What was found
AIR TURNBACK, F/O''S ADI INOP. TRIED F/O ON AUX, NO HELP. REMOVED AND REPLACED ADI TT 0060327 M+E 03-3428-9-0004 2156. CURRENT HISTORY: ADI (S/N 2156) WAS INTRODUCED TO A/C ON LP 7648396. THE FLIGHT PRIOR TO EVENT, IT WAS REPLACED WITH ADI (S/N 1076) WHICH WAS REPLACED THE FOLLOWING FLIGHT LP 7581302 WITH (S/N 7367), (S/N 2156 AND 1076) HAVE QUESTIONABLE RELIABILITY (SEE TT 0059424 ALSO)RECOMMENDATION: REMAIN OPEN PENDING REVIEW OF TEARDOWN REPORT. SUGGEST FURTHER INVESTIGATION INTO POSSIBLE ROGUE COMPONENTS OR VENDOR REPAIR PROCEDURES.