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magnetic interference. It contains the flux valve, which is
the direction-sensing device of the system. A concentration
of lines of magnetic force, after being amplified, becomes
a signal relayed to the heading indicator unit, which is also
remotely mounted. This signal operates a torque motor in
the heading indicator unit that processes the gyro unit until
it is aligned with the transmitter signal. The magnetic slaving
transmitter is connected electrically to the HSI.
There are a number of designs of the remote indicating
compass; therefore, only the basic features of the system are
covered here. Instrument pilots must become familiar with
the characteristics of the equipment in their aircraft.
As instrument panels become more crowded and the pilot’s
available scan time is reduced by a heavier flight deck
workload, instrument manufacturers have worked toward
combining instruments. One good example of this is the
RMI in Figure 3-26. The compass card is driven by signals
from the flux valve, and the two pointers are driven by an
automatic direction finder (ADF) and a very high frequency
omnidirectional range (VOR).
Gyroscopic Systems
Flight without reference to a visible horizon can be safely
accomplished by the use of gyroscopic instrument systems
and the two characteristics of gyroscopes, which are rigidity
and precession. These systems include attitude, heading,
and rate instruments, along with their power sources. These
instruments include a gyroscope (or gyro) that is a small wheel
with its weight concentrated around its periphery. When this
wheel is spun at high speed, it becomes rigid and resists tilting
or turning in any direction other than around its spin axis.
Attitude and heading instruments operate on the principle
of rigidity. For these instruments, the gyro remains rigid
in its case and the aircraft rotates about it. Rate indicators,
such as turn indicators and turn coordinators, operate on the
principle of precession. In this case, the gyro processes (or
rolls over) proportionate to the rate the aircraft rotates about
one or more of its axes.
Power Sources
Aircraft and instrument manufacturers have designed
redundancy in the flight instruments so that any single failure
will not deprive the pilot of the ability to safely conclude
the flight. Gyroscopic instruments are crucial for instrument
flight; therefore, they are powered by separate electrical or
pneumatic sources.
Pneumatic Systems
Pneumatic gyros are driven by a jet of air impinging on
buckets cut into the periphery of the wheel. On many aircraft
this stream of air is obtained by evacuating the instrument
case with a vacuum source and allowing filtered air to flow
into the case through a nozzle to spin the wheel.
Venturi Tube Systems
Aircraft that do not have a pneumatic pump to evacuate the
instrument case can use venturi tubes mounted on the outside
of the aircraft, similar to the system shown in Figure 3-27. Air
flowing through the venturi tube speeds up in the narrowest
part and, according to Bernoulli’s principle, the pressure
drops. This location is connected to the instrument case by
a piece of tubing. The two attitude instruments operate on
approximately 4" Hg of suction; the turn-and-slip indicator
needs only 2" Hg, so a pressure-reducing needle valve is
used to decrease the suction. Air flows into the instruments
through filters built into the instrument cases. In this system,
ice can clog the venturi tube and stop the instruments when
they are most needed.
3-17
Figure 3-27. A venturi tube system that provides necessary vacuum
to operate key instruments.
Figure 3-28. Single-engine instrument vacuum system using a steel-vane wet-type vacuum pump.
Vacuum Pump Systems
Wet-Type Vacuum Pump
Steel-vane air pumps have been used for many years to
evacuate the instrument cases. The vanes in these pumps
are lubricated by a small amount of engine oil metered into
the pump and discharged with the air. In some aircraft the
discharge air is used to inflate rubber deicer boots on the
wing and empennage leading edges. To keep the oil from
deteriorating the rubber boots, it must be removed with an
oil separator like the one in Figure 3-28.
The vacuum pump moves a greater volume of air than is
needed to supply the instruments with the suction needed,
so a suction-relief valve is installed in the inlet side of the
pump. This spring-loaded valve draws in just enough air to
maintain the required low pressure inside the instruments,
as is shown on the suction gauge in the instrument panel.
　
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