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descending has problems, too – if
you’re getting clear ice, it’s a fair bet
that the air is warmer above you,
since it may be freezing rain, which
means an inversion, probably within
1000 feet or so, as you might get
before a warm front. In this
position, landing on your first
attempt becomes more important as
you are unlikely to survive a goaround
without picking up more of
the stuff. You basically have three
choices, go up, down or back the
way you came. Going up is a good
first choice if you know the tops are
nearby, if only because you won’t
have a chance to do so later, but you
do present more of the airframe to
icing risk, which is why there is often
a minimum speed for climbing in
icing conditions.
Before going, check the freezing
level is well above minimum
altitudes, which will help get rid of
ice in the descent. Try to make sure
the cloud tops are within reach as
well, or you have plenty of holes.
Turbulence
This also exists high up, not so much
due to convection, but the passage
of fronts or mountain waves. You
can’t see the evidence of its existence
as there is little moisture to form
cloud, hence Clear Air Turbulence.
If turbulence is likely, mention it to
the cabin crew and advise the
passengers to return to, and/or
remain in their seats, ensuring their
seat belts/harnesses are securely
fastened. Catering and other loose
equipment should be stowed and
secured until the risk has passed. Fly
at recommended turbulence speed.
110 Operational Flying
Windshear
This concerns 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 is considered to
cause airspeed changes of greater
than 15 kts, or vertical speed
changes greater than 500 feet per
minute. Expect it to occur mostly
inside 1000 feet agl.
Although mostly associated with
thunderstorms (see above), 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, 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, the greater
the changes will be. Warm fronts
tend to have less windshear than
cold ones, but as they're slower
moving, you catch it for longer. In
general, the faster the front moves,
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.
One 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 from a
downburst out of 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 of the downburst, you
will not be in a position to cope with
the resulting loss. This has led to the
classification of windshear as either
performance increasing or performance
decreasing. Windshear encountered
near the ground (say below 1000
feet) is the most critical, mainly
because you can't quickly build up
airspeed—remember the old saying;
altitude is money in the bank, but
speed is money in the pocket.
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
　
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