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tropopause, where inversion
conditions stop the ascent and
strong winds produce the distinctive
anvil shape. When downdraughts hit
the ground, they spread out into a
gust front, ahead of which are more,
called downbursts, which may
themselves contain microbursts.
Expect lightning as well at this stage.
The main characteristic of dissipation
is a downdraught and disappearing
cloud (exam question).
A thunderstorm is actually a
collection of several cloud cells in
varying stages of development, with
varying diameters. 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 self-perpetuating.
Otherwise, development normally
takes 20-30 minutes, the mature
stage lies between 40-60 minutes and
dissipation starts after 2 hours.
The temperature band for lightning
is between +10°C to -10°C.
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.
Switch on the pitot heaters, deicing,
and continuous ignition system,
where fitted. Disregard any beacons
subject to interference, such as ADF
and OMEGA (although tuning the
former 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
Weather 135
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 (or land) if a
thunderstorm is overhead or
approaching within 5 nm.
Hurricanes
These are cyclonic (circular) storms
from tropical areas, or low latitudes
where the coriolis force is very small.
They are very destructive, and
characterised by high wind speeds,
but in the centre, or the eye, the wind
speed will be next to zero. They are
caused by masses of water vapour
condensing on to hygroscopic
nuclei, releasing massive amounts of
heat to cause vast updraughts, which
suck air from the surface.
Tornadoes
Rotating tunnel-shaped clouds
coming out of the bottom of a
thunderstorm and touching the
ground. They are capable of lifting
very heavy items and transporting
them for several miles. A waterspout is
the same thing, but over water.
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
　
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