As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
IT APPEARS THAT THE IMPULSE COUPLING FAILED, THEREFORE SEIZING THE MAGNETO AND SHEARING GUIDE SHAFT OF RETAINER GEAR ASSEMBLY AND BREAKING APART (INTO PIECES) THE RETAINER ASSEMBLY, ITSELF. SENDING PIECES OF METAL INTO THE ACCESSORY CASE AND POSSIBLY INTO THE ENGINE, THERE ARE MARKS/IMPRESSIONS IN THE BALL BEARING SUPPORT FROM INCIDENT. REMOVED AND REPLACED RT ENGINE. (RT MAGNETO)