Icing Requirements

AVIATION WEATHER

CHAPTER FIVE

Structural icing forms on the external structure of an aircraft. Structural ice forms on the wings,

fuselage, antennas, pitot tubes, rotor blades, and propellers. Significant structural icing on an

aircraft can cause control problems and dangerous performance degradation. The types of

structural icing are clear, rime, mixed, and frost.

Engine icing occurs when ice forms on the induction or compressor sections of an engine,

reducing its performance.

Icing Requirements

There are two requirements for the formation of aircraft icing. First, the atmosphere must have

super-cooled visible water droplets. Second, the free air temperature and the aircraft's surface

temperature must be below freezing.

Clouds are the most common form of visible liquid water and super-cooled water is liquid water

found at air temperatures below freezing. When super-cooled droplets strike an exposed object,

such as a wing, the impact induces freezing and results in aircraft icing. Therefore, when

penetrating a cloud at subzero temperatures, icing should be expected. Frozen precipitation in

solid form (hail, snow grains, ice pellets) does not cause structural icing.

Super-cooled water forms because, unlike bulk water, water droplets in the free air do not freeze

at 0C. Instead, their freezing temperature varies from 10 to 40 degrees C: the smaller the

droplets, the lower the freezing point. As a general rule, serious icing is rare in clouds with

temperatures below 20C since these clouds are almost completely composed of ice crystals.

However, icing is possible in any cloud when the temperature is 0C or below.

Structural Icing Conditions

Clear icing normally occurs at temperatures between 0 and 10 degrees C, where water droplets

are large because of unstable air, such as in cumulus clouds and in areas of freezing rain or

drizzle. Instead of freezing instantly upon contact with the aircraft's surface, these large water

droplets move along with the airflow, freeze gradually, and form a solid layer of ice. This layer

of clear ice can cover a large portion of the wing surface and is difficult to break off. Clear icing

is the most severe form of icing because it builds up fast, can freeze the flight controls, and

disrupts airflow over the wings.

Rime icing is rough, opaque, milky white in appearance and most likely to occur at temperatures

of 10 to 20 C. It is more dense and harder than frost, but lighter, softer, and less transparent

than clear ice. Rime ice occurs in stable conditions, clouds where the water droplets are small

and freeze instantaneously, such as stratiform clouds and the upper portions of cumulus clouds.

It is brittle and fairly easy to break off. Rime ice does not normally spread over an aircraft

surface, but protrudes forward into the air stream along the leading edges of airfoils.

Mixed icing is a combination of clear ice and rime ice, occurring where both large and small

water droplets are present, normally at temperatures of 8 to 15 degrees C. Because mixed

icing is a combination of large and small water droplets, it takes on the appearance of both rime

Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds

5-13