since 1940

Aircraft Compass Problems? Compass Degauss 

 Degauss Instructions

If you haven’t yet rented my Degaussing tool then be sure to read the section titled: Making your own Magnetic Field Meter.

It appeared to me a few years ago that there wasn't any good system of degaussing aircraft to get the compass working as it should. I came up with this system, not to make money but as a challenge, and since it seemed like there ought to be something better. The theory of this system is that it is not necessary to degauss the entire aircraft as magnetic fields don't travel far. You need only to eliminate the stray fields that act on the compass. To do this you need a simple, effective method of detecting these stray fields.

This system has been used on approximately 200 aircraft with good results over a period of 15 years. However, each case is unique and this may not work in all cases. If it doesn't work in your case, I apologize. I'm open to suggestions on how to improve the system. If you aren't satisfied with the degausser, then please let me know so that I can refund you the rental. As in life - there are no other warranties expressed or implied.

The compass is a very sensitive instrument. It reacts to a magnetic field that is approximately 1,000 times weaker than a small permanent magnet and 10,000 times weaker than an inexpensive ferrite magnet (the horizontal component of the earth's magnetic field is approximately .45 Gauss.) It is no wonder that even a small amount of magnetism around the compass can throw it off.

Your aircraft is made of permanent magnatism, induced magnetism (earth's field magnatism), and transient magnatism from the electrical currents flowing thru the aircraft's structure (return current from the starter travels not thru a wire but thru the airframe structure). All three affect the compass. By degaussing we are trying to reduce only the permanent magnatism. Compass problems may still exist.

What NOT to degauss

The first thing you should know is that this degausser doesn't know what NOT to degauss. This is up to you. The degaussing field extends outward from the probes; anything within the field will be demagnetized (or magnetized) to some extent. Magnetic fields don’t radiate far and the articulated legs of the degausser allows you to direct the field. Probably the biggest danger is accidentally activating the unit when it is touching something sensitive. Don’t worry about the magneto pole shoes that act as keepers.

Remove from the area (5 feet) that what you don't want exposed to a magnetic field. This includes avionics, instruments, and tachometers, and the compass. The magnetic field doesn’t radiate far when the degausser is pressed against the metal part; however, when you pull the degausser away from the part, while activated, the field radiates five feet or so. I have no idea how sensitive avionics are to spurious magnetic fields so the best thing to do is to be extra careful.

If you double the distance from a magnet the strength drops by a factor of 8 so you might be able to move the compass to a different spot. 

What to Expect

Sorry to say that in many cases it is not possible to completely rid a steel structure of stray magnetic fields (or keep them from returning). In the compass world this is called "permanent magnetism. Your goal is not to completely demagnetize the  structure but to reduce the stray magnetic field as much as possible. The magnetic compass must then be corrected. Here are the steps:

1. map out the "hot" spots with the meter
2. use the degausser to reduce as much as possible the stray magnetic field
3. check with the meter
4. correct the compass using permanent magnet correctors

Many times just doing steps 1 thru 3 will solve your compass problem. But, if you own a Bellanca for example,  you are probably going to need to do all 4 steps. This article and my degausser take care of steps 1 thru 3 so it is often not a "final" solution. If you need to proceed to step 4 then the "bible" on compass compensation is The Handbook of Magnetic Compass Adjustment published by the NATIONAL GEOSPATIAL-INTELLIGENCE AGENCY

Here’s how its done - using the Magnetometer

The secret of demagnetizing something is having a magnetic field meter that allows you to seek out the hot spots and to measure your progress. Sweep the area around the effected area with the meter to find hot spots. The more the meter needle moves, the more magnetized the object is. The sense point for the meter is on the lower side where the arrow points. Run this side along the metal objects in question; it doesn't matter which way the needle moves. When you find an area that moves the needle (hot spot), you can demagnetize it with the degausser.

 

Find the Field using meter	Degauss	Check for field

Here’s how its done - using the degausser

The articulated legs of the degausser consist of laminated transformer iron and are designed to move inward or outward to fit the part being degaussed. These legs direct the magnetic field to their ends. When the ends are placed on or near the object you wish to degauss, the field is concentrated at this point. The legs also reduce the amount of magnetic field radiating outward in other (undesirable) directions.

Orient the legs so that they make contact, or as close as you can get to the hot spots. Press the button and move the degausser away from the object. The speed at which you move the degausser away makes no difference. The degausser provides the alternating magnetic field; by moving the degausser away you make it decay. Push the button and move the degausser away to decay the field - this takes probably a second. You do not need to press the button for long periods of time.

