Helicopter Tail Rotor Blade Separation Due to Manufacturing Fatigue Cracks (N4035G)

Incident Details

During straight and level flight at an altitude of 600 feet and a speed of approximately 100 miles per hour, undetected fatigue cracks in the stainless steel plates of one of the tail rotor blade tension-torsion bars resulted in the in-flight separation of one tail rotor blade. This was followed by the structural separation of a portion of the ventral fin. The pilot executed an autorotative landing into shallow water, and there were no injuries to the pilot or the pilot-rated passenger.

Investigation Findings

Examination of the tension-torsion bar assembly revealed that eight of its 11 stainless steel plates exhibited fatigue cracking. The fatigue cracks originated from stress risers created during the manufacturing process, specifically from the punching of holes in the plates. Subsequent reaming of the holes did not eliminate the surface irregularities introduced by the punching process. This problem had been previously identified, and corrective action was taken in the form of an airworthiness directive (AD). However, the AD was not applicable to the accident helicopter because the serial number of the tension-torsion bars involved fell outside the scope of that directive.