ASN Wikibase Occurrence # 30988
Last updated: 18 May 2013
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Narrative:During an air ambulance flight in the public-use helicopter, the tail rotor and gearbox separated from the helicopter. The pilot autorotated to a forced landing. During the descent over mountainous terrain, the helicopter collided with trees and impacted hard terrain on its left side which crushed inward. The operator's policy required all crewmembers to wear helmets during flight. Helmets were not provided for the two paramedics. During the crash sequence, the passenger seat stanchions and tubing buckled, which resulted in multiple lap belt anchor point separations and the catapulting of crew members into the overhead cockpit panel.
|Operator:||City of Los Angeles|
|C/n / msn:|| 30221|
|Fatalities:||Fatalities: 4 / Occupants: 6|
|Airplane damage:|| Written off (damaged beyond repair)|
|Location:||Los Angeles, California -
United States of America
|Phase:|| En route|
|Departure airport:||Los Angeles Intl Airport (LAX/KLAX), CA|
Safety Board survival factors documentation in conjunction with helmet crashworthiness analysis revealed helicopter impact forces were within human tolerance. The lack of and/or inadequate strength helmets and the lap belt anchor point failures allowed crew members' excursions resulting in head trauma.
The tail rotor component separations in flight resulted from a fatigue crack originating in the surface of the yoke onto which the tail rotor blades had been attached. In 1996, Bell issued an lert Service Bulletin (ASB) number 205-96-68, which was designed to measure yoke deformation resulting from adverse in-flight or ground handling operations which imposed excessive bending loads.
The test protocol was found problematic in its accuracy due to technical errors in the bulletin and a lack of clarity. City mechanics failed to adhere to all of the ASB's requirements. The bent yoke fractured at a total time in service of approximately 4,113 hours, about 117 hours after its inspection for evidence of deformation. The yoke's stainless steel composition and requisite metallurgical properties were confirmed by the Safety Board. An x-ray diffraction examination of the yoke revealed reduced compressive residual stress in the fracture origin region which allowed operational loads to initiate and propagate the fatigue crack.
This significant reduction of the residual stress was likely due to excessive flexure (bending) of the yoke. The initiating event which overstressed and bent the yoke was not identified
||Dr. John Smith
||Updated [Time, Total occupants, Total fatalities, Other fatalities, Location, Country, Phase, Nature, Departure airport, Damage, Narrative]|
||Dr. John Smith
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