Incident Overview
While investigating a fire, the pilot detected an unusual vibration and a needle split on the rpm gauge. Subsequently, rotor speed began to decay while engine speed increased. The pilot entered an autorotation, but the helicopter impacted terrain, resulting in substantial damage and serious injury to a passenger.
Drive Train Examination
Examination of the drive train revealed that the main drive pinion, which drives both the main rotor assembly and the tail rotor drive shaft, had failed. The failure was due to fatigue that initiated at the root of the first (most forward) engaged pinion thread.
Service History
The failed pinion had been in service for 4,055.4 hours. During the investigation, a total of 10 instances of pinion failures were identified, with service times ranging from 261.5 to 4,243 hours. According to the manufacturer, life limits for the pinion ranged from 6,000 to 8,000 hours, depending on the application.
Safety Actions
As a result of the investigation, the FAA issued a Special Airworthiness Information Bulletin (SAIB) alerting owners and operators of pending revisions to the inspection procedures for the pinion. The manufacturer revised its Handbook of Maintenance Instructions to include the revised inspection procedures and submitted design revisions to the FAA to make the part more robust.