Types of Icing

Ice forms on aircraft structures and surfaces when supercooled droplets adhere to them and freeze. Small and/or narrow objects ice up more rapidly than large thick objects. This is why a small protuberance within sight of the pilot can be used as an ice evidence probe. It will generally be one of the first parts of the airplane on which an appreciable amount of ice will form. An aircraft’s tailplane will be a better collector than its wings, because the tailplane presents a thinner surface to the airstream. The type of ice that forms can be classified as clear, rime, or mixed, based on the structure and appearance of the ice. The type of ice that forms varies depending on the atmospheric and flight conditions in which it forms.

Clear Ice

Clear ice is glossy, transparent and is formed by the relatively slow freezing of large supercooled water.

Because of its large drop size, clear ice does not freeze instantly on contact with the aircraft surface but freezes gradually as it flows back across the surface of the wing, leaving a smooth, hard, glossy, and transparent covering of ice over the wing surface. Since it flows back along the wing and slowly freezes, clear ice might form on unprotected areas of a wing.

Rime Ice

Rime ice is created by the near instantaneous freezing of small supercooled liquid drops.

Rime ice is an opaque, or milky white deposit of ice that forms when the airplane is flying through stratiform clouds. For rime to form, the aircraft skin must be at a temperature below 0°C. The drop will then freeze completely and quickly without spreading from the point of impact.

Mixed Ice

Mixed icing, as the name implies, has the properties of both clear and rime icing. Large and small supercooled droplets coexist. Appearance is whitish, irregular and rough. Favourable conditions include liquid and frozen particles found in the colder portion of the cumuliform cloud and wet snowflakes. Mixed ice can accumulate rapidly and is difficult to remove.