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lift from either increased headwind
or decreased tailwind, taking you
above the glidepath – recovery
involves reducing power and
lowering the nose, with a higher
power setting than before when reestablished,
or the aircraft will sink.
The latter is the opposite, of course
– recover by increasing power and
setting it to less than the original
when established.
The effects also depend on the
aircraft and its situation, in that
propeller driven types suffer less
than jets, and light aircraft tend to be
less vulnerable than heavy ones—
those with a good power to weight
ratio will come off best. The take-off
leaves you most vulnerable because
of the small scope for energy
conversion, less amounts of excess
engine power and the amount of
drag from the gear and flaps, which
is not to say that landing is that
much better.
84 JAR Private Pilot Studies
In extremely simple terms, where
windshear is expected, you should
have a little extra airspeed in hand;
you can help with the following:
· On take-off, use the longest
runway, less flap and more
airspeed up to about 1,000 feet
AGL, but watch your gradient
and use about 10 kts more than
usual. If shear is indicated by
rapidly fluctuating airspeed
and/or rate of climb or descent,
apply full power and aim to
achieve maximum lift and
distance from the ground. Be
prepared to make relatively
harsh control movements and
power changes, using full
throttle if you have to—new
engines are cheaper than new
aircraft.
· Set prop RPM to maximum (to
get flat pitch).
Windshear should be reported as
soon as possible, for the benefit of
others.
Microbursts
These are small, intense
downdraughts that spread out in all
directions when they reach the
surface, commonly associated with
thunderstorms. You are most likely
to encounter them within 1,000' of
the ground, that is, right on the
approach. They are most dangerous
where the vertical push converts to
the horizontal, between the base of
the microburst and the ground – you
could get a vertical speed of over
6000 feet per minute and a
horizontal one over 45 kts. The
diameter will be up to 5 km, and the
duration anything between 1-5
minutes, or more, though the
maximum intensity will start on
touching the ground and only last
for a couple of minutes.
They are problematical because they
involve a performance-increasing
shear to start with, followed by a
performance decreasing one. Being
so close to the ground, you are likely
to be taking off or landing, and
therefore more vulnerable. The angle
of attack reduces inside a downburst,
because it changes with the relative
airflow, so the nose should be placed
into a high pitch attitude on entry,
and reduced (quickly) on exit.
Microbursts are rarely isolated – if
you meet one, watch for another.
Wake Turbulence
A by-product of lift behind every
aircraft, (including helicopters) in
forward flight, arising from induced
drag, particularly severe from heavy
machines, and worst at slow speeds,
as on takeoff or landing. Wake vortices
are horizontally concentrated
whirlwinds streaming from the
wingtips, from the separation point
between high pressure below and
low pressure above the wing. Air
flowing over the top of the wing
tends to flow inward due to the
reduced pressure sucking it in, while
that under the wing tends to flow
outwards because it is of higher
pressure and pushes outwards.
Where the lower air curls over the
wingtip, it combines with the upper
air to form a counter clockwise flow.
The distance between the vortices
will be about ¾ of the wingspan or
rotor disc.
The heavier and slower the aircraft,
the more severe they will be, and
flaps, etc. will only have a small
Weather 85
effect in breaking them up, so even
clean aircraft are dangerous. The
effects become undetectable after a
time, varying from a few seconds to
a few minutes after the departure or
arrival, although they have been
detected at 20 minutes. Vortices are
most hazardous to other aircraft
during take-off, initial climb, final
approach and landing, but you
should be careful any time you are
within 1,000 feet below and behind a
heavy aircraft.
Although there is a danger of
shockloading, the biggest problem is
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