CHART 3404. TROUBLESHOOTING (GYRO HORIZON INDICATOR) (cont)
Trouble Cause Remedy
Bar oscillates or shimmies Instrument loose in panel. Tighten mounting screws.
continuously.
Vacuum too high. Adjust valve.
Defective mechanism. Replace instrument.
Instrument does not indi- Instrument not level in Loosen screws and level
cate level flight. panel. instrument.
Aircraft out of trim. Trim aircraft.
Bar high after 180° Normal, if it does not
turn. exceed 1/16 inch.
Instrument tumbles in Low vacuum. Reset regulator.
flight.
Dirty filter. Clean or replace filter.
Line to filter restricted. Replace line.
Plug missing or loose in Replace or tighten plug.
instrument.
DIRECTIONAL GYRO.
The directional gyro is a flight instrument incorporating an air driven gyro stabilized in the vertical plane.
The gyro is rotated at high speed by lowering the pressure in the air tight case and simultaneously allowing
atmospheric air pressure to enter the instrument against the gyro buckets. Due to gyroscopic inertia, the spin
axis continues to point in the same direction even though the aircraft yaws to the right or left. This relative
motion between the gyro and the instrument case is shown on the instrument dial which is similar to a compass
card. The dial, when set to agree with the airplane magnetic compass, provides a positive indication free from
swing and turning error. However, the directional gyro has no sense of direction and must be set to the
magnetic compass, since the magnetic compass is subject to errors due to magnetic fields, electric instruments,
etc. The directional gyro is only accurate for the heading it has been set for. If the gyro is set on 270°, for
instance, and the aircraft is turned to some other heading, there can be a large error between the gyro and the
magnetic compass due to the error in compass compensation. This will appear as gyro precession. The gyro
should only be checked to the heading on which it was first set. Due to internal friction, spin axis error, air
turbulence and airflow, the gyro should be set at least every 15 minutes for accurate operation, whether it has
drifted or not.
34-10-05
Page 34-09
December 1, 1978
2G17
PIPER AIRCRAFT
PA-28RT-201 / 201T
MAINTENANCE MANUAL
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