曝光台 注意防骗
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as opposed to spreading backwards.
Weather 137
Variations on clear ice are freezing rain
and freezing drizzle, both of which
have larger droplets and are caused
by rain, snow or ice crystals falling
through warmer air at lower
altitudes. However, the latter’s
droplets have a higher water content.
Although aircraft are different,
expect icing to occur (in the engine
intake area, anyway) whenever the
OAT is below 4°C. Clear ice is
found most often in cumulus clouds
and unstable conditions between 0
and –10°C, and rime ice in
stratiform clouds between –10 and –
20°C (exam questions).
Ice is reported as:
· Trace, meaning slight, nonhazardous.
· Light, with occasional use of
deicing equipment.
· Moderate, where use of above
equipment is necessary.
· Severe, where the equipment is
useless and you must divert.
Pitot head, static vent and fuel vent
heaters should be on whenever you
encounter icing, together with
anything else you feel is appropriate.
Try not to use deicing boots until at
least ½ inch of solid (not slushy) ice
has formed, or they will merely
stretch the ice covering and operate
inside the resulting cocoon. Waiting
a while at least gives you the ability
to crack the ice off. I know that
some experts have determined that
this is not the case, but, trust me,
they’re wrong. If you operate the
boots too early, the ice coating on
them will merely flake and stay stuck
on. Boots on horizontal stabilizers,
by the way, will be less effective due
to their geometry. High propeller
RPM will give ice less of a chance to
form, as well.
You need warmer air to get rid of ice
effectively – just flying in clear air
can take hours, but you could at least
say you won’t get any more.
Aerodynamic heating comes from air
friction, which may get rid of ice, but
at high speeds, so will not likely
benefit light aircraft, except
helicopter rotor blades, which may
have a difference of 1° purely from
their speed of rotation.
Climbing out is often not possible,
due to lack of performance or ATC
considerations, and descending has
problems, too – if you’re getting
clear ice, it’s a fair bet that the air is
warmer above you, since it may be
freezing rain, which means an
inversion, probably within 1,000 feet
or so, as you might get before a
warm front.
In this position, landing on your first
attempt becomes more important as
you are unlikely to survive a goaround
without picking up more of
the stuff. You basically have three
choices, go up, down or back the
way you came. Going up is a good
first choice if you know the tops are
nearby, if only because you won’t
have a chance to do so later, but you
present more of the airframe to icing
risk, which is why there is often a
minimum speed for climbing in icing
conditions, slightly more than
normal. A zoom climb will help;
apply full power, then raise the nose.
To keep out of trouble, before
going, check that the freezing level is
well above any minimum altitudes,
which will help get rid of ice in the
descent. Try to make sure the cloud
138 Canadian Professional Pilot Studies
tops are within reach as well, or that
you have plenty of holes.
Keeping in mind the CYA (Cover
Your Ass) principle, try to get a
PIREP (pilot report) as well – the
way CARs is worded means that
very often you can't take off at all if
even a small amount of ice is
forecast, even though it's a clear day.
De-icing Fluids
The main types are what used to be
known as AEA (Association of
European Airlines) Type I
(unthickened) with a high glycol
content and low viscosity, and Type
II (thickened) with a minimum
glycol content of about 80% which,
with a thickening agent (one or two
teaspoons of corn flour), remains on
surfaces for longer, but remember it
has to blow off before you actually
get airborne. The idea is to decrease
the freezing point of water but, as
the ice melts, the fluid mixes with
the water, both diluting it and
making it more runny (what's left
after repeated applications to combat
this is of an unknown concentration,
and may refreeze quickly). Type III
lies somewhere between the two.
Type IV is similar to Type II, but
with significantly longer holdover times.
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