Go back with the field indicator and check the field; it should be reduced or hopefully gone. Move on to another hot spot. Do not keep working the same spot in an attempt to get the field indicator needle to completely stop moving. Your goal is to reduce the field as much as possible - one or two attempts at one spot is all you need. Keep working in each area that might affect the compass. Magnetic fields attenuate with the square of the distance (they don't go far) so you need not demagnetize areas that are far removed from the compass.

Keep the field meter far away from the degausser unless you want to buy me a replacement. While were at it, it probably isn't a good idea to the degausser with pacemakers or other such body electronics.

How does it demagnetize?

Alternating current from the power supply sets up a magnetic field in the degausser that reverses itself sixty times a second. This causes a magnetic field in the object you are trying to demagnetize that also reverses sixty times a second.

To reverse the magnetic field, the field must pass through a point of zero magnetism sixty times a second. Now the magnetic domains don't like changing direction so fast so they act differently during the positive cycle than during the negative cycle; this is called hysteresis.

 

By decaying the field the hysteresis curve becomes smaller and smaller (less magnetism). Eventually, 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.

Is the compass incorrect because of something magnetic or because the earth’s magnetic field is being distorted?

Here’s why the meter supplied with the degausser is important:  With the compass you are trying to measure the earth's magnetic field and not the magnetic field of your aircraft. This meter is not sensitive enough to react to the earth’s magnetic field so any meter movement is the result of something being magnetized that shouldn't be. If there is no movement with the meter then your compass error may be caused by the earth’s magnetic field inducing a magnetic field in nearby iron objects. 

A magnetic field is the vectored sum of all the fields present at any point. This means that the earth’s field traveling one direction through air acts on the compass and; The earth’s field induces magnetism in a nearby iron structure that also acts on the compass. The sum of the two fields vectors is where the compass points. You cannot remove the earth field from the iron structure as you cannot degauss the earth. You’re choices are to move the compass, move the iron structure, or use compensating magnets to change the field’s vector sum.

I received a call where the user complained that the degausser wasn't working. Even though the field meter indicated "0 field" the compass was still reacting to the metal tubing close to the compass; no matter how many times they used the degausser, the tubing still affected the compass because the tubing will always have an induced magnetism from the earth’s magnetic field.

The earth's magnetic field travels from pole to pole usually through the air. When the field approaches a ferromagnetic (iron) object it finds it much easier to travel through the iron than through the air. The same thing happens if you attach a piece of iron to a magnet, you distort the magnet's field because iron has greater permeability than the air. You cannot use a degausser to rid the tubing of the earth’s magnetic field because the earth's magnetic field is always there traveling in the tubing and distorting the surrounding field.

 

Compass "frozen" or "locked" to a heading	Compass points to magnetized part. Degauss should be effective
Compass deviation changes with heading and not "frozen"	Earth's field distorted by nearby iron object. Degauss not effective.

 Magnetic Fields caused by electrical currents traveling through the airframe

Another problem you might encounter is external magnetic fields caused by using the aircraft’s structure for the current return path (ground) back to the battery. Your aircraft becomes a solenoid and acts as a bar magnet. Such currents, if strong enough, can magnetize the aircraft structure.  Modern Light-weight starters in 12 volt aircraft draw very high current loads. Be sure that you have a adequate (robust) grounding path back to the starter. 

For example, when you turn on the landing light, the current travels through the aircraft structure on its way back to the battery. Current flow radiates a magnetic field that may influence the compass. This type of problem is diagnosed by turning on electrical appliances and observing how the compass reacts. To fix this problem you need to move the compass or move the electrical path away from the compass.

It is interesting to note that there are automotive compasses that automatically compensate for localized distortions in the magnetic field. The “heart” of this system is a compass-on-a-chip that you can now purchase as a compass kit. One wonders if the 50 year old aircraft compass is due for an upgrade.

The degausser draws approximately six amperes on steel and seven amperes in air.

Making your own Magnetic Field Meter  

Before you rent my degausser you might want to check to see if your compass problem is the result of something magnetized close to the compass. The degausser will take care of this type of problem. The degausser will not fix problems caused by the earth’s magnetic field inducing magnetism in an object nor will it correct for magnetic fields created by current traveling through the aircraft structure.

The easiest way of finding out if something is magnetized is with the field indicator that is included with the kit. You can make a simple field indicator with 12 inches of thread and a metal paper clip that works almost as easy. Straighten the paper clip and tie the end of the thread onto the wire such that the wire hangs horizontal. Bring the hanging paper clip close to items you think might be magnetic. The wire will swing and dip toward the magnetic field and will either be repelled or want to stick to the object. I was able to detect the magnetic field in the electrical cord going to my toaster when I turned it on. Make sure you don’t accidentally magnetize the wire. If you happen to touch the wire to a magnet then it will pick up some of the magnetism and then react to any iron object.

