On Saturday, March 22, 2014 5:14:57 PM UTC-6, Greg Delp wrote:
How about inside a bumpy Cumulus cloud?

The idea of using the magnetic compass "Northerly Turning Error" as an emergency letdown procedure has appeared in instrument flying books for the last 75 years - accompanied by the statement that it probably wouldn't work in a real emergency. It's used to help students understand one of the many compass errors.

A wet compass itself doesn't really work well in bumpy air. As one poster wrote, magnetic compasses are probably the most useless instrument found in gliders. GPS ground track is much more useful - and accurate.

Flying slow light airplanes under instrument rules, I found controllers aren't always well versed in the difference between track and heading. With fast jets, there isn't much difference. I found the controllers were much happier when I flew a GPS ground track instead of the "heading" in my clearance.