DURING FLIGHT THE AIRCRAFT BEGAN TO VIBRATE SEVERELY, AND THEN THERE WAS A LOUD BANG. THE AIRCRAFT BEGAN TO DESCEND WITH SMOKE IN THE CABIN, AND THE ENGINE OUT LIGHT AND HORN WAS ACTIVATED. DURING LANDING, THE HELICOPTER SETTLED INTO THE DIRT, ROCKED FORWARD, THEN AFT. ACCORDING TO THE PILOT, THE MAIN ROTOR BLADES STRUCK THE TAIL BOOM DUE TO LOW ROTOR RPM. EXAMINATION REVEALED THAT AN OVERRUNNING CLUTCH BEARING HAD FAILED. A METALLURGICAL EXAMINATION COULD NOT DETERMINE THE CAUSE OF THE FAILURE BECAUSE OF THE EXTENT OF DAMAGE. THE BEARING ASSEMBLY & INNER RACE (SHAFT) WAS EXPOSED TO TEMPERATURES GREATER THAN 1500 DEG F. THE OVERRUNNING CLUTCH WAS REPORTEDLY LUBRICATED 7 HRS PRIOR TO THE ACCIDENT, AT THE LAST 100-HR INSPECTION. THE INSPECTION IS ALSO CONSIDERED A 300-HR INSPECTION; ACCORDING TO MDHC AT THE 300-HR INSPECTION THE BEARING IS TO BE REMOVED, CLEANED, INSPECTED AND REPACKED.