heat of fusion. Some of this liberated heat is taken on
by the unfrozen portion of the drop; its temperature is
thereby increased, while another portion of the heat is
conducted away through the surface in which it lies.
The unfrozen drop now begins to evaporate due to its
increase in temperature, and in the process, it uses up
some of the heat, which, in turn, cools the drop. Due to
this cooling process by evaporation, the remainder of
the drop is frozen. Icing at 0°C will occur only if the air
is not saturated because the nonsaturated condition is
favorable for evaporation of part of the drop.
Evaporation cools the drop below freezing, and then ice
formation can take place.
INTENSITIES OF ICING
There are three intensities of aircraft icinglight,
moderate, and severe.
The rate of accumulation may create a problem if
flight is prolonged in this environment (over 1 hour).
Occasional use of deicing/anti-icing equipment
removes/prevents accumulation. It does not present a
problem if the deicing/anti-icing equipment is used.
The rate of accumulation is such that even short
encounters become potentially hazardous, and the use
of deicing/anti-icing equipment or diversion is
The rate of accumulation is such that
deicing/anti-icing fails to reduce or control the hazard.
Immediate diversion is necessary.
ICING HAZARDS NEAR THE GROUND
Certain icing hazards exist on or near the surface.
One hazard results when wet snow is falling during
takeoff. This situation can exist when the air
temperature at the surface is at or below 0°C. Wet snow
sticks tenaciously to aircraft components, and it freezes
if the aircraft encounters markedly colder temperatures
If not removed before takeoff, frost, sleet, freezing
rain, and snow accumulation on parked aircraft become
operational hazards. Another hazard arises from the
presence of puddles of water, slush, and/or mud on
airfields. When the temperature of the airframe is colder
than 0°C, water blown by the propellers or splashed by
wheels can form ice on control surfaces and windows.
Freezing mud is particularly dangerous because the dirt
may clog controls and cloud the windshield.
OPERATIONAL ASPECTS OF
Due to the large number of types and different
configurations of aircraft, this discussion is limited to
general aircraft types, rather than specific models.
These high speed aircraft generally cruise at
altitudes well above levels where severe icing exists.
The greatest problem will be on takeoff, climb, and
approach because of the greater probability of
encountering supercooled water droplets at low
altitudes. Also, the reduced speeds result in a decrease
of aerodynamic heating.
Turbojet engines experience icing both externally
and internally. All exposed surfaces are subject to
external airframe icing.
Internal icing may pose special problems to turbojet
aircraft engines. In flights through clouds that contain
supercooled water droplets, air intake duct icing is
similar towing icing. However, the ducts may ice when
skies are clear and temperatures are above freezing.
While taxiing and during takeoff and climb, reduced
pressure exists in the intake system, which lowers
temperatures to a point that condensation and/or
sublimation takes place, resulting in ice formation. This
temperature change varies considerably with different
types of engines. Therefore, if the free air temperature
is 10°C or less (especially near the freezing point) and
the relative humidity is high, the possibility of induction
icing definitely exists.
The problems of aircraft icing for this type of
aircraft combine those of conventional aircraft and
turbojet aircraft. Engine icing problems are similar to
those encountered by turbojet aircraft, while propeller
icing is similar to that encountered by conventional
Propeller icing is a very dangerous form of icing
because of the potential for a tremendous loss of power
and vibrations. Propeller icing varies along the blade
due to the differential velocity of the blade, causing a