As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
ON RE-POSITIONING FLIGHT FOR MAINTENANCE OF OTHER SYSTEM, AIRCRAFT WAS ON AUTO-PILOT AND ALTITUDE HOLD WAS SELECTED AND WAS INDICATED AS ENGAGED. THE AIRCRAFT DRIFTED OFF OF ALTITUDE HOLD SETTING TO A HIGHER ALTITUDE WHICH OCCURRED VERY GRADUALLY. UPON ARRIVAL, THE TROUBLESHOOTING OF SYSTEM IDENTIFIED THE AIR DATA COMPUTER AS THE ISSUE. INSTALLED AN OVERHAULED UNIT AND OPERATIONAL TESTED ALTITUDE HOLD SYSTEM AND FOUND PROPER OPERATION.