing roll, a critical situation can exist. So you're coming in faster, and then you can't brake as well on the runway. Faster approach plus worse braking — that combination is what makes it critical. And that's why it is obvious that great effort must be made to prevent the accumulation of ice during flight. Let me pull the whole chain together for you, because this is the examinable logic. Roughness increases skin-friction. Increased skin-friction reduces boundary layer kinetic energy. Reduced boundary layer energy causes early separation — incipient stall at lower angles of attack and lower lift coefficients. That lowers CLMAX, which raises stall speed, which raises landing speed. And if braking is poor on top of that, you have a critical landing situation. The frost case is especially insidious because the CLMAX reduction is smaller than with ice, so it's easy to underestimate — but it's still enough to bite you. That figure shows the areas most susceptible to ice formation, and it makes the point that the drag increase during take-off roll due to frost or ice is not considerable — the real danger is in the lift and stall behaviour we just discussed.