1.  SOURCES OF AIRCRAFT ICING CONDITIONS
1.1  Weather-related conditions
1.1.1  Icing conditions on the ground can be expected when air temperatures fall below freezing and when moisture or ice occurs in the form of either precipitation or condensation.
1.1.2  Precipitation may be rain, sleet or snow. Frost can occur due to the condensation of fog or mist. Frost occurs systematically when OAT is negative and sky is clear overnight.
1.1.3  To these weather conditions must be added further phenomenon that can also result in aircraft ice accretion on the ground.

1.2  Aircraft-related conditions
1.2.1  The concept of icing is commonly associated only with exposure to inclement weather. However, even if the OAT is above freezing point, ice or frost can form if the aircraft structure temperature is below 0° C (32° F) and moisture or relatively high humidity is present.
1.2.2  With rain or drizzle falling on sub-zero structure, a clear ice layer can form on the wing upper surfaces when the aircraft is on the ground. In most cases this is accompanied by frost on the under wing surface.

1.3  Sources of Icing
There are several sources, which will contribute to the accumulation of frost, ice, slush or snow on an aircraft.
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Precipitation - rain, fog , drizzle, ice pellets/crystals, hail, sleet, snow

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Ice accumulation in engine inlets

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Moisture freezing on the wing due to the cold soak effect

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Frost, ice or clear ice on wing surfaces due to the effect of cold soaked fuel. This is formed when the aircraft descends after a period of time at altitude where the ambient temperature is significantly lower than the ambient temperature at ground level. It is possible for these types of frost or ice to form on aircraft wings, even at ground temperatures of 10o C or higher

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Frost formation on the aircraft on clear nights

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Slush , snow and ice accumulations on landing gears, wheels and brakes during take off , landing and taxiing

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Aircraft flying through icing conditions can lead to formation of ice on the aircraft surface and in case of short turn around times ,the aircraft can remain contaminated before the next take off

Note : Icing conditions are far more frequent than effective ice accretion. Icing conditions do not systematically lead to ice accretion.

EFFECT OF AIRCRAFT CONTAMINATION ON PERFORMANCE
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Ice generally occurs on the protruding parts of the aircraft like the nose, wing, fin, tail plane leading edges, engine intakes, antennas, hinges, etc.

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Accumulation of ice on the wing leads to degradation in lift, increase in drag, increase in stall speed, and reduction in stalling angle.

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Icing can damage the engine and APU
　
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