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 wind nights when the air temperature and dew point are below freezing. 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.
Frontal Icing Conditions
Cold fronts and squall lines generally have a narrow band of both weather and icing. The
associated clouds will be cumuliform. The icing zone will be about 10,000 feet thick, 100 miles
wide, and the icing will be predominantly clear, accumulating rapidly.
Warm fronts and stationary fronts generally have a much wider band of weather and icing,
reflecting the size of the warm frontal zone. The icing will be found mainly inside stratiform
clouds, accumulating at a relatively low rate, due to the smaller size of the super-cooled water
droplets. The vertical depth of the icing zone will generally be about 3000 to 4000 feet thick,
possibly up to 10,000 feet. The type of icing will be predominantly rime, but may also contain
The most critical icing (rain or drizzle) area is where water is falling from warm air above to a
flight level temperature below freezing. In this case, severe clear ice would be encountered
below the cloud layer and the evasive action is to climb to an altitude where the temperature is
Occluded fronts often produce icing covering a very widespread area, containing both stratiform
and cumuliform type clouds. The depth of the icing zone will often be 20,000 feet, approximately
double the depth of icing zones with other type fronts. The types of icing will be clear, mixed,
and rime, with a very rapid and heavy rate of accumulation.
EFFECTS AND HAZARDS OF STRUCTURAL ICING
The most hazardous aspect of structural icing is its aerodynamic effects. The presence of ice on
an aircraft decreases lift, thrust, and range, and increases drag, weight, fuel consumption, and
stall speed. The added weight with reduced lift and thrust can be a dangerous combination
(Figure 5-10). Ice can alter the shape of an airfoil, changing the angle of attack at which the
aircraft stalls and therefore increasing the stall speed. Ice reduces lift and increases drag on an
Weather Hazards of Turbulence, Icing, Ceilings, Visibility, and Ash Clouds