Ice will form on an airframe if three conditions are met simultaneously — Page 291, Lesson 274

I want to walk you through the topic of icing. This is a critical subject for any professional pilot because ice on the airframe can seriously degrade the performance and handling of your aircraft. Let's start with the basic causes. Ice will form on an airframe if three conditions are met simultaneously. First, there must be water present in a liquid state, specifically in the form of supercooled water droplets. Second, the ambient air temperature must be below 0°C, though I'll mention hoar frost as a special case shortly. Third, the airframe temperature itself must be below 0°C. All three of these need to be true for ice to accrete on the aircraft. Now, what exactly is a supercooled water droplet? It's a droplet of water that remains in the liquid state even though its temperature is below 0°C. Normally, pure water freezes at 0°C, but it needs something called a freezing nucleus to trigger the phase change from liquid to ice. A freezing nucleus is a tiny particle in the atmosphere around which an ice crystal can form. The key point here is that the number of freezing nuclei in the atmosphere is considerably less than the number of condensation nuclei you learned about in Chapter 6. Because freezing nuclei are scarce, supercooling is a frequent occurrence in clouds. Supercooled water droplets can exist in clouds at temperatures as low as -40°C. However, the moment an aircraft strikes one of these droplets, the impact itself will cause the droplet to start freezing. So the aircraft's passage through the cloud provides the trigger. The size of supercooled water droplets depends on two factors: the size of the basic cloud droplet, which is controlled by cloud type, and the temperature. As temperature decreases, the water droplets evaporate, which reduces their size. This is the Bergeron process, which you'll find covered in Chapter 13. Let me give you the size breakdown by temperature and cloud type. Large supercooled water droplets exist in the temperature range from 0°C down to -20°C, and they are found in Cumulus, Cumulonimbus, and Nimbostratus clouds. Small supercooled water droplets are found in two regimes. First, in the upper levels of Cumulus, Cumulonimbus, and Nimbostratus clouds, from -20°C down to -40°C. Second, in Stratus, Stratocumulus, Altostratus, and Altocumulus clouds across the full range from 0°C down to -40°C. Below -40°C, only very tiny supercooled water droplets can exist. Now let's look at the effects of icing on the aircraft, starting with aerodynamics. Ice tends to form on the leading edges of wings, tail surfaces, and other forward-facing components. This spoils the aerodynamic shape of those surfaces. The result is a reduction in lift, an increase in drag, an increase in weight, an increase in stalling speed, and an increase in fuel consumption. To give you a concrete number, ice, frost, or snow with a thickness and roughness similar to coarse sandpaper can reduce lift by 30% and increase drag by 40%. That is a massive performance penalty. There is also a mechanical hazard. Pieces of ice can break off from other surfaces, such as the propeller or the fuselage, and then jam between the control surfaces and the wings or tail. This can restrict or even prevent normal control movement, which is a serious flight safety issue. Icing is difficult to forecast, which is why you need a full understanding of the processes involved. Let's move on to the different types of ice that can form.