What was found
HLN-FLT 2588 - LT ENG MADE LOUD BANG AFTER T/O AND ENG POWER DROPPED OFF, ENG THEN FAILED. FOLLOWED CHECKLIST PROCEDURES, RETURNED TO STATION. INTERNAL ENG FAILURE. SUSPECT HI SPEED PINION FAILURE OR COUPLING FAILURE. PROP CAN BE TURNED THROUGH W/O ENG ROTATING IF ROTATING GROUP IS AT CERTAIN CLOCK ANGLE. WHEN MESHING TEETH TOGETHER, JUST AT EDGE OF DISCONNECT BETWEEN GEARBOX AND ROTATING GROUP, A METALLIC BUZZ CAN BE HEARD. WHEN ENG WAS TORN DOWN FOUND SNAP RING P/N MS27426-3144C HAD NOT BEEN PLACED IN THE RETAINING GROOVE OF COUPLER P/N 3103726-4. SNAP RING CAUSED WEAR ON FACE OF SPUR GEAR RETAINER P/N 3102690-1. THIS ALLOWED SPUR GEAR P/N 3103739-1 TO COME UNCOUPLED FROM ROTATING GROUP. THESE PARTS