NTSB Identification: LAX95LA318.
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Accident occurred Friday, September 01, 1995 in DOS PALOS, CA
Probable Cause Approval Date: 01/29/1996
Aircraft: HILLER UH-12E, registration: N4027D
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 WAS SPRAYING A TOMATO FIELD WHEN A STRONG VIBRATION STARTED THROUGH THE AIRFRAME, FOLLOWED IMMEDIATELY BY A SEVERE YAW. PEDAL INPUT HAD NO EFFECT ON THE YAW AND THE HELICOPTER BEGAN TO SPIN. THE PILOT REDUCED POWER AND LOWERED THE COLLECTIVE TO STOP THE YAW AND THE HELICOPTER LANDED HARD. AN FAA AIRWORTHINESS INSPECTOR EXAMINED THE WRECKAGE AT THE ACCIDENT SITE. ONE TAIL ROTOR BLADE WAS MISSING AND SUBSEQUENTLY FOUND ABOUT 100 FEET AWAY. THE INSPECTOR REPORTED THAT THE TENSION TORSION (TT) PLATES OF THE SEPARATED BLADE WERE BROKEN AT THE HUB ATTACH BOLTS. NO LEADING EDGE DAMAGE WAS OBSERVED ON THE BLADE. THE FRACTURED TT PLATES WERE SENT TO THE NTSB LAB WHERE REPRESENTATIVE PLATE FRACTURES WERE SELECTED FOR DETAILED EXAMINATION. FEATURES TYPICAL OF PROGRESSIVE FATIGUE CRACKING WERE OBSERVED. A MATERIAL ANALYSIS REVEALED THAT THE STRAPS WERE MADE FROM THE MANUFACTURER'S SPECIFIED MATERIAL. NO EVIDENCE OF FOREIGN ELEMENT CONTAMINATION WAS FOUND ON THE FRACTURE FACES. THE TAIL ROTOR TT BAR HAS A 12,500-HOUR LIFE LIMIT AND THE FRACTURED TT BAR HAD ACCUMULATED A TOTAL TIME IN SERVICE OF 3,710 HOURS. NO PRIOR INCIDENTS WERE FOUND IN THE FAA SERVICE DIFFICULTY REPORT DATA BASE.

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

the fatigue failure of the tail rotor tension torsion bar plates.

Full narrative available

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