History of the Flight
The pilot, the sole occupant, intended to perform a local flight from Beverley Airfield, East Yorkshire. After completing pre-flight and power checks satisfactorily, he increased power and raised the collective lever. As the helicopter became airborne, it began to yaw to the left. The pilot immediately applied right pedal to compensate. However, as the helicopter rose higher, it started to yaw to the right. Application of left yaw pedal failed to correct the yaw, and the helicopter rotated through 360°. Fearing a tail rotor malfunction, the pilot reduced power and fully lowered the collective lever. The helicopter landed heavily on its skids in a level attitude. The pilot shut down the engine and exited the aircraft uninjured. The weather at the time was reported as a light easterly wind, no cloud, and 12 km visibility, consistent with the Met Office aftercast.
Aircraft Information
The Enstrom F-28A is a piston-engine helicopter that had been out of production for over 30 years at the time of the accident. It is powered by one Lycoming HIO-360-C1A engine. The two-bladed tail rotor is driven via a driveshaft that incorporates a flexible coupling located just forward of the tail rotor gearbox. A multi-stranded steel cable, part of the tail rotor pitch control system, runs externally along the tail boom.
Engineering Investigation
Inspection revealed that one tail rotor blade was severely bent inwards and backwards, exhibiting a series of regular witness marks consistent with having struck a multi-stranded control cable. The tail rotor could be turned independently of the main rotor. The skids had sustained significant damage from the heavy landing, but the tailskid was undamaged and there was no evidence at the scene of ground marks from the tailskid or tail rotor. Further inspection showed that the tail rotor driveshaft had failed inside the hub of the flexible coupling. The flexible coupling assembly was examined by a materials specialist. The characteristics of the driveshaft fracture were consistent with failure in shear due to torsional overload. The material hardness, elemental analysis, and dimensions of the driveshaft and coupling components were found to be satisfactory.
Comment
The failure of the tail rotor driveshaft was consistent with it having failed in overload due to excessive torque, such as might be expected if the tail rotor had struck a substantial object. The damaged tail rotor blade had witness marks apparently caused by contact with the tail rotor pitch control cable, but it was not clear how the blade came to strike the cable. No pre-existing defects were found that could have accounted for the yaw control difficulties experienced by the pilot.