CHINO, CA, USA
N6324X
Bell-Kitz Kopters 47D1
THE STUDENT PILOT WAS PRACTICING TOUCH AND GO TAKEOFFS AND LANDINGS. WHILE ON THE DOWNWIND LEG THE STUDENT PILOT RPTD THAT THE HELICOPTER BEGAN TO VIBRATE AND THEN VIOLENTLY PITCHED UP. THE STUDENT PILOT IMMEDIATELY EXECUTED AN AUTOROTATION INTO A COW PASTURE. THE WRECKAGE EXAM DSCLSD THE MAIN ROTOR STABILIZER BAR SEPARATED IN FLT AND IT WAS NOT FOUND. THE ATTACH SECTION OF THE BAR REMAINED CONNECTED AT ITS ATTACH POINTS. METALLURGICAL EXAM DSCLSD THE INNER BAR AND THE TIE ROD FRACTURED IN FATIGUE WHICH HAD PROPAGATED VERY SLOWLY, AND CRACKS WOULD HAVE BEEN EVIDENT. BELL HELICOPTER HAD ISSUED SERVICE BULLETIN 47-(04-2)73-1 REQUIRING DAILY INSPECTION OF THE BAR FOR CRACKS. THE TIE ROD NUT WAS A BOGUS PART AND EXCESSIVELY WORN.
A FATIGUE FAILURE OF BOTH THE MAIN ROTOR STABILIZER BAR TUBE AND THE TIE ROD. IN ADDITION, THE TIE ROD NUT WAS LOOSE AND DID NOT CONFORM TO ENGINEERING DRAWING SPECIFICATIONS. COMPANY PERSONNEL FAILED TO DETECT THE FATIGUE CRACKS DURING DAILY INSPECTIONS.
Source: NTSB Aviation Accident Database
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