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(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
Instruments 75
the wings are level or not. It is gyrodriven,
and the gyro is vertically
mounted so the housing can rotate
around the vertical axis.
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 (exam question).
Heading Indicator (DGI)
This works in a similar way to the
artificial horizon, except that the
gyro is horizontally mounted. The
casing turns around the gyro, which
has a compass card mounted on it. It
is also air driven.
Because of wander (real or apparent
- see above), unless you have a slaved
compass (meaning automatic), you
should align this instrument with the
magnetic compass every 15 minutes
or so, remembering, of course, to do
it in level, unaccelerated flight. You
may get erroneous readings if the
aircraft adopts unusual attitudes.
Turn Coordinator
This is actually a combination of two
instruments, one power driven, and
the other not. A small aircraft tilts to
indicate whether you are banking, so
it is a useful backup to the artificial
horizon, especially since the gyro is
electrically operated and not affected
if the suction system fails (although
it gives you a rudimentary indication
of bank, turns without the other
instruments are done with timing).
The instrument is sensitive to yaw
and roll, because the gyro's axis is
tilted upwards by about 30-35°.
When the wings in the little aircraft
hit one of the lower marks you are in
a Rate 1 turn, which takes two
minutes to go through 360°, making
3° per second (you can also add 7 kts
to 10% of your airspeed to get a
rough guide to the bank angle).
76 Canadian Private Pilot Studies
Underneath is a ball in a clear tube
containing fluid, for damping
purposes, called an inclinometer:
It is subject to gravity and centrifugal
force, and will be thrown one way or
another if the aircraft is not in a
coordinated turn. In a slip, the rate of
turn is too slow for the bank, so the
centrifugal force will be less, and the
ball will not be thrown out so much
(exam question).
It will therefore be on the inside of
the turn (decrease the angle or
increase the rate to correct, or both):
In a skid, the turn is too fast, so
more centrifugal force causes the
ball to be displaced more, to the
outside of the turn:
Corr
ection is the opposite of the slip,
above.
The turn indicator's gyro is laterally
mounted, so it can tilt about the
longitudinal axis. A linkage joins it to
the pointer, and there is a restraining
spring between it and the instrument
case. There are mechanical stops to
keep it from going more than 45°
either side of the centre.
As it is electrically driven, it is not
affected by suction failure. Although
it gives you a rudimentary indication
of bank, turns without the other
instruments are done with timing.
Rate of turn indications are only
accurate at the speed for which the
instrument has been calibrated,
　
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