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spread out into a gust front, ahead of
which are more, called downbursts,
which may themselves contain
microbursts. Expect lightning as well.
The characteristics of dissipation are
downdraughts and disappearing
cloud.
The different cells in a thunderstorm
may be developing, maturing or
dissipating at rates of their own,
which could form their own trigger
actions and make the storm selfperpetuating.
Approaching the area
Seat belts should be tightened, and
loose articles stowed. One pilot
should control the aircraft and the
other should monitor the flight
instruments. Select an altitude for
penetration that will keep you clear
of obstacles, and use the weather
radar to select the safest track. Set
the power for the recommended
turbulence speed, adjust the trim and
note its position, so any excessive
changes from autopilot or mach trim
can be quickly assessed. Height,
mach, rate of climb or descent and
airspeed locks should be disengaged
but the yaw damper(s) should be on.
Weather 97
Switch on the pitot heaters, deicing,
and continuous ignition system,
where fitted. Disregard any beacons
subject to interference, such as ADF
(although tuning it to its lowest
frequency will give you a primitive
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
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,
98 Canadian Private Pilot Studies
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.
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