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Filtered air enters the instrument cases from a central air
filter. As long as aircraft fly at relatively low altitudes, enough
air is drawn into the instrument cases to spin the gyros at a
sufficiently high speed.
Dry Air Vacuum Pump
As flight altitudes increase, the air is less dense and more air
must be forced through the instruments. Air pumps that do not
mix oil with the discharge air are used in high flying aircraft.
3-18
Figure 3-29. Twin-Engine Instrument Pressure System Using a Carbon-Vane Dry-Type Air Pump.
Steel vanes sliding in a steel housing need to be lubricated,
but vanes made of a special formulation of carbon sliding
inside carbon housing provide their own lubrication in a
microscopic amount as they wear.
Pressure Indicating Systems
Figure 3-29 is a diagram of the instrument pneumatic
system of a twin-engine general aviation airplane. Two dry
air pumps are used with filters in their inlet to filter out any
contaminants that could damage the fragile carbon vanes in
the pump. The discharge air from the pump flows through
a regulator, where excess air is bled off to maintain the
pressure in the system at the desired level. The regulated air
then flows through inline filters to remove any contamination
that could have been picked up from the pump, and from
there into a manifold check valve. If either engine should
become inoperative or either pump should fail, the check
valve isolates the inoperative system and the instruments are
driven by air from the operating system. After the air passes
through the instruments and drives the gyros, it is exhausted
from the case. The gyro pressure gauge measures the pressure
drop across the instruments.
Electrical Systems
Many general aviation aircraft that use pneumatic attitude
indicators use electric rate indicators and/or the reverse. Some
instruments identify their power source on their dial, but it
is extremely important that pilots consult the POH/AFM to
determine the power source of all instruments to know what
action to take in the event of an instrument failure. Direct
current (D.C.) electrical instruments are available in 14- or
28-volt models, depending upon the electrical system in
the aircraft. A.C. is used to operate some attitude gyros and
autopilots. Aircraft with only D.C. electrical systems can use
A.C. instruments via installation of a solid-state D.C. to A.C.
inverter, which changes 14 or 28 volts D.C. into three-phase
115-volt, 400-Hz A.C.
Gyroscopic Instruments
Attitude Indicators
The first attitude instrument (AI) was originally referred to as
an artificial horizon, later as a gyro horizon; now it is more
properly called an attitude indicator. Its operating mechanism
is a small brass wheel with a vertical spin axis, spun at a high
speed by either a stream of air impinging on buckets cut into
its periphery, or by an electric motor. The gyro is mounted in
a double gimbal, which allows the aircraft to pitch and roll
about the gyro as it remains fixed in space.
A horizon disk is attached to the gimbals so it remains in
the same plane as the gyro, and the aircraft pitches and
rolls about it. On early instruments, this was just a bar that
3-19
Figure 3-30. The dial of this attitude indicator has reference lines
to show pitch and roll.
represented the horizon, but now it is a disc with a line
representing the horizon and both pitch marks and bank-angle
lines. The top half of the instrument dial and horizon disc
is blue, representing the sky; and the bottom half is brown,
representing the ground. A bank index at the top of the
instrument shows the angle of bank marked on the banking
scale with lines that represent 10°, 20°, 30°, 45°, and 60°.
[Figure 3-30]
A small symbolic aircraft is mounted in the instrument case so it
appears to be flying relative to the horizon. A knob at the bottom
center of the instrument case raises or lowers the aircraft to
compensate for pitch trim changes as the airspeed changes. The
width of the wings of the symbolic aircraft and the dot in the center
of the wings represent a pitch change of approximately 2°.
For an AI to function properly, the gyro must remain
vertically upright while the aircraft rolls and pitches around
it. The bearings in these instruments have a minimum of
friction; however, even this small amount places a restraint
on the gyro producing precession and causing the gyro to tilt.
To minimize this tilting, an erection mechanism inside the
instrument case applies a force any time the gyro tilts from
its vertical position. This force acts in such a way to return
　
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