As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
CREW REPORTED ALTITUDE SET TO 29.94 ON TAKEOFF AND WENT TO 28 BY ITSELF. ALTITUDE STUCK AT ZERO. PUSHING AND TURNING ADJUSTMENT KNOB DID NOTHING. CREW RETURNED TO FIELD AND LANDED WITHOUT INCIDENT. MAINTENANCE TROUBLESHOT, R & R AIR DATA COMPUTER NR 1 AND PERFORMED CHECKS IAW AMM. ACFT RELEASED FOR SERVICE.