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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—new engines are
cheaper than new aircraft.
· In a jet, you can use higher
angles of attack and still get a
sizeable amount of lift for a
moderate increase in drag,
because the wings are designed
that way. Various methods are
used to inform you of the stall,
and you want to keep the thing
flying just above that point—
something that may require
some practice in a simulator.
Similarly, if the shear is
encountered during the
approach, positive application
of power and flying controls
should keep the speed and rate
of descent within normal limits;
if there is any doubt, abandon
the approach and act as above.
· Set max prop RPM (flat pitch).
Windshear should be reported
quickly, for the benefit of others. It
can be detected by radar, using
Doppler Shift to calculate how fast
raindrops are moving and
Weather 121
subsequently the pattern of air
movement, specifically looking for
headwind/tailwind combinations. In
theory, this could also be used to
detect turbulence at higher levels,
assuming raindrops are present.
Low level windshear is found under the
anvil of a cumulonimbus.
Windshear is occasional if it exits for
about 1/3 of the time, intermittent
between then and 2/3, and continuous
over that. The alert is given when
the mean surface wind is over 20 kts,
and the difference between it and
the gradient wind is over 40 kts.
There also needs to be a temperature
difference of 10° between the
surface and 1,000 feet and Cbs or
heavy showers within 5 nm.
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
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
122 Canadian Professional Pilot Studies
most hazardous to other aircraft
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