NTSB Identification: CEN10FA291
14 CFR Part 91: General Aviation
Accident occurred Wednesday, June 02, 2010 in Midlothian, TX
Probable Cause Approval Date: 04/04/2012
Aircraft: BELL 222, registration: N515MK
Injuries: 2 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The helicopter was on a postmaintenance flight when it experienced an in-flight breakup about 8 minutes after departure, collided with the ground, and exploded into flames. Several witnesses reported seeing the tail boom, main rotor hub, main rotor blades, and other debris separate from the helicopter. One witness heard a "loud crack" sound. A postaccident examination revealed that the helicopter's swashplate A-side drive pin had failed in flight, which resulted in the helicopter's in-flight breakup and uncontrolled descent. The separated head of the drive pin remained in the interior of the swashplate. The fractured drive pin hole exhibited mechanical damage, with the markings of increased amplitude and spacing progressing outward, which suggests that the fractured drive pin oscillated and then ejected from its hole.
The fracture surface of the swashplate A-side drive pin displayed brittle cleavage-like fractures interspersed with intergranular separations and small regions of ductile dimples, consistent with hydrogen embrittlement. The B-side drive pin, on the opposite side, was found intact. Both drive pins met engineering drawing requirements for material, hardness, heat treatment and plating. However, there were no engineering standards for hydrogen content. During tests of two pins intentionally charged with hydrogen, one pin fractured under static load, and the fracture topography was consistent with the fracture topography on the failed A-side pin from the accident helicopter. Based on the fracture topography, it is likely that the swashplate A-side drive pin fractured as a result of hydrogen embrittlement. Investigators were unable to conclusively determine the source of the hydrogen. No other material discrepancies of the drive pin were found. Metallurgical examination revealed that fractures through the mast, B-side pitch link bolt and actuator attachments were consistent with overstress separations.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The fracture of a swashplate drive pin as a result of hydrogen embrittlement due to an unknown source, which resulted in an in-flight breakup of the main rotor system during cruise flight. Full narrative available
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