How can a compass suddenly go out by 20 degrees?

Thanks for all the suggestions. I went out today and looked at the plane
myself (I had only heard about this via a couple of my partners. Turns out
that someone had riveted a compensation magnet to the windshield post behind
the compass, with a really crumby bracket made from very thin aluminum
sheet. Somehow this had been bent a few tenths out of line, probably by
someone parking their headset on the glare shield. It was obvious that it
was out of position, because you could see where it had originally been by
paint marks. When I pushed it back to near the original position, the
compass swung back to near the correct reading. Unfortunately, now that the
bracket is bent, it will not go back to the original position without being
removed and straightened. Since I don't want to drill out the rivet in 25
degree weather, I duct taped it back into position and will fix it properly
when spring finally gets here. Actually, I think I'll try and swing the
compass without it first, and see if it's really necessary at all.

Next question, can a non-A&P owner swing a compass, or must I have a real
mechanic do it?

BTW, the GPS antenna and cable has no effect on the compass at all, even
when right next to it with the GPS on. I knew the antenna was OK, because
the compass problem was reported before I changed to the glareshield-mounted
AirMap antenna.

Bob (Chief Pilot, White Knuckle Airways)

"Bob Chilcoat" wrote in message
...
Recently one of my partners reported that the compass (standard whiskey
compass, not a vertical card) on our Archer was reading 20-25 degrees off
of
runway heading at departure. Today a new partner was up with an
instructor
for his sign off and reported the same thing. I hadn't noticed, but we
seem
to have a definite problem. Nothing has changed in the plane for a long
time. We recently replaced the old Garmin GPS with a Lowrance AirMap 300
(which has its antenna on the top of the glare shield near the compass),
but
the first incident was before that substitution was made. Any idea how
something like this can happen?

--
Bob (Chief Pilot, White Knuckle Airways)