In article ,
"RST Engineering" wrote:

Mumetal shields are a patch for the real problem ... which is that you are
carrying a magnet around with you that is sucking the compass towards it on
all headings. Solve the problem; don't patch it.

Don't forget that people cary magnets into the plane with them too. Steel
tools. Electronic gizmos. Wris****ches. Not to mention sunglasses:

http://www.framesdirect.com/magnetic...r-gst-lgp.html

The solution to that is a fluxgate out on the wingtip, but that's a few
more AMU's than most people are willing to spend.