Icing Page: 13 of 22
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ANSWER: c. resist the deflective influence.
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The factors necessary to produce structural icing on an aircraft in flight are:
1. Free air temperature. Structural ice may form when the free air temperature is at or
below 0 degrees C. The most severe icing occurs with temperature between 0 degrees and -10
2. Visible liquid moisture. Clouds are the most common form of visible liquid moisture, but
not all clouds with temperatures below freezing produce serious ice formation. Although serious
icing is rare in temperatures below -20 degrees C, the aviator must recognize that icing is possible
in any clouds where the temperature is below 0 degrees C.
Freezing rain, which occurs in the cold air below a frontal inversion is another form of visible
liquid moisture that causes icing. Freezing rain is probably the most dangerous ic condition.
3. Sublimation (Transforming vapor to a solid without going through the liquid state).The
sublimation process apparently is intensified when large quantities of suitable nuclei are present.
The movement of an aircraft through an area of high humidity and high concentration of nuclei
often triggers the sublimation process and results in a rapid accumulation of ice. Any layer of air
at altitudes above the freezing level, even if clear, is a potential icing zone when its
temperature-dew point spread is small.
Which factor(s) below is (are) necessary to produce structural icing on aircraft in flight?
a. Free air temperature at or below freezing.
b. Free convection.
c. Visible liquid moisture.
) Both a and c.
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Snyder, Henry W. Icing, pamphlet, February 1971; Texas. (texashistory.unt.edu/ark:/67531/metapth46570/m1/13/: accessed May 24, 2017), University of North Texas Libraries, The Portal to Texas History, texashistory.unt.edu; crediting Boyce Ditto Public Library.