曝光台 注意防骗
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amongst other things.
Zero degrees is actually when water
becomes supercooled and capable of
freezing. Airframe icing happens
when supercooled water droplets
strike an airframe below that. Some
of the droplet freezes on impact,
releasing latent heat and warming the
remainder which then flows back,
turning into clear ice, which can
gather without noticeable vibration.
On the ground this can mean
ground resonance in a helicopter,
and bits of ice flying off rotor blades
or propellers. In flight, the extra
weight and drag could cause descent
and improper operation of flying
controls. So—it's a good idea to
avoid icing conditions but, in any
case, you shouldn't go if you haven't
got the equipment, which naturally
must be serviceable (see Certification
for Flight in Icing Conditions, above).
The trend now is towards a "clean
air concept" which, essentially,
means that nothing should be on the
outside of an aircraft that should not
be there, except, of course for
deicing fluid.
All ice should be removed from
critical areas before take-off,
including hoar frost on the fuselage,
because even a bad paint job will
increase drag, which is relevant if
you're heavy, and hoar frost will
have a similar effect. Deicing details
should be entered in the relevant
part of the Tech Log, including
start/end times, etc. The critical
areas include control surfaces,
rotors, stabilisers and the like.
The ability of an object to
accumulate ice is known as its catch
efficiency; a sharp-edged object is
better at it than a blunt-edged one,
due to its lesser deflection of air.
Speed is also a factor. Due to the
speed and geometry of a helicopter's
main rotor blades, their catch
efficiency is greater than that of the
fuselage, so ice on the outside of the
cabin doesn't relate to what you
might have on the blades. In fact,
Canadian Armed Forces tests show
that you can pick up a lethal load of
ice on a Kiowa (206) rotor blade
inside 1-6 minutes, although it’s true
to say that 206 blades, being fairly
crude, don’t catch as much as more
sophisticated ones, such as those on
the 407. Mind you, tailplanes have
sharper leading edges than wings,
and will collect ice more efficiently,
so you might see nothing on the
wing yet have it on the horizontal
stabiliser. Because of the ratio of ice
thickness to the chord length, the
effects will be more marked.
It’s the rate of accretion that's
important, not the characteristics of
the icing, although clear ice is
definitely worse than rime ice, since
the latter contains air bubbles and is
much lighter and slower to build. It
also builds forward from the leading
Operational Procedures 109
edge as opposed to spreading
backwards. 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 a layer of
warmer air at lower altitudes.
However, the latter’s droplets have a
much higher water content.
Although aircraft are different,
expect icing to occur (in the engine
intake area, anyway) whenever the
OAT is below 4oC. Otherwise, it can
form in clear air when humidity is
high. Clear ice is found most often
in cumulus clouds and unstable
conditions between 0 and –10 degs
C, and rime ice in stratiform clouds
between –10and –20.
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, otherwise 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.
You need warmer air to get rid of ice
effectively – just flying in clear air
can take hours, but I suppose you
could at least say you won’t get any
more. Climbing out is often not
possible, due to lack of performance
or ATC considerations, and
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