Dale wrote:
In article 1111697079.782652@sj-nntpcache-3, Dave Butler
wrote:

Anyway, my main point is that a magnetic compass in need of swinging can look
like HI precession.

Hmmm, hadn't thought of that. Was thinking a compass needing swung
would indicate a "fixed" heading...can't see why it would like like the
DG was precessing.

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.

A real quick way to check the DG is to fly a ground track using your
GPS, set the DG to the ground track...see how it does..

That should work, given a constant wind, I guess. I'd think you'd want to vary
your heading over the duration of the test, though, in case the error only
occurs during heading changes. I'd think that excessive friction in the gyro
bearings might cause an error during turning that doesn't occur during straight
flight, but maybe not.