JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
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
mixed icing.
The most critical freezing precipitation (rain or drizzle) area is where water is falling from warm
air above to a flight level temperature that is 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 above freezing.
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
DRAG & FUEL
CONSUMPTION
INCREASE
STALL SPEED
INCREASES
LIFT
DECREASES
WEIGHT
INCREASES
THRUST & RANGE
DECREASE
Figure 4-8 — Cumulative Effects of 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 4-8). Ice can alter the shape of an airfoil, changing the angle of attack at which the
aircraft stalls therefore increasing the stall speed. Ice reduces lift and increases drag on an airfoil.
Ice thickness is not the only factor determining the effect of icing. Location, roughness, and
shape are important, too. For example, a half-inch high ridge of ice on the upper surface of the
airfoil at 4% chord reduces maximum lift by over 50%. Yet, the same ridge of ice at 50% chord
decreases maximum lift by only 15%. On another airfoil, a distributed sandpaper-like roughness
on the leading edge of the wing may decrease lift by 35%. Along with this decrease in lift, it is
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