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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
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 moving warm air
hits a mountainside, it will be forced
downwards, but will not be able to
penetrate the cold air, so it is forced
to move over the top of the cold air
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, 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.
In extremely simple terms, where
windshear is expected, you should
84 Canadian Private Pilot Studies
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
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