Compass Maintenance and Degaussing Aircraft.
In essence, the compass is a piece of magnetized material hung so that it is free to turn and stay aligned with the earth's magnetic field.
Large masses of ferrous (iron) metals, such as the aircraft engine, tubular fuselage structure, or landing gear actuating handle, can disturb the earth's magnetic field causing the compass to give an erroneous reading. Small adjustable magnets, called compensators, allow much of this type of error to be adjusted out of a properly installed compass. However, if the airframe or landing gear handle or control column has become magnetized accidentally, the effect of this magnetic field on the compass may be greater than can be handled by the compensators. These large ferrous masses can be magnetized easily by accident.
TIPS ON COMPASS CARE:
Simple as the compass is, certain precautions must be observed if the full benefit of this vital instrument is to be realised.
DON'T use electric motors (AC or battery powered DC) such as buffers, drills, or vacuum cleaners in or around the cockpit. These motors generate magnetic fields that may induce magnetism into the aircraft structure.
DON'T route wires carrying high currents along with, or parallel to, tubes of the fuselage structure. Improperly routed wiring can induce large magnetic fields in the airframe.
DON'T use ferrous metal tools to adjust the compass compensators. A common screwdriver or knife will change the fields created by the compensators. A brass screwdriver is suggested.
DON'T drape any kind of AC power cord over any part of the aircraft structure.
DO swing the compass periodically. Replacement, addition, and/or removal of the engine, gear handle, gear motor, radio, navigational aids, or electronic autopilot also requires that the compass be swung.
DO place the aircraft on a compass rose when performing the compass swing.
DO be cautious when using electric powered service units such as engine heaters, oil immersion heaters, or battery chargers in or around the cockpit. These units may also induce magnetism into the structure, especially if their power cords are in contact or closes proximity to the airframe.
DO use a magnetic field indicator to locate areas of the airframe that need demagnetizing.
DO use only the proper tools and techniques in demagnetizing the airframe. Instructions for demagnetizing are found in other sections of this info.
DO check carefully the effects on the compass of recent optional equipment installations, recent wire routing changes, relocation, modification, or removal of any equipment using electricity; or the relocation, modification, installation, or removal of any large ferrous metal parts.
SWINGING THE COMPASS:
The procedure for swinging the compass is as follows:
1. Set the engine speed at 1800 to 2000 RPM.
2. Turn on rotating beacon and all radio equipment.
3. Position aircraft on compass rose and adjust N-S compensator screw on aircraft compass for 0 degree indication on north heading (use non-ferrous tools).
4. Taxi aircraft to east heading and adjust compass E-W compensator screw for 90 degree indication.
5. With aircraft on south heading,
adjust compensator to reduce indicated error by 2 degrees (if compass reads 184
degrees, adjust so it reads 182 degrees.
With aircraft on west heading, adjust compensator to reduce indicated error by 50%.
6. Return aircraft to north heading, reset directional gyro and, using DG check and record on compass Deviation Card the compass error every 30 degrees.
NOTE: During a compass swing, if any error greater than 20degrees (with compensator screws centred or compensators removed) persist, the cockpit area should he demagnetised. If this does not correct the error or if movement of controls cause compass deviation the whole aircraft should be demagnetised.
Demagnetising or degaussing can be done by following the method described in this instruction. The procedures used and the amounts of demagnetizing needed vary from aircraft to aircraft, depending on the location and the concentration of the magnetism. For this reason, all degaussing should be done by Wingweigh Aircraft Services experienced in performing such work. This method employs a modulating magnetic field generating device known as a degaussing instrument or "growler". Light duty equipment, such as tape recorder head demagnetizers as found on eBay are ineffective because of the weak magnetic field they produce.
CAUTION: Improper Use of the growler can induce more magnetism and cause extensive demagnetizing requirements.
NOTE: Remove compass from aircraft before trying to demagnetize cockpit.
1. Position aircraft at heading of East/West. i.e. 90 degrees or 270 degrees.
2. Make sure growler is turned off. Place growler in cockpit, position seats aft and support power cord so that it does not touch or pass within six inches of any structural member.
3. Hold growler inside cockpit in a low position. Turn growler on and slowly move it close to landing gear handle, up toward and along windshield centre post, and along overhead structure. Move growler in a circular motion down from behind shoulders to waist or mid-chest height before turning off.
4. Hold compass in place to see if headings now match. If too much heading error (more than 15 degrees) still exists, remove compass from aircraft and repeat.
5. Turn on growler and move it along centre cross structure from one side of cockpit, overhead, and down the other side.
6. Again hold compass in place and compare headings. If heading error has not been reduced sufficiently to less than 15 degrees, repeat all steps until heading error is less than 5 degrees.
When heading error has been reduced to less than 15 degrees, install compass, swing on compass rose using procedure described in this service instruction.
It is recommended that before adjustment begins put the compensator screws in centred position or compensators removed, depending on type of compass installed.
If compass will not compensate to within 10 degrees on any heading, it should be replaced.