Instrument Flying: Technique in Weather Page: 19
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Section 4
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T. O. No. 30-100D-1mains on the other wing can reduce the air speedl and
decrease the controllability of the airplane. Proper
procedure must take into account the type of airplane
and the expected duration and intensity of the icing
condition for the particular flight.
Centrifugal force will ordinarily keep the propeller
blades free of clear ice. Near the hub, where cen-
trifugal force is less, an accumulation of ice may de-
velop, causing vibration. However, serious power loss
is seldom experienced as a result of clear ice on the
propellers.
RIME MAY BE NO JOKE
2. RIME.
Porous, white, usually granular or feathery in
appearance, and not as dense as glaze. Rime forms
when water droplets freeze as fast as they accumu-
late. Since this freezing is almost instantaneous on
striking the airplane, there is no excess of liquid
water. Hence rime forms only where the droplets
hit. This will be principally on leading edges, but
rivet heads and other minor protuberances may also
accumulate rime. Because rime lacks the mechani-
cal strength of glaze and because it does not spread
back from the de-icer boot, it is more easily removed
than glaze. Since rime freezes rapidly, it may adhere
to the propeller blades, but centrifugal force will
usually dislodge it and maintain propeller efficiency.
However, the rime may not break simultaneously
from all the blades. This unbalances the propeller,
causing severe vibration and necessitating a reduc-
tion in rpm. Until balance is again established by
removal of ice from the other blades, the available
power is seriously diminished.
Occasionally a very light accumulation of rime may
form a narrow, sharp-edged bead of ice along the
leading edge of the wing. Such an accumulation ap-
pears to be too slight to be of any consequence; but
if the air speed is reduced, as in a landing approach,
the angle of attack changes and the tiny bead may then
have a profound effect upon the stalling speed. The
only safe rule is to maintain an excess of air speed
whenever there is any ice whatsoever upon the wings.
On structures not protected by de-icers, rime will
sometimes form in extreme and unusual patterns. Re-
ports are on record of such peculiar formations as a
tapered cone built forward from a loop antenna for a
distance of 14 to 16 inches, and an 8- to 10-inch pro-
jection from the air-speed pitot head supported at the
base on an area of less than 1/2 square inch. Icicle-
like formations appear on antenna wires, individual
spikes being 6 to 8 inches long. Such formations
greatly increase the drag and correspondingly cut
down the air speed.
Formation of rime is favored by low temperature,
small cloud droplets, and a small over-all quantity of
liquid water in a given volume of cloud. However, the
accumulation of rime can be very rapid if the tem-
perature is low ard the number of droplets great.FROST
3. FROST.
Individual crystals of ice adhering to the skin of
the airplane. Frost forms when a cold aircraft moves
through air which is relatively warm and moist or when
it passes through air which is supersaturated with water
vapor. It forms not only on leading edges, but on all
surfaces exposed to the air. Although the total weight
of the accumulation is negligible, the great number of
individual crystalling spikes of ice produces serious
drag and tends to disrupt air flow in the boundary layer
next to the airfoil. This may be sufficient to increase
stalling speed very unexpectedly.
Frost formation seldom occurs in middle latitudes.
In the Arctic, however, it is encountered often, par-
ticularly during periods of extreme cold. Although it
occurs most frequently at low altitudes it can also oc-
cur at higher altitudes, particularly in the lee of
mountain passes, or wherever the normal flow of
wind is restricted.
Frost forms when the air next to the airplane's skin
becomes supersaturated with water vapor. The vapor
then turns to ice without passing through the liquid
state. Frost can, therefore, form in clear air and its
formation cannot always be foreseen. Pilots operating
in the polar region must know the peculiarities of this
type of icing and take proper precautions.
Of course, a heavy accumulation of frost may
gather on the wings of an airplane parked in the open
overnight. When this happens, take-off should never
be attempted until all frost is removed from the wings.
There have been many instances of airplanes carrying
only a light accumulation of frost which were unable
to leave the ground.
MIXED FORMS OF ICE
Not all the ice encountered will be either glaze or
typical rime. As conditions vary between those form-
ing glaze and those producing rime, any intermediate
form of ice may result. If snow is encountered to-
gether with glazing conditions, snowflakes may adhere
and freeze to leading edges, producing a thick, rough
accumulation in a short time. It is even possible that
very wet sticky snow may pack on the leading edges,
producing accumulation not unlike ice in its effect.
However, dry snow or ice crystals will not adhere to
an aircraft, so that an ice cloud or a fall of dry snow
does not present any icing hazard.
WHERE TO EXPECT ICING
Since the rate at which ice accumulates can be no
greater than the rate at which liquid water is depos-
ited on the airplane, regions of greatest icing hazard
in the air are the regions where there is the most
liquid water at freezing temperatures. The most
liquid water will be present when air starts out warm
and moist but is cooled rapidly by expansion upon
rising to a higher altitude. Regions where rapid lifting
occurs are to be expected in the neighborhood of fronts
and within cumulonimbus or large cumulus clouds.RESTRICTED
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Army Air Forces. Instrument Flying: Technique in Weather, book, January 1, 1944; Ashland, Ohio. (https://texashistory.unt.edu/ark:/67531/metapth873973/m1/25/: accessed July 16, 2024), University of North Texas Libraries, The Portal to Texas History, https://texashistory.unt.edu; crediting National WASP WWII Museum.