I am a student pursuing CPL/MEIR at a flight school in Turkey that's equivalent of a part 141 in the US. Our instrument equipment check procedures for IFR flight contains comparing the compass heading with the heading indicator's heading. We have no six pack aircraft(G500 DV20'sand G1000 DA40NG's and DA42NG's) so we compare PFD heading with the compass heading and it is said that the difference must be +-6 degrees for IFR and +-10 degrees for VFR. I can't find the relevant regulation. Can somebody explain or at least point me in the right direction? Thank you so much. Fly safe.
The Earth constantly rotates at 15° per hour while the gyro is maintaining a position relative to space, thus causing an apparent drift in the displayed heading of 15° per hour. When using these instruments, it is standard practice to compare the heading indicated on the directional gyro with the magnetic compass at least every 15 minutes and to reset the heading as necessary to agree with the magnetic compass.
-- Instrument Flying Handbook FAA-H-8083-15B
The heading indicator should be checked at least every 15 minutes, and reset to the correct magnetic heading by reference to the magnetic compass (and compass correction card) during steady straight-and-level flight. For the heading indicator to be acceptable in normal operations, this correction should not exceed 3° in 15 minutes.
The Pilot’s Manual 3: Instrument Flying by Trevor Thom
The Pilot's Manual is not an official document, it is more along the lines of advice.
It is inevitable for the compass to differ from HI, the limit for 6 deg for IFR and 10 deg for VFR might be relative to a fix.(VOR, NDB)