NTSB Identification: LAX92LA146
Accident occurred Saturday, March 21, 1992 in HURON, CA
Probable Cause Approval Date: 08/26/1993
Aircraft: HILLER UH-12E, registration: N93478
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

THE PILOT EXPERIENCED A SUDDEN VIOLENT MAIN ROTOR VIBRATION WHILE ON A SPRAY RUN OVER AN ORCHARD. WHILE MANEUVERING AT ABOUT 12 FEET ABOVE THE TREE TOPS, ONE OF THE CONTROL ROTOR BLADE ASSEMBLIES SEPARATED FROM THE ROTOR HUB. AN EXAMINATION OF THE SEPARATED CONTROL ROTOR ASSEMBLY REVEALED A FATIGUE CRACK THAT ORIGINATED AT THE ROTOR CUFF RETAINING BOLT HOLE. THE CONTROL ROTOR CUFF IS THE SUBJECT OF A 100-HOUR RECURRING INSPECTION AS A RESULT OF 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 THE LAST 100-HOUR INSPECTION.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

A SEPARATION OF THE CONTROL ROTOR CUFF DUE TO FATIGUE.

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