Compass issues

[FBCK]

I had a PA 700 compass in my 231 and my old shop told me they took it out of the plane and worked on it but it was toast, and that I should replace it.  I purchased a new one a dn found a new shop for many reasons and when the new shop took the old one out and placed it nect to the new one they both read the same, telling me it was not the compass but something else.  I remember reading somewhere about the airframe can become magnetized, how do you demagnetize an airframe.

[Bob_Belville]

I have not seen anything about degaussing an airframe lately but I know we did it to my old E in the '80s. With our slaved magic these days the only time I look at the compass is to when I'm grabbing the support tube to move around the cabin! 

[Mooneymite]

Marauder told us about this:

http://www.aircraftspruce.com/search/search.php?s=COMPASS%20BALLS

Worked like magic for me. After years of a wet compass that could not be adjusted, this little gem allowed the compass to be dead on.

[N601RX]

If there is still any TV shops around in your area they will have a Deguass coil. Basically just back away slowly while waving the coil around in a circular motion. I think I've saw a Mooney service bulletin before that describes the process in detail.

I put a used pma7000 in several months ago and was able to get it pretty much dead on. I've found it better to adjust in calm air while flying. Be sure the mounting screws are all nonmagnetic. They also have a specially designed mounting bracket that limits the viberation that it sees. You can also send back for a overhaul for about 2/3 the price of a new one.

[BigTex]

When I purchased my plane the vertical card compass would spin around like it was possessed. I had a MSC degauss the airframe but it didn't make much difference. There's a Service Bulletin out that re-routes the main bus that was causing it to magnetize the tubing. I had the SB done then degaussed. I also had to install A SIRS compass. Now the compass works pretty well.

[MARZ]

Read something about not relying on the compass with the landing light on somewhere as well....

[Marauder]

The major cause of magnetism in our planes is aircraft power carrying wires running across the steel frame. Same effect like the experiment we did as kids wrapping a wire around a screwdriver and connecting it to a battery. You can also contribute by running electrical cords from devices like vacuum cleaners, portable heaters, etc. inside the airframe. If you have a vertical card compass, you can zero out most of the interference using the balancing balls Mooneymite provided a linkage for. He pointed out to me that large versions of these are used on ships! The other path to go is using a degaussing coil, but expect to see the problem again unless you fix the root cause.

[dcrogers11]

I had the same problem with my M20C.  I moved the compass up 3-4 inches and cured the problem.  You might try moving it by hand and see if that works.  My plane had sat in the same position in a hangar idle for 20 years prior to purchase and not moved.  We came up with the idea that the compass magnitized the area it was mounted on due to the long exposure without moving.

 

Don 

[DAVIDWH]

Looks like the "compass ball" trick would work well.

 

Solved my compass error problem by removing from glareshield and placing

 

the compass at the top of windshield. (first mounting with velcrow failed, second mount was a solid mount.)

 

Degaussing airframe, usually only temporary fix.

[FBCK]

I have the balls but I think the compass (both of them now) are so out of wack that they cant compensate for the field.

[Marauder]

I have the balls but I think the compass (both of them now) are so out of wack that they cant compensate for the field.

Do you see it change when you turn on the master and/or radio master? You may have a high current line running across the steel frame acting as a energizer coil. Also, what electronics do you have near it? Some portable antennae will impact it as will other electronics with high power requirements (like a Comm radio transmitting). I chased the demon for a couple of years with my old wet compass. I eventually moved wiring, degaussed the plane, installed the balancing balls with a vertical card compass and it works much better. I still need to swing it periodically, but it is not an excessive activity.

[takair]

See Mooney SI M20-95 and SB M20-150A. Both are available on their web site.

[John Pleisse]

Don't discount the back light wiring. Usually it is shielded, but can be chaffed or broken somehow allowing enough voltage exposed to mess with things.