We're starting a brand-new chapter now: Airframe Contamination — Page 235, Lesson 284

We're starting a brand-new chapter now: Airframe Contamination. This is a big one for your ATPL studies, because it directly links meteorology to aircraft performance and handling. Let's get into it. The core idea is simple to state but serious in its consequences: the airframe can become contaminated by ice, frost, or water. This can happen either while the aircraft is in flight, or when it's just standing on the ground. The meteorological conditions that actually cause the ice and frost to form are covered in a different part of your syllabus, but what we care about here is the effect. And the effect is an accumulation of ice or frost on the surface of the aircraft, which will directly affect its performance and its handling characteristics. Let's break down the types of contamination. First, we have frost. Frost can form in two distinct situations. One, when the aircraft is standing on the ground and the temperature falls below 0°C. Two, in flight, if the aircraft has been flying in a region where the temperature is below 0°C, and then moves into a warmer layer of air. What you get is a fairly thin coating of crystalline ice. So, frost is that thin, feathery, crystalline deposit. Now, the second type is ice, and this is where we get into the main forms. There are three main forms of icing you need to know: clear ice, rime ice, and rain ice. Let's take them one at a time. First, clear ice, which is also called glaze ice. This is a translucent layer of ice with a smooth surface. The mechanism is important: it's caused by large supercooled water droplets striking the leading edges of the airframe. Because the droplets are large, there is some delay in freezing. That delay means there is some flow back along the surface, behind the leading edge, before it freezes. So you get that smooth, glassy, translucent layer that can extend back from the leading edge. Now contrast that with rime ice. Rime ice forms when small supercooled water droplets strike the leading edges and freeze almost immediately. Because they freeze on contact, there is no flow back. The result is a white, opaque formation. So the key contrast here is droplet size and freezing speed: large droplets, delayed freezing, clear ice with runback; small droplets, immediate freezing, rime ice with no runback, white and opaque. That's the foundation of this chapter. We've got frost, and we've got the three main forms of ice: clear ice, rime ice, and rain ice. We've covered the first two in detail. Next, we'll look at rain ice, and then the effects of all this contamination on the aircraft, its instruments, and its controls.