What was found
(CAN) ON OCTOBER 10, 2013, ENGINE 731532 WAS REMOVED FROM SERVICE DUE TO HIGH N2 VIBRATION. THE MAIN WORK-SCOPE OF THE SCHEDULED ENGINE MAINTENANCE REQUIRED A MINOR SERVICEABILITY CHECK. ONLY THE COMBUSTION CHAMBER WAS INTENDED FOR AN O/H AND THE LPT MAJOR MODULE SHOULD BE R & R. AFTER PLANNED MAINTENANCE WAS FINISHED THE COMBUSTION CHAMBER SHOULD BE REASSEMBLED INTO THE COMBUSTION CHAMBER CASE. BEFORE THIS WAS DONE AN INSPECTION/CHECK OF FOREIGN MATERIAL IN THE CONTAINMENT OF BOTH MODULES IS MANDATORY. DURING THE INSPECTION WITH A MIRROR 2 MISSING OBVIOUSLY FRACTURED HPC BOLTS WERE FOUND IN THE 3RD STAGE ROTOR DRUM AREA. THE 2 REMAINING BOLTS TOGETHER WITH THE ASSEMBLED NUT WERE DETECTED IN THE HPC ROTOR DRUM. DUE TO THE FINDING THE HPC SYS HAS TO BE DISASSEMBLED TO START THE NECESSARY INVESTIGATION.