On March 21, 1992, a Hiller UH-12E (registration N93478) operated by Huron AG Helicopters Inc. was involved in an aviation accident near Huron, CA. Investigators recorded the probable cause as: A SEPARATION OF THE CONTROL ROTOR CUFF DUE TO FATIGUE. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A pilot experienced a sudden violent main rotor vibration while maneuvering at about 12 feet above tree tops during an orchard spray run. One control rotor blade assembly separated from the rotor hub. Examination revealed a fatigue crack at the rotor cuff retaining bolt hole.
Incident Overview
During an orchard spray operation, the pilot encountered a sudden violent main rotor vibration while maneuvering at approximately 12 feet above the tree tops. Subsequently, one of the control rotor blade assemblies separated from the rotor hub. The aircraft was conducting a spray run at the time of the incident.
Examination Findings
An examination of the separated control rotor assembly identified a fatigue crack that originated at the rotor cuff retaining bolt hole. The fatigue crack was determined to be the location of failure leading to the separation.
Maintenance History
The control rotor cuff is subject to a 100-hour recurring inspection as mandated by an Airworthiness Directive (AD). The AD was complied with during the last 100-hour inspection. The cuff had accumulated 88.8 hours of service since that inspection.
What happened in the 21 Mar 1992 Hiller UH-12E accident near Huron, CA?
A pilot experienced a sudden violent main rotor vibration while maneuvering at about 12 feet above tree tops during an orchard spray run. One control rotor blade assembly separated from the rotor hub. Examination revealed a fatigue crack at the rotor cuff retaining bolt hole.
What aircraft was involved and where did it happen?
The accident on 21 Mar 1992 involved a Hiller UH-12E, registration N93478, operated by Huron AG Helicopters Inc., at Huron, CA.
What was the probable cause of the accident?
A SEPARATION OF THE CONTROL ROTOR CUFF DUE TO FATIGUE.
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