Refer to figure.
It occurs when a sub-zero surface comes into contact with moist air (the ambient temperature is lowered to saturation level). It is a white crystal deposit which appears similar to frost on the ground. Hoar frost occurs in clear air.
Water vapour in contact with the airframe is converted to ice crystals without becoming liquid (sublimating).
This whole process required the presence of another type of ice nucleus – their composition is usually inorganic (ie. Dust, soil particles).
Hoar frost can occur:
- on the ground. This usually occurs at night and is similar to the frost which forms on a car. If frost is not removed from the wings before flight, it can cause an early airflow separation that decreases lift and increases drag. This causes the airplane to stall at a lower-than-normal angle of attack.
- in flight. Hoar frost can occur in flight in the following cases:
- If a rapid descent is made from a very cold region to a warm moist layer.
- If a climb is made from a temperature below 0°C through an inversion.
The icing is not severe. The effects can be overcome by flying in a region where the temperature is above 0°C or by flying faster to increase the kinetic heating.
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