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you from others. If you can't,
keep manoeuvres to a minimum,
and inform them ASAP.
Take-off and Landing
Do not take off if a
thunderstorm is overhead or
approaching within 5 nm. At
destination, hold clear or divert.
Use of Weather Radar
This detects rainfall to avoid (not
penetrate) severe weather, as
large raindrops in a small area
are a dead giveaway for
thunderstorms or, rather, their
activity is - turbulence is
proportional to the rate at which
rainfall increases or decreases
over a given distance. Your
decisions are therefore based on
a deduction from certain facts.
A clear area on the radar screen
doesn’t mean there isn’t any
cloud, as minute cloud droplets,
ice, dry snow and dry hail have
low reflective levels, if at all. It is
more likely to indicate large
water droplets, as they will
absorb energy as they approach
the size of the radar wave.
Operational Procedures 107
Operation is quite simple, but
full use on the ground should be
avoided (not below 500 feet, in
fact). Naturally, you've got to
check the equipment before
departure, but most sets have an
internal procedure for this.
When you do switch it on, it
should be set to Standby for at
least 3 minutes first, to allow
things to warm up. You will
have several scan ranges to
choose from, possibly from 250
miles down to 5, but 80 is
adequate, which is about what
you would get with a 10 inch
antenna, the usual fit in small
aircraft. The smaller it is, the
wider the beam and the dispersal
of energy, which means that a
lot of it will pass by whatever
storm is around, giving you an
indication very much less than
the true hazard. You would be
safe in assuming that whatever
you see is one or two levels
more severe.
Once airborne, there is a tilt
capability which will point the
antenna upwards or downwards
so you can get more detail about
any storm cells you may be
approaching, but don’t expect to
see the tops of a storm, because
the crystals there won’t reflect
the energy in the first place, and
your beam focussing will be too
narrow to include it. In the same
way, you will also get ground
echoes, which are good for
detecting the enemy coast ahead,
but only because the water will
absorb all the echoes and you
will see a big black hole in its
place. Buildings and the like
won’t reflect properly at all –
you might just see a mass of
confusing colours.
If you haven't got the luxury of
colour and computer controlled
echo highlighting (and have to
rely on steam), there are certain
distinctive storm patterns to
look out for, such as the hook,
finger and U-shape, which all look
similar to a figure 6. These, plus
scalloped edges round a cloud
outline, are all characteristic of
hail. They can change quickly, so
they need careful monitoring
(hail shows up better when the
gain is reduced). The heaviest
precipitation, and the heaviest
turbulence, will show up as
black holes, or red when using
colour, which will be best
detected in Contour mode.
Remember that radar signals
weaken, and might show the end
of the weather falsely.
Iso-Echo produces a hole in a
strong echo when the returned
signal is above a pre-set value.
The edge of the hole that
actually appears on the screen
has the same rainfall rate, and is
like a contour line, hence the
name. When it is narrow, there
is a strong intensity gradient, so
avoid hooked echoes, especially
rapidly changing ones.
Avoid the brightest returns (i.e.
those that are changing rapidly)
by at least 20 nm. Above the
freezing level, make it 5 nm and
10 nm when below. If you see
anything at all between 50-70
nm, keep well away from it.
108 Operational Flying
Icing Conditions
Ice adversely affects performance,
not only by adding weight, but also
altering the shape of lift producing
surfaces, which changes your stalling
speed – autorotation in a helicopter
could therefore be a lot more
interesting than normal. The US
Army found that half an inch on the
leading edge reduces your lifting
capacity by up to 50%, and increases
drag by the same amount. On top of
that, fuel could freeze in wing tanks,
as could control surfaces, and slush
picked up on take-off could stop the
landing gear from operating,
　
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