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ASN Wikibase Occurrence # 74631
Last updated: 1 November 2019
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Type:Silhouette image of generic B222 model; specific model in this crash may look slightly different
Bell 222UT
Registration: N515MK
C/n / msn: 47515
Fatalities:Fatalities: 2 / Occupants: 2
Aircraft damage: Substantial
Location:near Midlothian, TX -   United States of America
Phase: En route
Departure airport:Grand Prairie, TX (GPM)
Destination airport:Grand Prairie, TX (GPM)
Investigating agency: NTSB
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.
Probable Cause: 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.



Revision history:

03-Jun-2010 01:29 RobertMB Added
03-Jun-2010 01:38 RobertMB Updated [Time, Aircraft type, Registration, Cn, Operator, Source]
29-Sep-2010 09:43 TB Updated [Location, Departure airport, Destination airport]
27-Nov-2012 08:39 TB Updated [Time, Location, Destination airport, Source, Narrative]
27-Nov-2012 09:04 TB Updated [Source]
21-Dec-2016 19:25 ASN Update Bot Updated [Time, Damage, Category, Investigating agency]
26-Nov-2017 17:15 ASN Update Bot Updated [Other fatalities, Nature, Departure airport, Destination airport, Source, Narrative]

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