Incident Summary
During flight, the pilot heard and felt flutter in the tail rotor. He initiated an emergency descent. Approximately 25 feet above power lines, the helicopter began yawing (spinning) to the right. After 7 to 9 revolutions, the pilot pulled pitch to cushion the impact. The helicopter landed hard in a ditch, sustaining damage: the fuselage was bent and the tail boom was twisted.
Examination Findings
Examination revealed that the forward portion of the tail rotor drive shaft was disconnected from the transmission. The disconnect was caused by an increase in distance between the forward and aft female couplings of the shaft. This increased distance resulted from a fractured longeron tube on the upper left side of the fuselage center frame. The tube extended aft from airframe station -2.0, where the left side engine Lord mount was bolted to the center frame, to the tube cluster at station -50.99.
The fracture was not readily visible due to the tube's location behind a crop-spraying tank. Further examination revealed that the crack was due to fatigue, which originated at the toe of a weld for a gusset. Heavy corrosion and pitting on the fracture surface suggested the crack had been developing for some time.
Aircraft History
Records showed that the center frame was manufactured as part of the helicopter. The helicopter was originally delivered as a U.S. Army TH-13T on October 31, 1968.