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aircraft deice and anti-ice equipment. Coping with the hazards
of icing begins with preflight planning to determine where
icing may occur during a flight and ensuring the aircraft is
free of ice and frost prior to takeoff. This attention to detail
extends to managing deice and anti-ice systems properly
during the flight, because weather conditions may change
rapidly, and the pilot must be able to recognize when a change
of flight plan is required.
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Figure 2-16. Clear Ice.
Figure 2-17. Clear Ice Buildup.
Types of Icing
Structural Icing
Structural icing refers to the accumulation of ice on the
exterior of the aircraft. Ice forms on aircraft structures and
surfaces when super-cooled droplets impinge on them and
freeze. Small and/or narrow objects are the best collectors
of droplets and ice up most rapidly. This is why a small
protuberance within sight of the pilot can be used as an “ice
evidence probe.” It is generally one of the first parts of the
airplane on which an appreciable amount of ice forms. An
aircraft’s tailplane is a better collector than its wings, because
the tailplane presents a thinner surface to the airstream.
Induction Icing
Ice in the induction system can reduce the amount of air
available for combustion. The most common example of
reciprocating engine induction icing is carburetor ice. Most
pilots are familiar with this phenomenon, which occurs when
moist air passes through a carburetor venturi and is cooled. As
a result of this process, ice may form on the venturi walls and
throttle plate, restricting airflow to the engine. This may occur
at temperatures between 20° F (-7° C) and 70° F (21° C). The
problem is remedied by applying carburetor heat, which uses
the engine’s own exhaust as a heat source to melt the ice or
prevent its formation. On the other hand, fuel-injected aircraft
engines usually are less vulnerable to icing but still can be
affected if the engine’s air source becomes blocked with ice.
Manufacturers provide an alternate air source that may be
selected in case the normal system malfunctions.
In turbojet aircraft, air that is drawn into the engines creates
an area of reduced pressure at the inlet, which lowers the
temperature below that of the surrounding air. In marginal
icing conditions (i.e., conditions where icing is possible),
this reduction in temperature may be sufficient to cause ice
to form on the engine inlet, disrupting the airflow into the
engine. Another hazard occurs when ice breaks off and is
ingested into a running engine, which can cause damage to
fan blades, engine compressor stall, or combustor flameout.
When anti-icing systems are used, runback water also can
refreeze on unprotected surfaces of the inlet and, if excessive,
reduce airflow into the engine or distort the airflow pattern
in such a manner as to cause compressor or fan blades to
vibrate, possibly damaging the engine. Another problem
in turbine engines is the icing of engine probes used to set
power levels (for example, engine inlet temperature or engine
pressure ratio (EPR) probes), which can lead to erroneous
readings of engine instrumentation operational difficulties
or total power loss.
The type of ice that forms can be classified as clear, rime, or
mixed, based on the structure and appearance of the ice. The
type of ice that forms varies depending on the atmospheric
and flight conditions in which it forms. Significant structural
icing on an aircraft can cause serious aircraft control and
performance problems.
Clear Ice
A glossy, transparent ice formed by the relatively slow
freezing of super cooled water is referred to as clear ice.
[Figure 2-16] The terms “clear” and “glaze” have been used
for essentially the same type of ice accretion. This type of
ice is denser, harder, and sometimes more transparent than
rime ice. With larger accretions, clear ice may form “horns.”
[Figure 2-17] Temperatures close to the freezing point, large
amounts of liquid water, high aircraft velocities, and large
droplets are conducive to the formation of clear ice.
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Figure 2-19. Aerodynamic Effects of Icing.
Figure 2-18. Rime Ice.
Rime Ice
A rough, milky, opaque ice formed by the instantaneous or
very rapid freezing of super cooled droplets as they strike
the aircraft is known as rime ice. [Figure 2-18] The rapid
freezing results in the formation of air pockets in the ice,
giving it an opaque appearance and making it porous and
brittle. For larger accretions, rime ice may form a streamlined
　
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