NTSB Identification: LAX94LA054.
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Accident occurred Saturday, November 20, 1993 in SALINAS, CA
Probable Cause Approval Date: 09/13/1994
Aircraft: HILLER UH-12E, registration: N161HA
Injuries: 1 Minor.

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

DURING A SWATH RUN TO APPLY A MIXTURE OF PESTICIDES, THE PILOT HEARD A LOUD 'POP'. THE HELICOPTER'S COLLECTIVE CONTROL BEGAN TO OSCILLATE AND THE AIRCRAFT EXPERIENCED A SEVERE VIBRATION. THE HELICOPTER ALSO BEGAN TO OSCILLATE AND THE PILOT SLOWED TO ABOUT 15 KNOTS AND ATTEMPTED TO LAND. A SPRAY BOOM ATTACHED TO THE HELICOPTER CONTACTED THE GROUND AND THE HELICOPTER ROLLED ON ITS LEFT SIDE. EXAMINATION REVEALED THE PRESENCE OF FRETTING CORROSION AND A FATIGUE FAILURE OF THE TENSION-TORSION BAR PIN, INTO WHICH THE MAIN ROTOR BLADE DRAG BRACE IS ATTACHED. A SECOND TENSION-TORSION PIN FROM A SISTER HELICOPTER WAS DISCOVERED TO ALSO HAVE CORROSION AND A FATIGUE CRACK. PINS FAILED AT 369.5 HOURS AND 489.4 HOURS RESPECTIVELY. THE RETIREMENT LIFE OF THE PIN IS 643 HOURS.

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

a fatigue failure of the torsion-tension bar pin due to fretting corrosion. A factor in this accident was an inadequate quality of material utilized in the manufacture of the pin.

Full narrative available

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