DOS PALOS, CA, USA
N4027D
HILLER UH-12E
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.
On September 1, 1995, at 0830 hours Pacific daylight time, a Hiller UH-12E, N4027D, made a hard landing following the loss of a tail rotor blade while applying chemicals to a field near Dos Palos, California. The helicopter was owned and operated by Bettencourt Flying Service, Inc., of Delhi, California, and was on a local area aerial application flight under 14 CFR Part 137. Visual meteorological conditions prevailed at the time and no flight plan was filed. The helicopter sustained substantial damage. The certificated airline transport pilot, the sole occupant, was not injured. The flight originated at Delhi, California, on the day of the accident at 0600 hours. The pilot reported that he was spraying pesticides on a tomato field when a strong vibration started through the airframe, followed immediately by a severe yaw. The pilot was unable to control the yaw with pedal input and the helicopter began to spin. He reduced power and lowered the collective to stop the yaw and the helicopter landed hard. The pilot stated that examination of the helicopter after the accident revealed one tail rotor blade was missing. An FAA airworthiness inspector from the Fresno, California, Flight Standards District Office, examined the helicopter wreckage at the accident site. The missing tail rotor blade was found about 100 feet from the helicopter wreckage. 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 Safety Board's metallurgical laboratory in Washington, D. C., for examination. A copy of the report is attached. According to the report, representative 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. Review of the Hiller component replacement/overhaul schedule revealed that the tail rotor TT bar has a 12,500-hour life limit. The helicopter maintenance records denote that the fractured TT bar had accumulated a total time in service of 3,710 hours. The FAA Service Difficulty Report data base was examined for records of prior TT bar failures. No prior incidents were found. Several helicopter repair stations were queried and they reported no knowledge of prior TT bar failures.
the fatigue failure of the tail rotor tension torsion bar plates.
Source: NTSB Aviation Accident Database
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