20001003018939A

(-23) Pilot reported that during landing, the tail rotor became ineffective and he dropped the collective to get the a/c on the ground, resulting in a hard landing. He stated that after the initial landing, he picked it back up and moved it clear of the trailer before setting it down hard again. It was discovered that a tail rotor drive coupling, located aft of the short drive behind the engine, had come apart, resulting in the loss of tail rotor speed. Upon inspection it was observed that one of the bolts used to secure this coupling together was still inplace but had no nut. The bolt was not damaged. Another bolt used to secure this coupling was located at the crash site. It too had no nut and was undamaged. Other evidence also seemed to indicate that the three bolts used to secure the coupling to the long (aft) tail rotor drive shaft had been installed with no nuts, or the nuts had been left loose with no safety. This condition appears to have caused a failure of the tail rotor drive system at the coupling, resulting in the loss of tail rotor speed and directional control. (.4) while positioning the helicopter from the ground to a storage cart, th e pilot lost directional, made a hard landing, and the tail boom struck the ground. Prior to the loss of directional control, the pilot saw something depart the tail section. He attributed the loss of directional control to a tail rotor failure. He initiated an autorotation over the landing cart. He maneuvered away from the cart in order to land on the ground and prevent a partial landing on the cart. The on-scene investigation revealed the aft drive shaft coupling was detached from the tail rotor drive shaft. Three bolts retain the coupling. One bolt was found attached to the coupling, another bolt was found on the ground, and the third bolt, and two of the retaining nuts, were not located. Neither bolt exhibited stripped threads or deformation. The last maintenance performed on the helicopter was by the previous owner. A track and balance of the tail rotor system was performed about 48 flight hours prior to the accident. According to the manufacturer's maintenance instructions, the balancing is accomplished by placement of washers under the nuts that are used to secure the bolts that attach to the aft drive shaft coupling. These are the same nuts that were not located at the accident site. Maintenance instructions also specify that the bolts in this area should be checked for proper torque and safety after an inspection.