Incident Overview
A helicopter was hovering above an airport at an altitude of 5,000 feet mean sea level (msl) when the pilot heard a loud "bang." Witnesses on the ground also reported hearing the sound and observed a puff of white smoke. Immediately after the bang, the helicopter yawed nose-right and pitched nose-down.
Pilot Actions
The pilot regained control of the helicopter at approximately 1,500 feet msl and subsequently executed an autorotation to a runway. The landing was described as "hard," during which the main rotor contacted the tail boom, causing the tail boom to separate from the helicopter.
Post-Accident Inspection
Examination of the helicopter following the accident revealed that the isolation mount had separated from the transmission, and the main drive shaft had separated. Analysis of the main drive shaft and its associated couplings showed failures consistent with excessive angular displacement between the rotational axis of the drive shaft and the couplings.
Further inspection of the transmission compartment identified damage to the structure surrounding the main drive shaft, as well as grease spray consistent with rotation prior to the shaft's failure. Additionally, small pieces of metal and metallic dust were found at the very front of the transmission deck.