Icing

Icing is a problem that can effect any aircraft at almost anytime. Icing is a cumulative hazard to airplanes. When ice builds up on the surface of an airplane, it increases weight and drag while reducing lift and trust. These factors tend to slow the airplane and/or force it to decend. Icing can also be a detriment to engines and insturments. There are three principle types of icing that can effect an airplane: rime ice, clear ice, and mixed ice.

Rime ice forms a brittle and frost-like surface. It develops when small drops, like those in stratiform clouds or in light drizzle, freeze on impact without spreading. It is rough and opaque. An example is like the frost in a home freezer.

Clear ice forms a hard and glossy surface. It develops when water droplets which touch the airplane flow across the surface before freezing. This ice can accumulate as a smooth sheet. Clear ice is likely to form in areas of large water droplets, such as in rain or cumuliform clouds.

Mixed ice forms a hard rough conglomerate. It develops when precipitation droplets vary in size. It has an opaque and very rough surface.

Icing can occur in clouds any time subfreezing temperatures are reached and is governed by water droplet size and distribution. Small water droplets are the most common in fog and low-level clouds. This can be seen as drizzle or light rain. Rime ice can generally be found here. Areas that form thick, stratified clouds, with long durations of precipitation contain large water droplets. These droplets present serious icing problems on long flights and are most prevalent in the winter. Clear ice, along with large water droplets, can be formed in cumuliform clouds upward currents.
Other factors in icing include fronts, terrain, and variations in the seasons.

Additional information on icing may be attained at NASA's Lewis Research Center Icing Branch.

Icing information referenced from: Gleim, Irvin N. Private Pilot Handbook. Aviation Publications,Inc. Florida. 1986.

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