AIR DRIVEN DIRECTIONAL GYRO
The air driven D.G. is driven by the engine driven dry-pneumatic pumps either on pressure or vacuum. Air
volume, and not air pressure, spins the gyro rotor. The air filter can become contaminated and restrict airflow,
reducing gyro rotor speed, while the pressure regulator will automatically adjust air pressure within proper
limits.
—CAUTION—
The gyro air filter must be clean or replaced before adjusting
gyro air pressure.
Since the D.G. has no reference to Magnetic North, it must be set from the magnetic compass. Due to
precession,the D.G. must be caged at least every 15 minutes while in a level attitude, even though drift may
not appear, to ensure rotor position is corrcct in relation to Earth's surface.
ELECTRICALLY DRIVEN DIRECTIONAL GYRO
These gyros contain rotors which are eclectically driven, with the gyro rotor acting as the armature of an
electric motor. To eliminate the friction of brush assemblies, which would induce abnormal precession, the
rotor/armature is inductively excited. The electric D.G. is subject to the same operational requirements of the
air driven D.G., except for the method of obtaining rotor rotation and the design of the erection mechanism.
TROUBLESHOOTING
Unless an obvious malfunction of the instrument (such as constantly spinning dial) requires repair or
replacement of the directional gyro, service is restricted to instrument installation and power (air/electric)
requirements. Typical installation problems include: restricted airflow due to air line kinks or leaks,
contaminated air filters, deteriorating electrical grounds, sagging instrument panel shock mounts, systems
regulators, faulty vacuum/pressure gauges, etc. (air pressure must be 5.5 plus or minus 0.5 PSIG). Excessive
precession is a common complaint and usually results from installation problems as described above, or can be
result of pilot operating error.
While D.G. precession can be exactly measured only under closely controlled conditions in an approved
gyro overhaul facility, any complaint of abnormal precession should be initially confirmed on the compass
rose. (Normal precession of the D.G. is plus or minus 3° in 10 minutes, or plus or minus 4° in l 0 minutes if
four cardinal headings are used and the total precession does not exceed 12°.)
When confirming precession complaints on the compass rose, aircraft position must be established by nose
wheel alignment with the compass rose lines. Under no conditions should the magnetic compass be used for
comparison. Only after abnormal precession has been confirmed and the system installation proven good,
should the instrument be pulled for replacement or repair.
34 - 15 - 00
Page 34 - 10
Reissued: August 1, 1986
2D18
PIPER AIRCRAFT
PA-28-236
MAINTENANCE MANUAL
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