JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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 (measured by the aircraft’s
outside air temperature gauge) 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.
Super-cooled water forms because, unlike bulk water, water droplets in the free air do not freeze
at 0° C. Instead, their freezing temperature varies from –10 to –40° C: the smaller the droplets,
the lower the freezing point. As a general rule, serious icing is rare in clouds with temperatures
below –20° C since these clouds are almost completely composed of ice crystals. However, be
aware that icing is possible in any cloud when the temperature is 0° C or below.
Structural Icing Conditions
Clear icing normally occurs at temperatures between 0° C and –10° 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
extremely hazardous because it builds up fast, can freeze the flight controls, and disrupts airflow
over the wings.
Rime icing is milky white in appearance and is 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° C. Because mixed icing is a
combination of large and small water droplets, it takes on the appearance of both rime and clear
icing. It is lumpy, like rime ice, but also hard and dense, like clear ice. The most frequent type of
icing encountered is usually a form of mixed icing.
Frost is a thin layer of crystalline ice that forms on exposed surfaces. It normally occurs on clear,
calm winter nights on aircraft surfaces just as it does on automobiles. Frost also forms in flight
when a cold aircraft descends from a zone of freezing temperatures into high relative humidity.
The moist air is chilled suddenly to below freezing temperatures by contact with the cold
surfaces of the aircraft, and deposition occurs. Frost, like other forms of icing, disrupts the
smooth boundary layer flow over airfoils, and thus increases drag, causes a loss of lift, and
increases stall speed. Though it is unlikely to add considerable weight to an aircraft, any amount
of frost is hazardous and must be removed prior to takeoff.
Aircrews should anticipate and plan for some type of icing on every flight conducted in below
freezing temperatures and should be familiar with the icing generally associated with different
atmospheric conditions, as discussed in the next section.
Version 3.2/Dec 08 4-11