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Pilot's Handbook of Aeronautical Knowledge
Flight Instruments
Gyroscopic Flight Instruments

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Pilot's Handbook of Aeronautical Knowledge



Table of Contents

Chapter 1, Introduction To Flying
Chapter 2, Aircraft Structure
Chapter 3, Principles of Flight
Chapter 4, Aerodynamics of Flight
Chapter 5, Flight Controls
Chapter 6, Aircraft Systems
Chapter 7, Flight Instruments
Chapter 8, Flight Manuals and Other Documents
Chapter 9, Weight and Balance
Chapter 10, Aircraft Performance
Chapter 11, Weather Theory
Chapter 12, Aviation Weather Services
Chapter 13, Airport Operation
Chapter 14, Airspace
Chapter 15, Navigation
Chapter 16, Aeromedical Factors
Chapter 17, Aeronautical Decision Making




Using the example of the bicycle, precession acts on the
wheels in order to allow the bicycle to turn. While riding
at normal speed, it is not necessary to turn the handle bars
in the direction of the desired turn. A rider simply leans in
the direction that he or she wishes to go. Since the wheels
are rotating in a clockwise direction when viewed from the
right side of the bicycle, if a rider leans to the left, a force is
applied to the top of the wheel to the left. The force actually
acts 90° in the direction of rotation, which has the effect of
applying a force to the front of the tire, causing the bicycle
to move to the left. There is a need to turn the handlebars at
low speeds because of the instability of the slowly turning
gyros, and also to increase the rate of turn.

Precession can also create some minor errors in some
instruments. [Figure 7-19] Precession can cause a freely
spinning gyro to become displaced from its intended plane
of rotation through bearing friction, etc. Certain instruments
may require corrective realignment during flight, such as the
heading indicator.

Precession of a gyroscope resulting from an applied deflective force.
Figure 7-19. Precession of a gyroscope resulting from an applied
deflective force.

Sources of Power
In some aircraft, all the gyros are vacuum, pressure, or
electrically operated. In other aircraft, vacuum or pressure
systems provide the power for the heading and attitude
indicators, while the electrical system provides the power for
the turn coordinator. Most aircraft have at least two sources
of power to ensure at least one source of bank information is
available if one power source fails. The vacuum or pressure
system spins the gyro by drawing a stream of air against the
rotor vanes to spin the rotor at high speed, much like the
operation of a waterwheel or turbine. The amount of vacuum
or pressure required for instrument operation varies, but is
usually between 4.5 "Hg and 5.5 "Hg.

One source of vacuum for the gyros is a vane-type enginedriven
pump that is mounted on the accessory case of
the engine. Pump capacity varies in different airplanes,
depending on the number of gyros.

A typical vacuum system consists of an engine-driven
vacuum pump, relief valve, air filter, gauge, and tubing
necessary to complete the connections. The gauge is mounted
in the aircraft's instrument panel and indicates the amount
of pressure in the system (vacuum is measured in inches of
mercury less than ambient pressure).

As shown in Figure 7-20, air is drawn into the vacuum system
by the engine-driven vacuum pump. It first goes through
a filter, which prevents foreign matter from entering the
vacuum or pressure system. The air then moves through the
attitude and heading indicators, where it causes the gyros
to spin. A relief valve prevents the vacuum pressure, or
suction, from exceeding prescribed limits. After that, the air
is expelled overboard or used in other systems, such as for
inflating pneumatic deicing boots.

It is important to monitor vacuum pressure during flight,
because the attitude and heading indicators may not provide
reliable information when suction pressure is low. The
vacuum, or suction, gauge is generally marked to indicate
the normal range. Some aircraft are equipped with a warning
light that illuminates when the vacuum pressure drops below
the acceptable level.

When the vacuum pressure drops below the normal operating
range, the gyroscopic instruments may become unstable and
inaccurate. Cross checking the instruments routinely is a
good habit to develop.

Turn Indicators
Aircraft use two types of turn indicators: turn-and-slip
indicator and turn coordinator. Because of the way the gyro
is mounted, the turn-and-slip indicator shows only the rate of
turn in degrees per second. The turn coordinator is mounted
at an angle, or canted, so it can initially show roll rate. When
the roll stabilizes, it indicates rate of turn. Both instruments
indicate turn direction and quality (coordination), and also
serve as a backup source of bank information in the event an
attitude indicator fails. Coordination is achieved by referring
to the inclinometer, which consists of a liquid-filled curved
tube with a ball inside. [Figure 7-21]