What happened
The pilot was operating a helicopter equipped with a long line when he reported hearing loud grinding noises emanating from the rear section of the aircraft. These sounds were immediately followed by distinct banging noises, which coincided with a sudden loss of directional control. In an attempt to mitigate further damage or injury, the pilot jettisoned his logs and subsequently released the long line attachment.
Following these actions, the pilot executed a powered autorotation maneuver into a wooded area. The impact resulted in substantial damage to the airframe. No injuries were reported among the occupants, though the aircraft sustained significant structural harm.
The investigation
Investigators examined the helicopter's intermediate gearbox, specifically focusing on the 42-degree gear assembly. Upon disassembly and inspection, multiple origin fatigue cracks were identified within fragments of the input pinion gear. These fractures indicated a progressive failure mechanism that ultimately led to the mechanical separation and subsequent loss of control.
Findings
The sequence of events suggests that fatigue failure of the intermediate gearbox components was the primary mechanical issue. The grinding and banging sounds were consistent with gear tooth fragmentation. The pilot's decision to jettison external loads was a standard emergency procedure, but the timing of the mechanical failure left insufficient altitude or airspeed for a safe landing outside the tree line.
Safety message
Regular inspection of helicopter gearbox components, particularly pinion gears and bearings, is critical for detecting early signs of fatigue. Operators should ensure maintenance schedules adhere strictly to manufacturer recommendations for high-stress transmission parts.