Piper CHEROKEE WARRIOR PA-28-151 1995 Maintenance Manual page 339

PIPER CHEROKEE WARRIOR SERVICE MANUAL
10-20. HEADING INDICATOR.
10-21. GENERAL.
The 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
i n e rtia, 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, on older
models, is similar to the magnetic compass card. On later models, and on all heading indicators used on the
Warrior III, the compass card has a 360° face.
The the heading indicator 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 heading indicator 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 heading indicators 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 operat-
ing instrument can be erected, after returning to level flight, by caging the gyro and resetting it.
10-22 TROUBLESHOOTING HEADING INDICATOR.
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 heading indicator m a lfunctions 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.
TABLE X-II TROUBLESHOOTING HEADING (DIRECTIONAL GYRO) INDICATOR
Trouble
Excess precession (drift) in
either direction.
Revised: February 28, 1995
Cause
Setting error.
Defective instrument.
High or low vacuum. If vacuum
is not correct, check for the
f o l l o wing:
a. Relief valve improperly
adjusted.
b. Incorrect gauge reading.
c. Pump failure.
d. Vacuum line kinked or
l e a king.
2C3
Island Enterprises
Remedy
Review paragraph 10-21 above.
Replace instrument.
a. Adjust.
b. Replace gauge.
c. Repair or replace.
d. Check and repair. Check for
collapsed inner wall of hoses.
INSTRUMENTS