How to fix Aircraft Compass Errors

Hello John

I am pulling my hair out with my compass problems.  I built a new panel for my biplane and put a vertical card compass in.  When I took the plane outside to run it, I noticed I was pointed West and the compass said North.. I borrowed an AC hoop that a friend of mine made.  It did not work.  I bought some special 80% nickel material to shield the magnetized area.. That did not work..

What I see is lacking in your efforts is precisely locating the source of the offending magnetic field and being able to measure its strength. Otherwise its like trying to find your shoes in the dark. A simple field indicator can be purchased from Aircraft Tool Company. 

Steps:

1. Eliminate any artificial (non-earth) magnetic fields affecting the compass using a field indicator and degausser.

2. Use compensator magnets to correct for distortion of the earth's field from nearby iron based structure.

Nearby Iron

It appears from your picture that you have iron structure in close proximity to your compass. Since the permeability of your structure is >air, the earth's magnetic field is going to be sucked into the iron-based structure, travel through it and exit at some point with its strength and direction slightly altered. This is what the compass's compensating magnets are there to do -- alter the field back to what it should be. You can't degauss this as the structure is always being re-magnetized by the earth. You can't shield (I hate that term - bend is more descriptive) the compass from the earth's field and expect the compass to still react to it. 

Field Indicator

Magnetic Field Indicator

This type of indicator works well because it is not sensitive enough to detect the earth's field but is sensitive enough to detect anything stronger -- what you want to degauss. It is used to both detect artificial fields and to see if your degaussing is effective.

Degausser

Degausser is available for rental

The degausser has no effect on natural magnetic fields radiating from close-by iron parts It's also possible that the tac cable where it enters the tac is magnetized. If this cable end is within 3-4 inches of the compass then it will pull on the compass. A Bendix or King VOR head that is within 3-4 inches will also pull on the compass. 

Compensators

Airpath Compass Compensator

Compass Compensator Neutral setting (no compensation) when dots (shown in green boxes) are in alignment.

If the compensator magnets are not up to the job then you have two choices: move the compass away from the iron structure or correct the earth's field by bending it back into shape. This is the method they use in ships where the compass is surrounded by iron. Usually, an expert is hired who from knowledge and experience knows where and how much magic to apply. 

N-S Compensator

Corrects C type errors in horizontal component of permanent magnetic field. Permanent magnet is located to the side of the compass. Adjusting the compensator  moves the compensator magnets to one side or the other to correct.

C type errors reverse direction every 180 degrees and form sine or cosine curves. Deviations will vary with the magnetic latitude so re-adjustment is necessary if there are significant changes in the latitude. Also corrects for fields produced by vertically induced magnetism.

E-W Compensator

Corrects for B type errors in horzontal component of permanent magnetic field. Permanent magnet is located in front or behind the compass. Adjusting the compensator moves the compensator magnets forward or back to correct.

B type errors reverse direction every 180 degrees and form sine or cosine curves. Deviations will vary with the magnetic latitude so re-adjustment is necessary if there are significant changes in the latitude. Also corrects for fields produced by vertically induced magnetism.

Compensator Limitations

If the compensator magnets are not up to the job then you have two choices: move the compass away from the iron structure or correct the earth's field by bending it back into shape. This is the method they use in ships where the compass is surrounded by iron. Usually, an expert is hired who from knowledge and experience knows where and how much magic to apply. 

Extra Credit

How does a degausser work

Hysteresis Curve

The degaussser like the one shown in this article produces a magnetic-field that alternates between N and S 60 times a second.  The hard iron, when placed in this field, is first magnetized in one direction, and then another 120 times a second. However, the iron resists this change. This resistance is called "hysteresis.

When we pull the degausser away from the hard iron we are reducing (decaying) the magnetic field as the distance increases.  By decaying the field the hysteresis curve becomes smaller and smaller (less magnetism). By the time we have pulled the degausser to arm's length, the part should be degaussed. We need not pull the degausser too far from the part because for every doubling of the separation distance, the field strength reduces to 1/4 then to 1/16 ..." At the end of the cycle, the magnetic domains orientation becomes random. The magnetic fields are always present in any object. It's just that you mixed up all the magnetic domains so that they have random directions and all their little fields cancel.

For AC magnetic-degaussing to work the initial magnetic field intensity generated by the degausser has to be greater than the part you are demagnetizing. The larger the part or the stronger the magnetism, the stronger your degaussiing tool needs to be. Actual demagnetization occurs only during the time the degausser is being separated from the part. Leaving the degausser in the degaussing field only heats up the degausser and the steel part. There must be no interruption in power during the separation or the incremental reduction will be stopped and the part will be left magnetized. If a power interruption occurs then go back and repeat the process. Complete demagnetization will not occur if the part is still subject to an external (bias) magnetic field. The earth's magnetic field will still be present in the part! 

In most of the USA the vertical component of the earth's magnetic field is stronger than the horizontal component. The compass only responds to the weaker horizontal component. The ends of vertical steel or iron parts will have strong magnetic poles and affect the compass if close-by.