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dip and other errors. Due to precession, inherent or apparent, the directional gyro 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.
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 directional gyro is subject to the same operational requirements of the air driven directional gyro, except for the method of obtaining rotor rotation and the design of the erection mechanism.
TROUBLESHOOTING - DIRECTIONAL GYRO.
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 malfunctions 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, system regulators, faulty vacuum/pressure gauges, etc.
— Note —
Air pressure must be 4.5 - 5.2 hg.
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 directional gyro 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.
— Note —
Normal precession of the directional gyro is ±3° in 10 minutes or ±4° in 10 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. 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.
DIRECTIONAL GYRO - REMOVAL AND REPLACEMENT.
Refer to Chapter 39, Removal and Replacement of Face Mounted Instruments.
34-20-00 Page 34-15 Issued: April 1, 1997
AIR DRIVEN GYROS - FITTING INSTALLATION PROCEDURE.
The following procedure applies to aircraft equipped with Edo-Aire air driven gyro instruments. The use of tenon tape on fitting threads is recommended.
— CAUTION —
PERMIT NO OIL, GREASE, PIPE COMPOUND OR ANY FOREIGN M ATERIAL TO ENTER PA RTS PRIOR TO INSTA L L ATION OF FITTINGS. MAKE CERTAIN ALL AIR LINES ARE CLEAN AND FREE OF FOREIGN PA RTICLES AND/OR RESIDUE BEFORE CONNECTING LINES TO GYRO.
— Note —
Use of thread lube on fittings or in parts will create a “warranty void” condition.
1.
Carefully lay teflon tape on the threads allowing one thread to be visible from the end of the fitting. Hold in place and wrap in the direction of the threads so tape will remain tight when fitting is installed.
2.
Apply sufficient tension while winding to assure that tape forms into thread grooves. One full wrap plus 1/2 inch overlap is sufficient.
3.
After wrap is completed, maintain tension and tear tape by pulling in direction of wrap. The resulting ragged end is the key to the tape staying in place.
4.
Press the tape well into the threads.
5.
Screw fitting into port being careful not to exceed torque requirements as noted on decal on gyro cover.
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34-20-00 Page 34-16 Issued: April 1, 1997
MAGNETIC COMPASS.
The magnetic compass is a self-contained instrument. This instrument has an individual light which is connected to the instrument lighting circuit. The compass correction card is located in the card holder mounted on the instrument. The compass should be swung whenever instruments or radios are changed and at least once a year.
ADJUSTMENT OF COMPASS.
Before attempting to compensate compass, every effort should be made to place the aircraft in simulated flight conditions; check to see that the doors are closed, flaps in retracted position, engines running, throttles set at cruise position and aircraft in level flight attitude. Aircraft master switch, alternators, radios (except for combustion heater) should be on. All other cockpit controlled electrical switches should be in the off position.
1.
Set adjustment screws of compensator on zero. Zero position of adjusting screws is when the dot of the screw is lined up with the dot of the frame.
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