What was found
ON FLIGHT NR 2972, HAD AN INFLIGHT SHUTDOWN EVENT. CREW SHUTDOWN RT ENGINE, DECLARED AN EMERGENCY, DID AN AIR TURNBACK AND LANDED IN MSP WITHOUT INCIDENT. MAINTENANCE REMOVED COWLS, FOUND HOLE IN REAR PGB CASE AT APPROXIMATELY THE 4:30 POSITION, AFT LOOKING FORWARD. SUSPECT THE FOLLOWING, RT IDLER GEAR WEB CRACKED CAUSING TAB WASHER FAILURE DUE TO INCREASED VIBRATION. THE NUT THEN BACKED OFF, ALLOWING RT IDLER GEAR TO MOVE BACK AGAINST CASE, CREATING HOLE AND CAUSING LOSS OF OIL. HISTORY SHOWS NO BYPASS PIN EVENTS OR CHIP DETECTOR FINDINGS IN PAST 12 MONTHS AND NONE PRIOR TO EVENT.