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lightning detector). Turn the cockpit
lighting fully on and lower crew seats
and visors to minimise the blinding
effect of lightning flashes.
Within the Area
As the speed of vertical air currents
may well exceed the capabilities of
the aircraft, fly by attitude at the
recommended turbulence speed and
maintain your original heading—do
not correct for altitude, except for
obstacles; avoid harsh or excessive
control movements, particularly with
power, except to restore margins
from stall warnings or high speed
buffets. Do not be misled by
conflicting indications on other
instruments, and don’t roll too
much. If auto-trim variations are
large, disengage the autopilot
(movement of the mach trim, where
it occurs, though, is necessary and
desirable). Check the yaw-damper
remains engaged. You might get
temporary warnings (e.g. low oil
pressure) from negative G, which
may be ignored at your discretion.
Air Traffic Control Considerations
Obtain clearance from, or notify,
ATC so they can separate you from
others. If you can't, keep
manoeuvres to a minimum, and
inform them ASAP.
Take-off and Landing
Do not take off if a thunderstorm is
overhead or within 5 nm. At
destination, hold clear or divert.
Icing
Ice adversely affects performance,
not only by adding weight, but also
altering the shape of lift producing
surfaces, which changes your stalling
speed – autorotation in a helicopter
could therefore be a lot more
98 JAR Private Pilot Studies
interesting than normal (the US
Army found that half an inch on the
leading edge reduces your lifting
capacity by up to 50%, and increases
drag by the same amount) – if your
engine stops, you could really fall out
of the sky!
On top of that, fuel could freeze in
wing tanks, as could control
surfaces, and slush picked up on
take-off could stop the landing gear
from operating, as well as flight
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 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,
because of 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
around in the air, such as industrial
haze or salt particles, but freezing
requires ice nuclei.
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
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
　
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