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instruments.
In fact, accident studies show that
wings can stall 30% above normal
stalling speed, and be undetected
longitudinally. In addition, lateral
control problems could lead to
severe roll rates, up to ± 80° of roll.
Zero degrees is actually when water
becomes capable of freezing, from
which you can infer that it doesn't
necessarily do so. A Supercooled Water
Droplet is one below freezing, but not
frozen. It gets away with this from
the absence of ice nuclei to bind on to
(hygroscopic nuclei are needed for
normal condensation of water,
which could be almost anything
microscopic floating in the air, such
as industrial haze or particles).
When such a droplet strikes an
airframe, however, just below 0°,
some of it will freeze on impact,
releasing latent heat and warming the
remainder, which then flows back,
turning into clear ice, which can
gather without noticeable vibration
(the airframe will act one giant ice
nucleus, in other words). 1/80th part
of a SWD will freeze on impact for
each degree below zero. The worst
136 Canadian Professional Pilot Studies
place to penetrate cumuliform cloud
is between 0 to -10°C, where most
SWDs are.
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 areas include
control surfaces, rotors, propellers,
stabilisers, control linkages, etc.
The ability of an object to
accumulate ice is known as its catch
or collection efficiency, which is inversely
proportional to the shape of the
surface. This means that a sharpedged
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, although they
rotate so fast they are always a
degree or so warmer. 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 a 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 type of 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 edge
　
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