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The movement of air over a crest
line has a venturi effect, giving an
increased windspeed over the
summit and a corresponding
reduction of pressure, which could
cause your altimeter to over-read.
On passing over or round an
obstacle, the air may become
turbulent or have formed into rolls
with a vertical or horizontal axis.
The general effect of a series of
ridges is to form rolls between the
crest lines, possibly causing a
226 Operational Flying
dangerous situation where a
downdraught can exist on an
upslope where an updraught would
normally be expected:
As a result, on top of steep ridges
there may be an area of nil or reverse
winds which is difficult to locate on
the first recce. The vertical distance
to which a mountain chain will
influence the movement of air is
about 3-5 times its height, changing
with the windspeed.
Horizontally, the effect is variable
and most noticeable in stable
conditions with more than 20 knots
of wind, when standing waves will
form. As you probably know, you
can recognise the existence of these
by lenticular clouds, but you will also
see ragged cloud around the peak.
These should be avoided at all costs
due to their turbulence, especially at
the wind speeds that lead to their
creation. As well as shockloading,
momentary loss of control may
occur, not to mention coffee all over
the place.
A couple of thoughts for when
you’re very high up; how much time
it takes to get down if you have a
problem, and meeting anyone else at
that height on an airway who doesn’t
expect you. And oxygen.
Landing Sites
Those on peaks or crests usually
present you with more escape routes
than any on flanks or valley bottoms
so, wherever possible, landings
should be made on ground higher
than the immediate surroundings, so
you can vary the approach according
to the wind and have a clear
overshoot path. Customers, though,
have this annoying habit of wanting
to land on the most obscure sites!
Use the windward sides of a slope;
leeward sides should only be used in
operational necessity, because wind
flowing down the slope can increase
its apparent angle (you need more
lateral cyclic to hold the helicopter in
place, and you could run out when
you reduce power to lower the
downwind skid). Don't forget you
will not have the full effects of a
ground cushion, if at all. Where
conditions allow, go as far to the
windward edge as possible, to avoid
suddenly finding yourself in dead or
reversed airflow (as if on a leeslope)
and make overshooting easier. The
wind coming over the peak will have
increased in speed, due to Venturi
effects, so a 15 knot wind can easily
become double that, aside from your
altimeter misreading.
Finding the wind direction can be
interesting if the site is bare and
gives you no information, and it
doesn’t help that mountain flying
tends to take place in high pressure
conditions, that is, where the winds
are light and variable. We are now
talking about local winds, caused by
convection, for instance, or katabatic
effects, combined with the prevailing
wind influenced by the ground, or
even a mixture of them all. Even a
cloud shadow can increase the speed
of a downflowing wind from a cold
surface. You could judge its effects
on the machine itself, flying round
the site with a constant speed and
Special Use Of Aircraft 227
power setting, or a constant altitude,
which is otherwise known as a contour
crawl, because you use one contour
all the way round.
Look at your power settings,
whether the air is turbulent, your
groundspeed varies, whether you
drift or whether the nose yaws into
or out from the slope. How much
pedal you use to keep straight is a
good help – a lot of right pedal
means the wind is from the left, for
example, and a fair amount of
vibration means it is behind you, but
it may be a good idea, if you can’t
have it at the front, to get the wind
off to the side that requires the use
of the power pedal (the left, in a
206), in case tail rotor authority
becomes a problem. Aft cyclic would
indicate a tailwind as well.
So, with constant power and
airspeed (say 40-50 kts), when you
rise, you will be on the windward
side, and vice versa. On the other
hand, you would use less power on
the windward side if you kept a
constant height. However, use
　
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