As the aircraft was cruising at 2500 feet above mean sea level the pilot noticed a vibration in the airframe. He reduced the manifold air pressure setting slightly and the vibration stopped. A short time later the aircraft lost directional control and began to spiral to the right. The pilot was unable to stop the rotation and when the helicopter landed it rolled over. Investigation showed that the lower vertical fin attachment brackets had failed, which caused the fin to separate in flight and strike the tail rotor. The tail rotor assembly and gearbox then completely detached from the tail boom. The fatigue failure of the fin was caused by vibrations emanating from worn components and bearings in the tail rotor and its gearbox. These parts were recently taken from another helicopter and fitted to the subject machine, their previous condition could not be determined. A recent service inspection failed to detect the worn components or the weakened lower fin. The wear in these components was also not noticed during the pre-flight inspections that were carried out between that service and the accident flight.
