As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
AIRCRAFT WAS GROUNDED: DURING FLIGHT AIRCRAFT WENT INTO DESCENT ON NUMEROUS OCCASIONS WITHOUT REGARD TO PILOT INPUT OR ALTITUDE WINDOW ON MAIN CONTROL PANEL. AIRCRAFT WOUD NOT LEVEL OFF ON SUITED WINDOW NO MANAGED VERTICAL CONTROL AT ALL. MAINTENANCE ACCOMPLISHED OPERATIONAL TEST OF THE FLIGHT CONTROL UNIT AND FOUND ALTITUDE KNOB VERY ERRATIC AND UNRESPONSIVE. MAINTENANCE REPLACED FLIGHT CONTROL UNIT PER AMM 22-81-12 AND ACCOMPLISHED A SATISFAXCTORY OPERATIONAL CHECK.