
10-31. DIRECTIONAL GYRO.
10-32 GENERAL. 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 simular 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
on the heading on which it was first set also 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.
10-33. TROUBLESHOOTING.
TABLE X-II. DIRECTIONAL GYRO INDICATOR
Trouble
Cause
Remedy
Excess drift in either
Setting error.
Reset per
direction.
Defective instrument
Replace instrument.
High or low vacuum.
If vacuum is not correct
check for the following:
a. Relief valve
a. Adjust.
properly adjusted.
b. Incorrect gauge
b. Replace gauge.
reading.
c. Pump failure.
c. Repair or replace.
d. Vacuum line kinked
d. Check and repair.
or leaking.
Check for collapsed
inner wall of hose.
Dial spins during turn
Limits (55 ° bank) of
Recage gyro in level
gimbal exceeded.
flight.
Dial spins continuously
Defective mechanism.
Replace.
PIPER AIRCRAFT
PA-28-161 CADET
AIRPLANE MAINTENANCE MANUAL
Issued: FEBRUARY. 28, 1989
I N S T R U M E N T
2B21