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Rime ice comes from smaller SWDs
well below 0° (actually from -10° to -
40°C). It is opaque and granular and
moves forward as it builds up on sharp
surfaces like antennae. On a
helicopter rotor blade, it is more
likely to occur on the top rather than
the leading edge. Below -40°C, you
will encounter ice crystals only,
which will not stick to the aircraft.
So—it's a good idea to avoid icing
conditions but, in any case, having
the equipment doesn't mean you can
fly in icing conditions. On small
twins it may just mean it produces
no adverse effects on normal flight
(though they might be nearly always
overweight), and no-one could be
bothered to take it off. Some aircraft
are simply not happy in icing, even if
the stuff is there (particularly true of
older Barons and PA31s). Icing
equipment is not certified if you are
carrying deposits from ground
operations or storage, so ensure that
all frost, ice and snow is removed
before you get airborne, if only
because the aircraft systems don't get
really under way till then
The trend now is towards a "clean
aircraft concept" which, essentially,
means that nothing should be on the
outside of an aircraft that should not
be there, except, perhaps, for deicing
fluid, but even that is suspect.
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 it will have a
similar effect (hoar frost is a light
frosty deposit that typically appears
on a parked aircraft after a clear cold
night. It can usually be seen
through). These include control
surfaces, rotors, propellers,
stabilisers, control linkages, etc.
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 have to divert.
To keep out of trouble, before
going, check that the freezing level is
well above any minimum altitudes.
Try to make sure the cloud tops are
within reach as well, or that you have
plenty of holes.
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
Weather 99
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.
It is green, and needs care to provide
uniform cover, especially over Type
I fluid already there.
Type I fluids have good de-icing
properties, but may refreeze - they
are for de-icing, not anti-icing. Union
Carbide Ultra fluid (i.e. Type IV)
appears to increase holdover times
by 1.5 over Type II and way more
for Type I. The holdover time is
how long the effects of the fluid
should last – it can be affected by
high winds or jet blasts damaging the
fluid film, and temperature,
humidity, etc.
General Precautions
Deposits must be swept from
hinge areas and system intakes,
and the sprays 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
　
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