The excerpt refers to Figure 16.5, which illustrates airflow around thin… — Page 291, Lesson 282

I want to walk you through a key part of aircraft icing — the factors that determine how quickly ice builds up on your airframe. We're looking at two major influences: the shape of the aircraft and the temperature at the cloud base. Let's start with shape. The excerpt refers to Figure 16.5, which illustrates airflow around thin versus thick wing shapes. Here's the critical point: thin shapes collect ice more rapidly than high-drag ones. By "high-drag" shapes, we mean thicker, blunter airfoils. So a thin wing — like you'd find on a fast jet or a glider — is more prone to rapid icing. The same applies to pressure heads, which are the pitot-static probes that measure airspeed and altitude. Those are thin, exposed, and therefore liable to rapid icing. Now, speed matters too. High speeds result in a greater ice hazard because the airframe strikes a greater number of supercooled water droplets in unit time — every second, you're hitting more droplets. However, there's a compensating effect: kinetic heating. At high speed, the friction of air against the skin heats the surface, which can raise the temperature above freezing and melt or prevent ice. So kinetic heating may cancel the increased droplet collection. Let's contrast two aircraft types. The wing of a small aircraft has a relatively broad cross-section — it's thick compared to its chord. That means it will accumulate ice relatively slowly. But other parts of the aircraft — the tail-plane, undercarriage struts, and antenna — have relatively narrow cross-sections, so they will accumulate ice more rapidly. This leads to a crucial operational warning: a small amount of ice on the mainplane could indicate more serious icing elsewhere on the aircraft. So when you see just a little ice on the wing, don't relax — check the tail, the struts, the antenna. They may be building ice much faster. Now the second factor: cloud base temperature. The principle here is simple: the higher the temperature, the greater the water vapour content the air can hold. So at a warmer cloud base, there's more moisture available. Condensation first occurs at the base of the cloud, and therefore there is a greater amount of free water available to become ice on an airframe. The free water content at any level in the cloud increases with base temperature. As you go up through the cloud, the amount of liquid water available depends on how warm it was at the bottom. The consequence: concentration of drops will increase and so will icing severity. Warmer cloud base means more supercooled droplets aloft, and that means more severe icing for you. The excerpt also references Figure 16.4, which illustrates the concentration of supercooled water droplets — that's the visual you'd see on the page showing how droplet density varies. So to summarise what I want you to take away: thin shapes ice faster than thick ones; high speed increases droplet impact but kinetic heating can offset it; and a warmer cloud base means more free water and more severe icing. And always remember — ice on the wing may be just the tip of the iceberg for the tail and other narrow parts.