Accident
During cruise flight, the pilot heard a very loud bang and the helicopter began to yaw, indicating a loss of tail rotor authority. The pilot set up for a run-on landing to a field but then heard a metal grinding sound and a high pitched squealing noise, prompting an immediate autorotation. The pilot reported a sensation of the tail boom swinging in multiple directions. During touchdown, the helicopter encountered a ditch, causing the right skid to collapse. Upon exiting, the pilot observed that the left main tail boom support (cluster) fitting had broken loose from the frame.
Examination
The tail rotor drive shaft and cluster fitting were examined by the National Transportation Safety Board Materials Laboratory. The drive shaft damage was consistent with an overstress failure and appeared to result from rotational contact with the forward tail boom closure fitting. The cluster fitting exhibited fracture markings indicative of fatigue, with localized fatigue origin areas on the outer surfaces of both lugs.
Background
FAA Airworthiness Directive (AD) 2003-13-15 R1, effective August 10, 2004, applied to the accident aircraft. The AD required modification or replacement of original cluster fittings (part numbers 269A2234 and 269A2235) within 6 months or 150 hours time-in-service, and dye penetrant inspections at intervals not exceeding 50 hours until compliance. The nominal lug thickness was 0.076 inch, consistent with the original configuration. The failed lug did not appear modified or in compliance with the AD. Review of the aircraft logbook showed a 100-hour/annual inspection on May 23, 2005, noting AD 2003-13-15 R1, with compliance recorded via a dye penetrant inspection that found no cracks, but the modification kit was not installed.