Hi all !

What is the reason for the need to regularly adjust the heading card in a DG?
a) Apparent precession
b) Earth transport wander precession
c) Real precession

It must be (c) real precession.
Real precession is the change in the gyro axis direction caused by real forces acting on the gyro, like acceleration/deceleration and turning of the aircraft. Apparent precession is the apparent change in the gyro axis caused by the rotation of the earth: even if the gyro is holding its axis direction perfectly in 3D space, the gyro appears to be precessing because the earth is rotating (the direction of "up" is changing). Transport precession is the same thing when you fly from one spot on earth to another spot. Apparent precession is about 15 degrees per hour maximum; transport precession typically slower than that. So the reason must be real precession of the directional gyro, which happens a little bit every time you bank. Anyway a DG at the equator, correctly aligned with north, doesn't experience apparent precession or transport precession.

Thanks a lot for your fantastic reply! I believe it's the answer CASA want.

PS: in Bob's AGK book this is described under 'drift' ( 'real drift', 'apparent drift' etc.)