Directional Gyro - Air Driven/Electric; Air Driven D.g.; Electrically Drive Directional Gyro; Directional Gyro Troubleshooting - Piper Navajo Chieftain PA-31-350 Service Manual

NAVAJO CHIEFTAIN SERVICE MANUAL
10-25. DIRECTIONAL GYRO - AIR DRIVEN/ELECTRIC.
10-26. GENERAL.
Both air and electric Directional Gyros are used depending on the option package
installed. Both types are displacement type gyros with "FREE" rotors mounted in Gimbal Assemblies. It
is important that if a Magnetic Slaving System Flight Director, or AutoPilot is coupled to the D.G., the
A.F.C.S. manufacturer's Service Manual should be consulted.
10-26a. AIR DRIVEN D.G.: The air driven D.G. is driven by the pneumatic system, which is supplied
by engine driven dry pneumatic pumps either on pressure or vacuum. or from cooled and regulated bleed
air on Turbine engine installations. It is of prime importance to realize that 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. (THE GYRO AIR
FII TER M UST BE CLEAN OR REPLACED. BEFORE ADJUSTING GYRO AIR PRESSURE.) Airflow
directed at the gyro rotor vanes, caused the rotor to spin approximately 17.000 to 22.000 R.P.M.. thus.
providing the gyroscopic ability to remain rigid in space. The instrument case moves freely about the spinning
gyro rotor in three dimensions by the use of a Gimbal Assembly, and the displacement or Azimuth readings
are presented on the instrument face. This results in a positive and stable presentation.
Since the D.G. has no reference to Magnetic North. it must be set from the Magnetic Compass. The D.G.
will agree only with the Magnetic Heading from which it wasset, since all othersubsequent MagneticCompass
Headings are subject to deviation. Northerly Turning. acceleration, deceleration, dip and other errors. Due
to precession. inherent or apparent, 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 correct in relation to Earth's surface.
10-26b. ELECTRICALLY
DRIVEN DIRECTIONAL
GYRO - These gyros contain rotors which are
electrically 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.
10-27. 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 the instrument
installation and power (air electric) requirements. Typical installation examples of gyro instrument mal-
functions are due to installation system problems such as: 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 .5 P.S.I.G.) Excessive
precession is a common complaint and usually results from installation problems such as described above, or
can be the result of Pilot operating error. While D.G. precession can only be exactly measured 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 10 minutes if (4) 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, otherwise. the deviation of the Magnetic Compass Heading can be read mistakenly
as precession. Finally. only after abnormal precession has been confirmed and the system installation proven
good. should the instrument be "pulled" for replacement or repair.
Revised: 10/15/82
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INSTRUMENTS
3B8