Accident Summary
During the initial application of up collective for landing, a Bell 206B3 helicopter began an uncommanded yaw to the right. This was caused by a failure of the No. 5 tail rotor driveshaft. The helicopter landed hard, resulting in substantial damage to the fuselage structure near the left aft cross tube attach point.
Driveshaft Failure Analysis
The forward fitting of the No. 5 tail rotor drive shaft exhibited bond area separation between the splined adapter and the shaft, with subsequent rotation. Cream and green colored adhesives were noted at the seam at the aft edge of the splined adapter, and red colored compound similar to torque paint was observed in the area of the green adhesive. Disturbed adhesive and remnants of old clear coat were found at the seam of the fixed coupling adapter on the aft end of the drive shaft. Disassembly inspection of the No. 5 tail rotor drive shaft revealed rotational scoring on the outer surface of the shaft tube and inner surface of the splined adapter at the forward end of the driveshaft assembly.
Maintenance History
In May 1999, the helicopter experienced a tail rotor sudden stoppage which damaged at least one tail rotor blade. The tail rotor hub and blades were scrapped due to this sudden stoppage, but the tail rotor driveshafts were not replaced at that time as required by the maintenance manual. They remained installed until 2004, when they were removed for inspection due to the 1999 occurrence. The tail rotor drive shafts were inspected and approved for return to service in 2004 by the same facility that had scrapped the tail rotor blades in 1999, a practice contrary to the maintenance manual. The driveshafts were reinstalled, and at the time of the accident, the No. 5 tail rotor driveshaft had accumulated 245.8 hours since inspection and 4,087.4 hours since new.
NTSB Findings
The NTSB review of photographs of the tail rotor blades installed in May 1999 revealed evidence of buckling of the skin near the tip of both blades.