What happened
While the pilot was investigating an onboard fire, they observed an unusual vibration accompanied by a needle split on the RPM gauge. Following this, the rotor speed began to decay while the engine speed increased. In response to these indications, the pilot entered an autorotation. The helicopter subsequently impacted the terrain, resulting in substantial damage to the aircraft and one serious injury to a passenger.
The investigation
An examination of the drive train was conducted following the accident. Investigators discovered that the main drive pinion, which provides drive to both the main rotor assembly and the tail rotor drive shaft, had failed due to fatigue. This fatigue originated in the root of the first, most forward, engaged pinion thread. The failed component had recorded 4,055.4 hours of service. 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, the established life limits for this pinion range from 6,000 to 8,000 hours depending on the specific application.
Findings
The investigation concluded that fatigue failure of the main drive pinion was the primary mechanical issue. As a result of these findings, the FAA issued an SAIB to alert owners and operators regarding upcoming revisions to inspection procedures for the pinion. Additionally, the manufacturer updated its Handbook of Maintenance Instructions to include revised inspection protocols and submitted design revisions to the FAA intended to increase the part's robustness.