Mag Compass


Well-Known Member
I was debating with my instructor how off your mag compass could be. Such as lining up on the runway and seeing it deflected...can anyone answer this question for me???


Gone West
swing freely and indicate known headings.....

I would have it checked if it indicated anything more than a few degrees

Definitely would not go IMC

Edit: The only place I remember seeing an actual number of degrees is the Instrument Flying Handbook
Heading indicator—before takeoff, recheck the heading indicator. If your magnetic compass and deviation card are accurate, the heading indicator should show the known taxiway or runway direction when the airplane is aligned with them (within 5°).
That is referring to the heading indicator being within 5, but it also assumes your mag compass is correct.


Well-Known Member
Roger that, on checking the deviation card. You may also want to check under what ‘condition’ the deviation card was verified. Were the strobes, gps, radios, etc. on at the time of the check. The mag compass is obviously a very sensitive unit that can be upset by the slightest change in electrical charges. A new starter in the plane can even disrupt the compass’ accuracy.