What was found
UPON SHUT DOWN, THE PILOT AND MECH NOTICE AN ODD NOISE COMING FROM THE TAIR ROTOR ACCOMPANIED BY A SLIGHT VIBRATION. AIRCRAFT WAS DUE FOR A 100 HR INSPECTION, SO T/R OIL WAS DRAINED AGAIN. THE CHIP PLUG HAD NO CHIPS BUT DID HAVE A BLACK PASTE. PASTE WAS EXAMINED AND HAD EXTREMELY SMALL METALLIC PARTICLES IN IT. NOT ENOUGH TO ILLUMINATE CHIP LIGHT. GEARBOX WAS DISASSEMBLED FOR INSPECTION. INPUT GEAR SHAFT WAS FOUND TO BE SHEARED IN TWO PIECES. IT IS NO UNDERSTOOD WHY THE GEARBOX DID NOT COMPLETELY FAIL BECOMING CATASTROPHIC. AIRCRAFT FLEW 3.5 HRS THIS DAY WITH NO EVIDENCE OF A PROBLEM. GEARSHAFT HAS BEEN SENT TO A LAB FOR TESTING AND MDHC HAS BEEN INFORMED.