A recent compass swing on our plane has turned up some opinions about
magnetic compasses that are surprising to me..

A club member has asked me why we spent money to have a 14 degree error
removed from our compass since it is just a back up instrument if everything
else quits. He just sets the DG to the runway heading on takeoff and uses
that. A couple of 360 in our 172 to look at something on the ground will
put our DG 15 -20 degrees off and it drifts about that much each hour. That
doesn't seem to concern him.

An A&P I asked in another forum said he hopes his customers don't expect him
to get the compass closer than about 10 degrees. Our shop says 10 degrees
is what is allowed.

I used to do a lot of sailing and a degree or two in a compass is a big deal
to me. Even though I can do direct to with the Loran or GPS, I like to be
able to start out in the right direction. If I'm looking for an airport or
landmark, knowing pretty accurately where the aircraft is pointed helps. If
everything else quits, I'd really like to know where the plane is pointed
while I try to find a place to land.

I agree that the compass is pretty fuzzy in an airplane. By the time you
get it to settle down, set the DG, and add in the difficulty of figuring out
exactly where the axis of the airplane is, 10 degrees may be the best you
can do. However, my experience with both navigation and engineering tells
me that it's still worth being precise where you can. If you accept a 10
degree error in the compass itself and then add the 10 degrees of other
factors, you could be up to 20 degrees. That seems like a lot to me.

Am I being overly compulsive about this? I know that everything the
magnetic compass tells you has to be verified with all other available
information but it is still the primary source of direction information in a
simple aircraft like our 172.

Are these casual attitudes towards compass accuracy I'm encountering common?


--
Roger Long