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Instrument Flying Handbook
Flight Instruments
Gyroscopic Systems

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Instrument Flying


Table of Contents

Chapter 1. Human Factors
Chapter 2. Aerodynamic Factors
Chapter 3. Flight Instruments
Chapter 4. Section I
Airplane Attitude Instrument
Using Analog Instrumentation
Chapter 4. Section II
Airplane Attitude Instrument
Using an Electronic Flight

Chapter 5. Section I
Airplane Basic
Flight Maneuvers
Using Analog Instrumentation
Chapter 5. Section II
Airplane Basic
Flight Maneuvers
Using an Electronic Flight

Chapter 6. Helicopter
Attitude Instrument Flying

Chapter 7. Navigation Systems
Chapter 8. The National
Airspace System

Chapter 9. The Air Traffic
Control System

Chapter 10. IFR Flight
Chapter 11. Emergency

The slaving control and compensator unit has a push button
that provides a means of selecting either the "slaved gyro"
or "free gyro" mode. This unit also has a slaving meter
and two manual heading-drive buttons. The slaving meter
indicates the difference between the displayed heading and
the magnetic heading. A tight deflection indicates a clockwise
error of the compass card; a left deflection indicates a
counterclockwise error. Whenever the aircraft is in a turn
and the card rotates, the slaving meter shows a full deflection
to one side or the other. When the system is in "free gyro"
mode, the compass card may be adjusted by depressing the
appropriate heading-drive button.

A separate unit, the magnetic slaving transmitter is mounted
remotely; usually in a wingtip to eliminate the possibility of
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
omni directional range (VOR).

Figure 3-26. Driven by signals from flux valve, the compass card
in this RMI indicates the heading of the aircraft opposite the upper
center index mark. The green pointer is driven by the ADF.

Gyroscopic Systems

Flights without reference to a visible horizon can he 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 ease, 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.