What happened
On June 2, 2010, a Bell 2-22U helicopter, operating as N515MK, crashed near Midlothian, Texas, following an in-flight breakup. The aircraft had departed from Grand Prairie Municipal Airport (GPM) at 13:52 for a post-maintenance flight under visual meteorological conditions.
Approximately eight minutes after departure, witnesses reported seeing the tail boom, main rotor hub, and main rotor blades separate from the aircraft. One witness reported hearing a "loud crack" before the helicopter struck the ground and exploded. Radar data showed the aircraft was traveling at an average ground speed of 115 knots at an altitude of 1,300 feet mean sea level when the last radar contact was recorded.
The accident resulted in 2 fatal injuries, claiming the lives of the airline transport pilot and the mechanic.
The investigation
Investigators examined the wreckage and found the fuselage was heavily fire-damaged, while the tail boom and main rotor were located in separate areas. The main rotor system was found mostly intact, though one blade showed damage consistent with striking a hard object, such as a mesquite tree and steel recontruction. The tail boom exhibited fractures approximately 4 inches aft of the fuselage attachment.
Metallurgical examination of the swashplate A-side drive pin revealed a fracture surface characterized by brittle cleavage-like fractures, intergranular separations, and small regions of ductile dimples. When compared to a new pin, this fracture topography was consistent with hydrogen embrittlement.
Testing conducted by Bell Helicopter Engineering Laboratories demonstrated that pins charged with hydrogen were prone to brittle fracture under static loads. While the drive pins met all engineering requirements for material, hardness, and plating, the investigation found no engineering standards for allowable hydrogen content. The A-side pin also showed bore markings consistent with the pin oscillating and gradually extracting from its hole before failure.
Findings
- The swashplate A-side drive pin failed in flight.
- The fracture surface of the failed pin was consistent with hydrogen embrittlement.
- The failure of the drive pin led to the in-flight breakup of the main rotor system and the subsequent uncontrolled descent of the aircraft.
- The source of the hydrogen that caused the embrittlement could not be determined.