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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


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