曝光台 注意防骗
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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, anyway) whenever the OAT
is below 4°C. Otherwise, it can form
in clear air when humidity is high—
anti-icing should be switched on well
in advance. Pitot head, static and fuel
vent heaters should be on whenever
you encounter icing, together with
anything else appropriate.
You need warmer air to get rid of ice
effectively – just flying in clear air
can take hours. Climbing out is often
not possible, due to 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 1000 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 go-around without picking
up more of the stuff.
Ground De-icing
Use either soft brushes, fluids, or a
combination (try parking behind a
friendly jet with its engines idling).
Priorities are control surface hinges,
engine intakes or static ports. Some
Operational Stuff 109
manufacturers, however, don't
recommend using fluids at all
because of the possible effect on the
bonding of composite materials.
Warm water can be used, but I'm
not so sure about that – I know if
you want to freeze water quickly,
you put it in the freezer hot.
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 cornflour), 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 I fluids have good de-icing
properties, but may refreeze—they
are for de-icing, not anti-icing. Union
Carbide Ultra fluid (potentially Type
IV, and green in colour) appears to
increase the times given by the tables
below by 1.5 over Type II and way
more for Type I – they give general
guidance on the use of I and II, and
the likely protection you might get on
the ground (most fluids are gone by
rotation speed):
General Precautions
Deposits must be swept away
from hinge areas and system
intakes, and the sprays
themselves should not be
directed to them, since the fluid
may be further diluted by the
melting ice it is designed to
remove, and may refreeze. It
may also cause smearing on
cockpit windows and loss of
vision during take-off.
Afterwards, confirm that flying
and control surfaces are clear
and move over their full range,
and intake and drain holes are
free of obstructions. Jet engine
compressors should be rotated
by hand to ensure they are not
frozen in position.
Further Precautions
Holdover time can be affected
by high winds or jet blasts
damaging the fluid film, and
skin temperatures can be
significantly lower than the
OAT, which makes them a
more representative entry point
into the tables above.
Recording Of Flight Times
Flight times in personal logbooks are
from first movement under power
until rotor rundown. Those in Tech
or Journey Logs, by contrast, are
from take-off to landing only,
sometimes entered in decimal hours.
It's common practice, when several
flights are made per hour without
closing down (as with pleasure
flying), to record the first take-off
and last landing times and to note
the actual airborne time in between.
There are many ways of doing this,
the most accurate using a stopwatch,
but there is an unofficial and widely
used practice (by arrangement with
your Inspector), when doing lots of
110 The Helicopter Pilot’s Handbook
sectors between engine starts, of
using two thirds of the total time
between first take-off and last
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The Helicopter Pilot’s Handbook(73)
