The Comair crash reminds me...

Jim Macklin wrote:
The slaved compass is corrected so as many errors
(deviation) are corrected and variation is also corrected as
much as possible too, so the slaved compass may be reading
different from the whiskey compass unless you apply the
compass correction card FOR-Steer to get a more accurate
setting for the HI. But the whiskey compass is not supposed
to have any error greater than 10 on any heading. Often the
compass is swung with the electrical and radios running. It
seems that swinging the compass should be done with the
electrical system dead and the radios off, that is when I'd
want the most accurate magnetic compass.

Why do you want your compass to be most accurate when your airplane
isn't being used?

Matt