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there are always residuals to resolve.
This is done by measuring the
effects on the aircraft's compass
against a Master compass, and
introducing fields of equal
magnitude but opposite polarity
deliberately to counteract them.
Airfields and maintenance areas have
clear areas in which this can be done.
The aircraft is taxied there and
everything that would be used in
flight turned on. Then the aircraft
compass is compared against a
landing compass on several headings, in
a correcting swing. Then the deviations
are reduced by adjusting the magnets
inside the compass and a calibration
swing is done to see what deviations
are left. These figures are written
down on the deviation card.
Gyroscopes
Typically, three cockpit instruments
are under gyroscopic influence.
These are the attitude indicator
(artificial horizon), heading indicator
(DGI) and turn indicator/coordinator.
A gyroscope is a rotating mass on an
axis, which may be vertical or
horizontal. The spinning allows the
gyro to maintain its own position in
74 JAR Private Pilot Studies
space (rigidity), regardless of whatever
it is attached to is doing. In other
words, it resists attempts to displace
it from its position. If you attached
one to a camera, for example, and
used the camera in a helicopter, the
helicopter could be bumping around
all over the place due to wind or
pilot input, and the camera would
not move from where the operator
put it. The same principle is used
with the instruments mentioned
above, as we shall shortly see.
Another property gyroscopes have is
precession, meaning that a force
applied to the spinning mass is felt
90° away from where it is applied, in
the direction of rotation:
The control inputs on a helicopter
have to allow for this, because the
rotor disc is nothing but a large gyro
– even though you make an input
for forward flight, the actual
movement applied to the rotor head
is done several degrees beforehand.
A more mundane example comes
from riding a bicycle – when you
apply a force to turn one way or
another, it is done at the top of the
wheels, but the turning movement
appears 90° later, hence the turn.
If a primary precession is impeded
for any reason, the impeding force
will produce a secondary precession in
the direction of the original force (as
in the turn and slip indicator, below).
Gyroscopic instruments are made to
spin through suction (heading and
attitude indicator) or electricity (turn
instruments). With the former, air is
sucked out of the casing, and vanes
(small bucket-shapes) on the gyro
mass catch the movement and force
it to go round (vacuum system). There
might be a pump, or a venturi
system (on older aircraft) to reduce
the pressure. Since the venturi
system depends on a tube aligned
with the airflow outside the aircraft,
it is only effective above about 90
kts, and therefore not good enough
for IFR. The suction gauge on the
instrument panel is always part of
the checklist before IFR flight to
ensure you have enough for the
instruments to work properly. The
rest of the vacuum system has a
pump driven by the engine, a relief
valve, an air filter, and enough
tubing for the connections.
During startup checks, pull and hold
any erection or caging knobs before
turning the power on, as the parts
inside can clash against each other as
they spin up (just one of those little
things a pilot can do to save longterm
maintenance costs).
Artificial Horizon
Otherwise known as the attitude
indicator, this instrument represents
the natural horizon and indicates the
pitch and bank attitudes, that is,
whether the nose is up or down, or
the wings are level or not. It is gyrodriven,
and the gyro is vertically
mounted so the housing can rotate
around the vertical axis.
Instruments 75
The horizon bar is connected to the
rear of the gyroscope frame and the
housing with a pin, so when the
housing moves, the bar stays rigid.
With all these rotating parts, there is
bound to be some friction, which
will cause some errors. Others
include acceleration error, found
during forward movement (as in a
takeoff) where a false climb is
indicated. Deceleration will show a
false descent.
Heading Indicator (DGI)
This works in a similar way to the
artificial horizon, except that the
gyro is horizontally mounted. The
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