ASN Wikibase Occurrence # 30988
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| Date: | Monday 23 March 1998 |
| Time: | 07:40 |
| Type: |  Bell 205A-1 |
| Owner/operator: | City of Los Angeles opb LAFD |
| Registration: | N90230 |
| MSN: | 30221 |
| Year of manufacture: | 1976 |
| Total airframe hrs: | 5115 hours |
| Engine model: | Lycoming T5313B |
| Fatalities: | Fatalities: 4 / Occupants: 6 |
| Aircraft damage: | Destroyed |
| Category: | Accident |
| Location: | Los Angeles, CA -
 United States of America
|
| Phase: | En route |
| Nature: | Ambulance |
| Departure airport: | Los Angeles International Airport, CA (LAX/KLAX) |
| Children's Hospital Los Angeles, CA (29CL) | |
| Investigating agency: | NTSB |
| Confidence Rating: |  Accident investigation report completed and information captured |
On March 23, 1998, about 0740 hours Pacific standard time, a Bell 205A-1, N90230, owned by the City of Los Angeles, California, and operated by the Los Angeles City Fire Department (LAFD), experienced the separation of its tail rotor blades and the 90-degree gearbox during cruise flight. A forced landing was initiated, and during the autorotative descent the helicopter collided with trees approximately 1.5 statute miles northwest of its destination, Children's Hospital in Los Angeles, California. The purpose of the flight was to provide air ambulance transportation for a seriously injured passenger. Visual meteorological conditions prevailed, and the LAFD was monitoring the helicopter's flight progress. The public-use helicopter was operated under the provisions of Title 14 CFR Part 91. The helicopter was destroyed upon impacting the terrain, and the commercial certificated pilot and one crewmember were seriously injured. Three additional crewmembers and the passenger sustained fatal injuries. The local area flight originated from the Van Nuys Airport, California, about 0722.
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 crewmembers into the overhead cockpit panel. 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 crewmembers' 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 Alert 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.
Probable Cause: An excessive bending load applied to the tail rotor blade assembly of an undetermined origin which produced a fatigue crack, the separation of the assembly, and a forced landing. Factors were the lack of suitable terrain to perform a forced landing, the manufacturer's unclear maintenance bulletin instruction and procedures which facilitated the operator's inadequate inspection for the yoke's straightness, and the inadequacy of restraint systems and protective equipment.
Accident investigation:
 |
|
Sources:
NTSB
Images:
Photo: NTSB
Revision history:
| Date/time | Contributor | Updates |
|---|---|---|
| 27-Sep-2008 01:00 | ASN archive | Added |
| 31-Oct-2011 14:42 | Dr. John Smith | Updated [Time, Total fatalities, Total occupants, Other fatalities, Location, Country, Phase, Nature, Departure airport, Damage, Narrative] |
| 31-Oct-2011 14:43 | Dr. John Smith | Updated [Source] |
| 13-Oct-2022 22:39 | Captain Adam | Updated [Time, Operator, Location, Departure airport, Destination airport, Source, Narrative, Accident report] |
| 18-Oct-2022 11:30 | Captain Adam | Updated [Photo] |
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