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turbulence and windshear (below)
may be encountered. The quickest
way out of severe turbulence is up,
with the next best directly away from
the range. Flying parallel to the range
in an updraught, avoiding peaks,
gives the most comfort.
Windshear
This is the name for airspeed
changes over about 10 kts resulting
from sudden horizontal or vertical
changes in wind velocity—more
severe examples will change not only
airspeed, but vertical speed and
aircraft attitude as well. Officially, it
becomes dangerous when the
variations cause enough
displacement from your flight path
for substantial corrective action;
severe windshear causes airspeed
changes greater than 15 kts, or
vertical speed changes over 500 feet
per minute. Expect it to occur
mostly inside 1,000 feet AGL, where
it is most critical, because you can't
quickly build up airspeed—
remember that altitude is money in
the bank, but speed is money in the
pocket. However, it can occur at
high altitudes, near jetstreams.
Although mostly associated with
thunderstorms (see below), where
you have the unpredictability of
microbursts to contend with, it's also
present with wake vortices,
temperature inversions, mountain
waves and the passage of fronts, not
forgetting obstructions near the
runway, and can occur over any size
of area. You can even get it where
rain is falling from a cumulus cloud,
as the air is getting dense from the
cooling, and will therefore fall
quicker. It's not restricted to
aeroplanes, either—helicopters can
suffer from it above and below tree
top level in forest clearings, when a
backlash effect can convert any
headwind to tailwind.
All fronts are zones of windshear—
the greater the temperature
difference across them (over 10°C),
the greater the changes will be.
Warm fronts tend to have less than
cold ones, but as they're slower
moving, you catch it for longer. In
general, the faster the front moves
(say, over 30 kts), the more vigorous
the weather associated with it; if it
goes slower, the visibility will be
worse, but you can still get
windshear even then and always for
up to an hour after its passage.
Warm air moving horizontally above
cold air can produce turbulence
where they join, as would be typical
with an inversion. In a valley, in
particular, when the moving warm
air hits a mountainside, it will be
forced downwards, but unable to
penetrate the cold air, so it is forced
to move over the top of the cold air
120 Canadian Professional Pilot Studies
in the valley bottom, so watch out
on those cold, clear mornings.
The most significant effect of
windshear is, of course, loss of
airspeed at a critical moment, similar
to an effect in mountain flying,
where a wind reversal could result in
none at all! You would typically get
this with a downburst from a
convective type cloud, where,
initially, you get an increase in
airspeed from the extra headwind,
but if you don’t anticipate the
reverse to happen as you get to the
other side, you will not be in a
position to cope with the resulting
loss. This has led to the
classifications of performance increasing
or performance decreasing. With the
former, you get more airspeed and
lift from either increased headwind
or decreased tailwind, taking you
above the glidepath – recovery
involves reducing power and
lowering the nose, and using 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.
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
　
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