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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. After either of these limits was exceeded,
the gyro housing contacted the gimbals, applying such a
precessing force that the gyro tumbled. Because of 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-erecting 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, but 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 be 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 180° 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, but 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 flow into the case and
out through a nozzle, blowing against buckets cut in the
3-20
Figure 3-31. The heading indicator is not north seeking, but must
be set periodically (about every 15 minutes) to agree with the
magnetic compass.
Figure 3-32. Precession causes a force applied to a spinning
wheel to be felt 90° from the point of application in the direction
of rotation.
Figure 3-33. Turn-and-Slip Indicator.
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 15° 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.
Heading indicators like the one in Figure 3-31 work on the
same principle as the older horizontal card indicators, except
that the gyro drives a vertical dial that looks much like the
dial of a vertical card magnetic compass. The heading of the
aircraft is shown against the nose of the symbolic aircraft on
the instrument glass, which serves as the lubber line. A knob
in the front of the instrument may be pushed in and turned
to rotate the gyro and dial. The knob is spring loaded so it
disengages from the gimbals as soon as it is released. This
instrument should be checked about every 15 minutes to see
if it agrees with the magnetic compass.
Turn Indicators
Attitude and heading indicators function on the principle
of rigidity, but rate instruments such as the turn-andslip
　
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