Rime ice forms when supercooled water droplets are small — Page 291, Lesson 278

I want to walk you through the three main types of ice that can form on an aircraft in flight: rime ice, mixed ice, and rain ice. Let's start with rime ice. Rime ice forms when supercooled water droplets are small. That happens at very low temperatures, or when the cloud droplets themselves are small. When these tiny droplets hit a surface, the whole droplet freezes on impact. Each droplet sticks to the surface it strikes and becomes solid almost at once. Because the droplets freeze instantly, air gets trapped between each frozen droplet. That trapped air makes the ice opaque. So rime ice is a white, opaque deposit with a light texture. It's caused by those small, supercooled water droplets freezing quickly. There is little or no flowback — meaning the water doesn't run back along the surface before freezing; it stays right where it hits. The ice grows outward from the leading edges of the wing or tail, and it gets compacted by the airstream. Some loss of aerofoil shape can occur, and air intakes can be affected. Rime ice can happen in any cloud where there are small supercooled water droplets — that includes nimbostratus, altostratus, altocumulus, stratocumulus, stratus, and the parts of heap clouds where supercooled water droplets are small. Now, mixed ice. Very often in cloud, at temperatures between 0°C and -20°C, we find a mixture of both large and small supercooled water droplets. This produces a build-up of ice on the leading edges from the small droplets — that's the rime ice component — and the flowback from the large droplets — that's the clear ice component. So mixed ice gives you a combination of the worst effects of both clear ice and rime ice. Finally, rain ice. Rain ice occurs in rain that becomes supercooled by falling from an inversion into air below 0°C. The rain does not freeze immediately in the air, but it can impact the aerofoil to form either clear ice or rime ice. Rain ice builds up very quickly, and a pilot's action should be to turn onto a reciprocal heading immediately — that means turn 180 degrees to fly back out of the conditions. Rain ice occurs in a narrow range of altitudes at low level, about 1,000 feet, ahead of a warm front or occlusion. It's associated particularly with the moderate continuous rain that often falls from nimbostratus cloud. This is illustrated in Figure 16.3, and you can see the formation of rime ice in Figure 16.2.