Dave Butler wrote:

Because the mag compass deviation (is that the right word?) is generally
different on different headings, so as you keep changing headings, you keep
resetting your perfectly good HI to match it, all the while muttering about how
your HI is precessing.

One way to test this is to fly one heading for a while, setting the DG to the
magnetic compass. Then change headings. Change headings as often as needed to
duplicate the "precession", but never reset the DG. Now return to the original
heading. If the DG now closely matches the magnetic compass, the compass needs
swinging. If not, the DG is drifting.

George Patterson
Drink up, Socrates -- it's all-natural.