As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
(CAN) DURING AN ICE PATROL FLIGHT AND FOLLOWING FMS 45 (FLIGHT MANUAL SUPPLEMENT 45 - AREA OF MAGNETIC DISTURBANCE OPERATION WITH ADAHRS), DURING STRAIGHT AND LEVEL FLIGHT, ONLY FOUR MINUTES AFTER CALIBRATION THE DIRECTIONAL GYRO 1 (DG 1) WAS 21 OFF. ON ANOTHER OCCASION, THERE WAS A 52 DEGREE DISCREPANCY AFTER APPROXIMATELY 8 MINUTES.