Ice and Rain Protection — Page 239, Lesson 300

I want to walk you through the start of the Ice and Rain Protection chapter, and I want to begin with a hard operational rule that governs everything else in this lesson: in no circumstances should a formation of ice or frost be allowed to remain on the aircraft wing surfaces prior to take-off. That is not a suggestion — it is a prohibition, and I want you to understand exactly why it exists. Let's start with the subtle danger of frost, because it is easy to underestimate. We all know ice is bad, but frost is more subtle. If a heavy coat of hard frost exists on the wing upper surface, the typical reduction in maximum lift coefficient — that is CLMAX, the maximum lift coefficient the wing can produce — causes a 5 to 10 percent increase in the aircraft's stall speed. So your stall speed goes up by 5 to 10 percent just from that hard frost. Now here is the trap. Because of the magnitude of that effect, the effect of frost on take-off performance may not be realized until too late. Let me explain the numbers. The take-off speed of an aircraft is generally some 5 to 25 percent greater than the stall speed. That means the take-off lift coefficient will be a value from 90 to 65 percent of CLMAX. Think about that: at take-off you are operating at 90 to 65 percent of your maximum lift coefficient. If frost has raised your stall speed by 5 to 10 percent, you can see how the margin collapses. It is possible that the aircraft with frost cannot become airborne at the specified take-off speed because of premature stalling. Even if the aircraft with frost were to become airborne at the specified take-off speed, it could have insufficient margin of airspeed above the stall. Then turbulence, gusts, and/or turning flight could produce incipient or complete stalling of the aircraft. Incipient stalling means the stall is just beginning; complete stalling means the wing has fully stalled. Now, there is a common misconception I want to correct. The increase in drag during the take-off roll due to frost or ice is not considerable, and there will not be any significant effect on the initial acceleration during take-off. So the aircraft accelerates normally at first. That is why the effect of frost or ice will be most apparent during the later portions of take-off — if the aircraft is unable to become airborne, or if insufficient margin above the stall speed prevents a successful initial climb. So the danger is not that you feel sluggish on the runway; the danger is that you run out of runway or you lift off and cannot climb because you are too close to the stall. Let me bring in the figure here — — which shows the areas most susceptible to ice formation. That is the map of where ice builds up on the airframe. Now let's move to icing in flight. Icing on aircraft in flight is caused primarily by the presence of super-cooled water droplets in the atmosphere. Super-cooled means liquid water that is below 0°C but has not yet frozen. If those droplets impinge on the forward facing surfaces of an aircraft — that is, they strike the leading edges — they freeze and cause a build-up of ice which may seriously alter the aerodynamic qualities. This applies particularly to small objects, which have a higher catch rate efficiency than large ones. Catch rate efficiency is the proportion of droplets in the air that actually strike the surface. Small objects catch a higher proportion, and small amounts of ice will produce relatively bigger changes in shape. So a thin antenna or a small probe is more vulnerable proportionally than a large wing. The actual amount and shape of the ice build-up depends on the surface temperature. And that surface temperature results from an energy change caused by heat variations to the skin of the aircraft. Let me list the factors that contribute to that energy balance. There are heating effects, marked Plus, and cooling effects, marked Minus. The heating effects are: kinetic air heating, which is the frictional heating of the air against the skin; kinetic heating by water droplets, which is the heating from the droplets striking the surface; and latent heat of fusion, which is caused by the water droplets changing from liquid to solid upon impact — that phase change releases heat. The cooling effects are: evaporation, which takes heat away, and convection, which carries heat away from the surface. Now here is the key relationship. Three different situations arise, depending on whether the surface temperature is less than, equal to, or greater than 0°C. When the temperature is less than 0°C, all the impinging water droplets are frozen — that is rime ice, all frozen on impact. When it is above 0°C, none are frozen — that is clear ice, the droplets stay liquid and run back before freezing. And when it is equal to 0°C, you are at the boundary between those two regimes. Let me show you where this happens on the airframe — — which illustrates where airframe icing occurs. So to pull it together: on the ground, frost is the silent killer because it raises stall speed by 5 to 10 percent without noticeably affecting acceleration, and you may not realize the problem until you are committed to take-off. In flight, ice builds from super-cooled droplets striking forward-facing surfaces, and whether it freezes on impact or runs back depends entirely on the surface temperature relative to 0°C, which is set by that balance of kinetic heating, latent heat, evaporation, and convection.