Compass Swinging

Consult the aircraft maintenance manual for compass swinging and calibration instructions. Advisory Circular AC43-7 "Calibration of Compasses" also contains interesting compass swinging and calibration instructions

  

Degauss Information from  Air Force T.O. 33B-1-1 NONDESTRUCTIVE INSPECTION METHODS Section VII  Demagnetization February 1966	Comments by John Schwaner
Since alternating current does not penetrate very deeply below the surface of magnetic materials, some parts may be difficult to demagnetize completely...This is particularly true with large heavy parts, and may also be the case with parts of unusual shape. Demagnetization accomplished with direct current is usually the most complete and effective possible." page 2-60	Our degausser uses alternating current. Most engine shops use AC to demagnetize engine parts after Magnaflux Inspection (unfortunately). Technically this is Half-Wave or Alternating Current This type of equipment is inadequate and over time part of the magnetic field returns. A direct current demagnetization. Technically a phase full-wave rectified (DC) alternating current (whether produced by a battery bank or rectified from 220v) is the preferred method of damaging crankshafts after Magnaflux inspection. Another reason most standard aircraft engine shop AC Magnaflux machines are inadequate is  that DC Magnaflux  has the deepest possible penetration and must be used for inspection for defects below the surface when using the wet magnetic particle method. "Head Shot" methods of magnetization (required by both Lycoming and Continental) are more difficult to permanently demagnetize, especially with AC equipment.  Spot checking of steel engine parts over the years in our shop found almost all crankshafts at the accessory end  were magnetized.
Sometimes parts difficult to demagnetize can be effectively demagnetized by hammering the part during the demagnetizing operation. To use this technique the part is placed in the demagnetizing coil and the current is turned on. The part is then hammered and withdrawn from the coil field while the hammering is continued. Care must be taken that the part is not damaged by the hammering. page 2-60
Demagnetizing coils sometimes work better if they are positioned so that the part of the part as it is drawn through the coil is in an east-and-west direction rather than north-and-south. This is particularly true for long parts which may be influenced by the earth's magnetic field. page 2-60	If you park your steel tube aircraft in a north-and-south direction and you hit the starter -- will the magnetic field created by the return path of the current  going through your tubing be more likely to magnetize the tubing?
Sometimes the residual field can best be removed by a technique known as the transient method of demagnetization. To perform this technique the part is placed in the demagnetizing coil and the current turned on and off five to ten times. The current is then turned on and left on while the part is withdrawn from the magnetic field of the coil. page 2-61
..most aircraft parts can be satisfactorily demagnetized through the use of standard alternating current demagnetizers. page 2-61	except firewalls!

  Stealth magnetic fields

We’ve heard about stealth airplanes that are hard for radar systems to spot but did you know that ships and submarines use stealth technology to evade magnetometer detection. Most sea mines use disturbances in the nearby magnetic field caused by an approaching ship to detonate. Submarines can be detected by accurately measuring disturbances in the earth’s magnetic field that the submarine causes. The DEGS system uses computers and long current carrying wires along the ship to create an opposite magnetic field that effectively cancels any magnetic disturbances. Pretty interesting.

  Feedback of Interest

John,
Just for your information...we managed to get all the magnetism out of the steel engine mount using a large bench-top type stepdown coil. It was about 2 1/2' across the inside of this thing. it took them several hours as the magnetism was constantly moving. they literally "chased" it through the structure of this PA-28 engine mount till they cornered it in the lower end of the nose gear strut housing. They pushed it off with many repeated treatments....left it for an hour and did it some more.
Now all we have to do is get it out of the four steel forgings on the inside of the firewall where the mount attaches to the airframe. We hope that patience and perseverance will prevail and overcome or at least reduce the magnetism effect enough to not adversely affect the compass. Will let you know how it works.
Regards,
Gary

John Schwaner wrote:

I've found that our degausser will not demag a firewall although it should work on the engine mount but may take some work. You've outlined the problem with AC style Magnaflux machines - I have no solution to your problem.  John Schwaner
President
Sacramento Sky Ranch Inc.
www.sacskyranch.com
john@sacskyranch.com

Copyright 2003 by Sacramento Sky Ranch Inc. All rights reserved.  Prices subject to change without notice. Not responsible for typographical or misprint.
Disclaimer: sacskyranch.com contains abundant information relating to aircraft maintenance. The information provided  is not intended to supercede or supplement the F.A.A. approved  maintenance and/or operator’s manuals. Those F.A.A. approved manuals must be utilized when performing maintenance and/or operating aircraft.