Piper PA-28-161, Maintenance Manual

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PIPER AIRCRAFT, INC.
PA-28-161, WARRIOR III
MAINTENANCE MANUAL
PAGE 11
Nov 30/06 3H11
34-20-00
4. Directional Gyro
A. Description and Operation
The directional gyro (or heading indicator) 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 and be
directed 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 has a 360° face.
The directional gyro has no sense of direction and must be set to the magnetic compass. The card,
when set to agree with the airplane magnetic compass, provides a positive heading indication, free
from acceleration/deceleration and turning error. However, precession forces applied to the gyro
during turns may cause the gyro to “drift” and, upon completion of the turn, result in a difference in
readings between heading indicator and the magnetic compass, necessitating resetting the gyro.
Even while maintaining a given heading, the compass card tends to precess (drift) due to internal
friction, spin axis error, air turbulence, and airflow. Therefore, the directional gyro should be checked
against the magnetic compass (at least every 15 minutes), and reset as necessary. Maximum
acceptable precession is 5° in 15 minutes.
Some directional gyros are limited to 55° of roll and pitch. Should these limits be exceeded, the gyro
will “tumble”. This is evidenced by a rapid spinning of the compass card. The gyro in a properly
operating instrument can be erected, after returning to level flight, by caging the gyro and resetting it.
B. Troubleshooting
See Chart 2.
Service is restricted to instrument installation and power source. Obvious malfunctions, such as
failure to erect, or compass card spinning that cannot be corrected by manually caging the
instrument, require repair or replacement. Typical installation examples of directional gyro
malfunctions are due to restricted air flow from air line kinks or leaks, contaminated air filters,
deteriorating electrical grounds, sagging instrument panel shock mounts, systems regulators, faulty
vacuum/pressure gauges. (Air pressure must be 5.5 plus or minus 0.5 in. Hg). Replace or repair the
instrument only after the operating system has proven to be good.