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the input from several sources and
interfaces it with the autopilot, aside
from storing commonly used
coordinates and routes.
In helicopters, a Stability Control
Augmentation System (SCAS) is used
for short term control assistance,
just to reduce the workload. It is
found in many machines, particularly
the Hughes 500, which uses a beep
trim on the cyclic control. Rate gyros
generate electrical impulses that
control actuators that resist
movement from where the controls
should be. To stop it going out of
control, there will be a limiter.
The Automatic Flight Control System
(AFCS) is a step above, using a
computer to memories control
positions and keep them there.
Control Wheel Steering, as fitted to
Boeings, allows inputs from the
control column to be made with
autopilot engaged, and the new
attitude will be held automatically.
Touch Control Steering needs the
autopilot to be disengaged with a
switch whilst a attitude is set. It is
maintained on re-engagment.
Interlocks stop the autopilot taking
over if the system isn't ready, and
cause it to disengage on failures.
Vibrations
In helicopters, vibrations come from
many sources, not least the
operation of the machine itself –
there are many spinning parts that
must be finely balanced. One good
source is being downwind. Various
components can induce vibrations in
others, for example, a tail rotor can
vary them in the main rotors. They
may be felt as lateral, longitudinal or
vertical, or a combination. One way
of testing which types are affecting
you is to sit in the cockpit and rest
your wrist on your knee to see which
way your hand moves.
Vibrations fall into three ranges:
·  Low, with large amplitude,
between 100-400 cycles per
minute, generally associated
with the main rotor. They can
usually be felt through the
cyclic. A wumper is one kick per
revolution (one per). A
vibration in the stick and
fuselage is possibly from the
rotors or the rotor support
system, particularly friction
dampers. If felt in the controls
only, look in the linkages.
·  Medium, 1,000-2000 cycles per
minute, usually stemming from
the tail rotor after improper
rigging or imbalances.
·  High, over 2000 cycles per
minute, usually from the engine.
Aircraft Husbandry
You can tell how well pilots treat
their machines by the maintenance
costs. For example, not using brakes
excessively while taxying can save a
lot of money in a large fleet.
Similarly, whilst the manufacturers
tell you that you can use the rotor
brake on a helicopter all the time, do
you really need to? They tell you
that, certainly, but they want to sell
you spare parts! Even something as
small as caging the gyros before you
turn them on can make a lot of
difference long-term.
Aircraft should not be parked on
soft or sloping ground, and suitable
chocks should be under the main
Airframes, Engines & Systems 231
gear wheels of aeroplanes. They
should be parked into wind, with the
nosewheel in line fore and aft.
Control locks are devices attached to
flying controls (external) or control
columns (internal), to stop them
moving on the ground and protect
them from gust and high wind
damage. Together with covers, they
should be used whenever convenient
(especially when winds will be high,
in which case consider picketing as
well), and all doors, hatches and
windows should be closed when the
aircraft is left unattended.
Control stops are devices that restrict a
control's range of operation.
Anti-collision lights should be
switched on immediately before
starting engines, but it is suggested
(like the military) that this be done
immediately the aircraft is occupied,
always having due regard for the
capabilities of your battery. Speaking
of which, always leave the anti-col
light switches on when leaving the
aircraft, because that lets you know
you've left the Master switch on.
Don't forget taxying procedures,
such as not using the brakes too
much, or using aircraft momentum
when turning corners to save using
the engine. Engine runups (like on
power checks) should be done into
wind for better engine cooling and
least strain on the prop, and away
from loose items on the ground,
both to protect people behind and
the prop, as the airflow around the
tips will tend to pull bits of gravel etc
　
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