MILPITAS, CA, USA
N91338
PIPER PA-28-161
WHILE CLIMBING TO CRUISE PLT HEARD A LOUD KNOCKING NOISE FOLLOWED BY 'VIOLENT VIBRATIONS' & A LOSS OF POWER. PLT MADE A FRCD LNDING IN A NEWLY EXCAVATED CONSTRUCTION SITE & WENT OVER AN EMBANKMENT. INVES REVEALED DEFORMATION OF THE #4 CYLINDER VALVE COVER. #4 EXHAUST VALVE ROCKER ARM WAS FOUND TO BE BROKEN. REVIEW OF ACFT MAINTENANCE RECORDS REVEALED THAT AD 87-10-06 WHICH INCORPORATES LYCOMING SERVICE BULLETIN (SB) 477A WAS ACCOMP 78.7 HRS PRIOR TO THE ACCIDENT. THE SB REQUIRES ROCKER ARMS THAT DO NOT HAVE A WALL THICKNESS OF.075 BE REPLACED. IF THE ROCKER ARM PASSES THE THICKNESS EXAM THE ROCKER ARM OIL DRIP HOLE OUTER EDGE MUST BE 'BURRED' TO THE APPROX .030 RADIUS PRIOR TO REASSEMBLY. THE ROCKER ARM WAS MEASURED USING A ROUND ANVIL MICROMETER. THE MEASUREMENT MET THE MIN DIMENSION OF .075. AFTER THE ACCIDENT, MET LAB EXAM REVEALED FRACTURE FEATURES OF HIGH CYCLE FATIGUE & OVERSTRESS. THE ORIGIN OF THE FATIGUE WAS TRACED TO THE REWORKED OIL DRIP HOLE WHICH DID NOT MEET .030 RADIUS. THE WALL THICKNESS MEASURED .055 USING A MEASURING MICROSCOPE.
Source: NTSB Aviation Accident Database
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