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power settings will be required (in
some helicopters, like the 500C, the
rotor blades will stall before you
reach engine limits). The dynamic
pressure applied to the ASI is also
reduced, so IAS will read less in
relation to TAS, so, if you maintain a
particular airspeed, your
groundspeed will increase
accordingly, and you will be going
faster than you think. The ASI will
also be slower to react.
Density Altitude is your real altitude
resulting from height, temperature
and humidity. The more the density
of the air decreases for any of those
reasons, the higher your machine
thinks it is. The effects are found at
sea level, as well as in mountains,
Specialised Tasks 67
when temperatures are high – for
example, 90° (F) at sea level is really
1900 feet as far as your aircraft is
concerned. In extreme situations,
you may have to restrict your
operations to early morning or late
afternoon (see also Performance).
Larger control movements will be
needed, with more lag, so controls
must be moved smoothly and
gradually, or the effects may well
cancel each other out – you may be
on the ground well before that large
handful of collective pitch even takes
effect! Rotor RPM will rise very
quickly with the least excuse.
Your maximum weight for a given
altitude (and vice versa), as well as
cruising speed in relation to them
both should be known, at least
approximately, in advance. You also
need to know the Hover ceiling In
and Out of Ground Effect
(HIGE/HOGE) for any weight, so
you know you can come to a low
hover properly, however briefly, and
recover from an unsuitable landing
site (hovering should actually be
minimised, partly because you can’t
rely on ground effect being present,
and you have less power anyway, but
also because you need to keep a little
up your sleeve if the wind shifts, or
you begin to lose tail rotor authority.
Having said that, no-hover landings
are not recommended, because of
the chances of snagging the skids on
something). Check the performance
charts in the back of the Flight
Manual, and start practising hovers
about 1-2 feet off the ground,
bearing in mind, of course, that the
said charts were established by test
pilots, in controlled situations.
If you allow for these effects as part
of your flight planning, fine, but it's
easy to get used to a particular place
and air density and a corresponding
take-off run, base leg, etc., and you
may get caught out one day when
things change.
Illusions
There is a psychological aspect to
mountain flying. In the initial stages,
it requires a good deal of selfcontrol,
as you overcome a certain
amount of fear and tension, which is
not good when you really need to be
relaxed on the controls. You will
also have to cope with some optical
illusions.
Almost the first thing you will notice
is the lack of a natural horizon, and
will maybe want to use the mountain
tops or sides as a substitute. This,
however, will probably cause a
climb, or other exaggerated attitudes,
and make it difficult to estimate the
height of distant ground, either from
a cockpit or on the ground itself, so
you will find it best to superimpose a
horizon of your own below the
peaks. This is where using your
instruments will help, both to keep
attitude and give you a good idea of
your height and speed (however,
you’re not supposed to be
instrument flying!).
Close to the ground, you will get an
impression of increased speed,
especially near to a ridge. For
example, climbing along a long
shallow slope is often coupled with
an unconscious attempt to maintain
height without increasing power so,
unless you keep an eye on the ASI,
you will be in danger of gradually
reducing speed—if your airspeed is
reducing, then either the nose has
been lifted or you're in a
downdraught (downdraughts will be
68 The Helicopter Pilot’s Handbook
associated with a loss of height or
airspeed for the same power). You
can also tell if you're in a
downdraught by watching the
position of the nose—if it yaws into
the slope, the air is flowing
downwards and vice versa. A lack of
cloud above, i.e. descending air, is
also a possible indication.
Downdraughts can frequently
exceed your climbing capabilities.
Strong updraughts can suspend you
in mid-air with zero power – if the
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The Helicopter Pilot’s Handbook(45)
