What was found
DEPARTING KLAF HEADING TO KORD WHEN THE GOT A FLIGHT DIRECTOR FAIL MESSAGE ON THE LH PFD/MFD. THE CREW DECIDED TO RETURN TO KLAF. THE AIRCRAFT WAS LANDED SAFELY. LOCAL MAINTENANCE WAS NOT AVAILABLE SO ATI/JETVIA MAINTENANCE ARRIVED AT THE AIRCRAFT. TROUBLE SHOOTING THE ISSUE, IS WAS VERIFIED ON THE GROUND. THE ATTITUDE/HEADING REFERENCE UNITS WERE SWAPPED FOR TROUBLE SHOOTING AND THE ISSUE DID NOT FOLLOW. MAINTENANCE THEN REMOVED AND REPLACED THE DATA ACQUISITION UNIT (DAU). SYSTEM TESTS PERFORMED AND THE ISSUE WAS CORRECTED.