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Instrument Flying Handbook
Flight Instruments
Gyroscopic Instruments

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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

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
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]

attitude indicator
Figure 3-30. The dial of this attitude indicator has reference tines
to show pitch and roll.

A small symbolic aircraft is mounted in the instrument ease 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 Al 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
the spinning wheel to its upright position.

The older artificial horizons were limited in the amount of
pitch or roll they could tolerate, normally about 60° in pitch
and 100° in roll. Alter either of these limits was exceeded,
the gyro housing contacted the gimbals, applying such a
precessing force that the gyro tumbled. Because 0f this
limitation, these instruments had a caging mechanism that
locked the gyro in its vertical position during any maneuvers
that exceeded the instrument limits. Newer instruments do
not have these restrictive tumble limits; therefore, they do
not have a caging mechanism.

When an aircraft engine is first started and pneumatic or
electric power is supplied to the instruments, the gyro is
not erect. A self-correcting mechanism inside the instrument
actuated by the force of gravity applies a precessing force,
causing the gyro to rise to its vertical position. This erection
can take as long as 5 minutes, hut is normally done within
2 to 3 minutes.

Attitude indicators are free from most errors, but depending
upon the speed with which the erection system functions,
there may he a slight nose-up indication during a rapid
acceleration and a nose-down indication during a rapid
deceleration. There is also a possibility of a small bank angle
and pitch error after a ISO° turn. These inherent errors are
small and correct themselves within a minute or so after
returning to straight-and-level flight.

Heading Indicators
A magnetic compass is a dependable instrument used as a
backup instrument. Although very reliable, it has so many
inherent errors that it has been supplemented with gyroscopic
heading indicators.

The gyro in a heading indicator is mounted in a double gimbal,
as in an attitude indicator, hut its spin axis is horizontal
permitting sensing of rotation about the vertical axis of the
aircraft. Gyro heading indicators, with the exception of slaved
gyro indicators, are not north seeking, therefore they must
be manually set to the appropriate heading by referring to
a magnetic compass. Rigidity causes them to maintain this
heading indication, without the oscillation and other errors
inherent in a magnetic compass.

Older directional gyros use a drum-like card marked in the
same way as the magnetic compass card. The gyro and the
card remain rigid inside the case with the pilot viewing the
card from the back. This creates the possibility the pilot might
start a turn in the wrong direction similar to using a magnetic
compass. A knob on the front of the instrument, below the
dial, can be pushed in to engage the gimbals. This locks the
gimbals allowing the pilot to rotate the gyro and card until
the number opposite the lubber line agrees with the magnetic
compass. When the knob is pulled out, the gyro remains rigid
and the aircraft is free to turn around the card.

Directional gyros are almost all air-driven by evacuating
the case and allowing filtered air to how into the case and
out through a nozzle, blowing against buckets cut in the
periphery of the wheel. The Earth constantly rotates at 15°
per hour while the gyro is maintaining a position relative
to space, thus causing an apparent drift in the displayed
heading of 15º per hour. When using these instruments, it
is standard practice to compare the heading indicated on the
directional gyro with the magnetic compass at least every 15
minutes and to reset the heading as necessary to agree with
the magnetic compass.