NTSB Identification: LAX00FA169.
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Accident occurred Saturday, April 22, 2000 in QUEEN CREEK, AZ
Probable Cause Approval Date: 12/18/2001
Aircraft: Air Tractor 502A, registration: N15466
Injuries: 1 Fatal.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
The airplane was spraying chemicals on a field of crops when the left wing separated in flight. Ground witnesses said that the airplane was making a level spray pass over a potato crop when the left wing failed. No unusual meteorological phenomena were observed in the area, and the airplane did not perform any unusual maneuvers prior to the failure. The left wing's lower spar cap was found to have fractured in fatigue at a point that transected the inboard attach block bolt hole where the spar attaches to the center section spar carry through member. Multiple origin sites were identified within the bolt hole bore, with corrosion pitting evident at the origin points. The spar caps met the hardness, chemistry, and microstructure of the production drawing specified material. Air Tractor Service Letter 118 and the maintenance manual specify an inspection procedure, which calls for the removal of the hopper for a detailed inspection of the spar caps for cracks; one area identified as critical for cracking was the inboard bolt hole bore. The inspection interval was specified as every 2,000 hours or 3 years. The number of crack arrest striations in the fatigue region was determined from the first point the crack would be visible from beneath the attach block during the specified inspection, and the mean time for the fatigue crack to progress from detectible during the inspection to failure was calculated to be about 105 hours. Examination of the right wing lower spar cap revealed the presence of multiple fatigue cracks emanating from corrosion pits within the outboard bolt hole bore. No maintenance records were located for either the airframe or engine. A mechanic was located who performed some periodic maintenance on the aircraft. He stated that during the removal of the hopper in May 1997, 2 years 11 months prior to the accident, he had accomplished the 2,000-hour check of the wing spars in the area of the attach bolts as required by Service Letter 118. Based on other records, the aircraft was believed to have flown an average of 500 hours per year and would have accumulated only 1,500 hours since the inspection.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The fatigue failure and in-flight separation of the left wing due to the manufacturer's underestimation during the aircraft design process of the time interval from fatigue crack initiation to failure, and, the manufacturer's subsequent specification of an inadequate inspection interval. Full narrative available
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