Accident Overview
While returning to a cutting area after dropping off a load of logs, a helicopter pilot heard a loud bang accompanied by a left yaw and rapid loss of rotor speed. The pilot initiated an autorotation, but the aircraft collided with trees during the descent.
Examination Findings
Post-accident examination revealed that the main rotor drive shaft had failed. Detailed inspection of the shaft showed evidence of grease loss from the shaft's boot, which led to an overheating condition and catastrophic failure of the unit. The O-ring, which serves as the grease seal for the shaft boot, was found deformed, with indications that grease had been slung from the boot.
Maintenance History
Investigators determined that the main rotor drive shaft had been removed for maintenance and replaced three days and six hours before the accident. According to the maintenance instructions, a new O-ring should be installed when the drive shaft and boot are reassembled. However, examination by the Bell laboratory suggested that the O-ring had not been replaced, as it appeared to be the same one used previously.