Bob,

This happens with precision when I turn on the airconditioning in my
Bonanza. (It's described in the POH.) While I realize you're probably
not running AC in the winter, it does show how equipment can affect
the compass. Have you noticed a difference with and without the master
turned on?

Rob


Mark Mallory wrote in message ...
Bob Chilcoat wrote:

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?


I had a problem with the Airpath compass in my '182, which at first seemed very
similar to what you describe. Turns out it wasn't a magnetic-field-related
problem; rather, the compass was simply "sticking". The compass has a flexible
diaphragm at the rear of the housing to allow for fluid expansion; the diaphragm
is separated from the main volume of the housing by a thin (non-magnetic) metal
plate. Apparently, pressure changes had caused the diaphragm to force the plate
forward, such that it would occasionally come in contact with the compass card,
causing the card to stick. The sticking would only happen on certain headings
with certain pitch attitudes; it worked fine most of the time.

I replaced the original compass with the same model Airpath, but after about two
years the replacement compass developed the same problem! I finally replaced IT
with one of these:

http://www.vargaair.com/navigator.htm

which works great (looks cool at night too.)

Mark/C182L