temperature increase from the hub to the propeller tip.
Today’s turboprop aircraft have deicers on the
propellers.
However, these deicers are curative, not
preventive, and the danger remains.
Rotary-Wing Aircraft
When helicopters encounter icing conditions, the
icing threat is similar, but potentially more hazardous
than with fixed-wing aircraft. When icing forms on the
rotor blades while hovering, conditions become
hazardous because the helicopter is operating near peak
operational limits.
Icing also affects the tail rotor,
control rods and links, and air intakes and filters.
PRELIMINARY CONSIDERATIONS
The first phase in the preparation of an aircraft icing
forecast consists of making certain preliminary
determinations. These are essential regardless of the
technique employed in making the forecast.
Clouds
Determine the present and forecast future
distribution, type, and vertical extent of clouds along the
flight path. The influence of local effects, such as terrain
features, should not be overlooked.
Temperature
Determine those legs of the proposed flight path that
will be in clouds colder than 0°C. A reasonable estimate
of the freezing level can be made from the data
contained on freezing level charts, constant-pressure
charts, rawinsonde observations, and airways reports
(AIREP) observations, or by extrapolation from surface
temperatures.
Precipitation
Check surface reports for precipitation along the
proposed flight path, and forecast the precipitation
character and pattern during the flight. Special
consideration should be given to the possibility of
freezing precipitation.
Note that each of the following methods and
forecast rules assumes that two basic conditions must
exist for the formation of icing. These assumptions are
1. the surface of the aircraft must be colder than
0°C, and
2. supercooled liquid-water droplets, clouds, or
precipitation must be present along the fight path.
THE ICING FORECAST
The following text discusses icing intensity
forecasts by using upper air data, surface data, and
precipitation data, as well as icing formed due to
orographic and frontal lifting.
Intensity Forecasts From Upper Air
Data
Check upper air charts, pilot reports, and
rawinsonde reports for the dewpoint spread at the flight
level. Also check the upper air charts for the type of
temperature advection along the route. One study,
which considered only the dewpoint spread aloft, found
that there was an 84 percent probability that there would
be no icing if the spread were greater than 3°C, and an
80 percent probability that there would be icing if the
spread were less than 3°C.
When the dewpoint spread was 3°C or less in areas
of warm air advection at flight level, there was a 67
percent probability of no icing and a 33 percent
probability of light or moderate icing. However, with a
dewpoint spread of 3°C or less in a cold frontal zone,
the probability of icing reached 100 percent. There was
also a 100-percent probability y of icing in building
cumuliform clouds when the dewpoint spread was 3°C
or less. With the spread greater than 3°C, light icing was
probable in about 40 percent of the region of cold air
advection with a 100-percent probability of no icing in
regions of warm or neutral advection. However, it
appears on the basis of further experience that a more
realistic spread of 4°C at temperatures near –10° to
-15°C should be indicative of probable clouds, and that
spreads of about 2° or 3°C should be indicative of
probable icing. At other temperatures use the values in
the following rules:
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