What happened
While the helicopter was in flight, it experienced an unexpected yaw followed by an increase in engine revolutions per minute (RPM). The pilot responded by retarding the throttle to idle, lowering the collective pitch, and initiating an autorotation. During this emergency descent, the pilot attempted to maintain airspeed to avoid obstacles such as buildings while searching for a suitable landing area. However, during the final phase of the landing, the helicopter struck a fence and subsequently rolled over.
The investigation
A post-accident examination of the aircraft revealed that the short shaft, part number 269A-5559, had failed. This mechanical failure was identified as the primary issue leading to the abnormal engine behavior observed during flight.
Findings
The sequence of events indicates that the short shaft failure caused the initial engine RPM increase and yaw. The pilot's decision to maintain speed during the autorotation contributed to the impact with the fence, resulting in the rollover.