NTSB Identification: LAX96LA196.
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Accident occurred Saturday, May 11, 1996 in WILLIAMS, CA
Probable Cause Approval Date: 12/16/1996
Aircraft: Ayres S2R-G6, registration: N447AT
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 reported that as the aircraft reached the takeoff rotation point, the engine's turbine section disintegrated, throwing large chunks of turbine wheels and other debris through the top and side of the engine cowling. The pilot lost directional control while attempting to stop the aircraft on the 2,700 foot-long dirt runway and it veered off the right side of the airstrip, crossed a drainage canal, and collided with the opposite embankment. Engine disassembly revealed that the second stage turbine wheel rim separated over almost its entire circumference. Detailed examination of the hub's fracture found machining type scoring to a depth of about 0.1 inches on the forward face of the wheel, which reduced the strength in the wheel web until overload failure occurred. Inspection of the second stage turbine stator assembly found that the sheet metal support for the turbine wheel to stator air seal was separated from the stator and had shifted forward to contact the failed wheel. Twenty-two out of 31 cooling passages which pass through the stator vanes from the outside diameter of the stator to the seal support area were completely plugged by braze repair material. The braze material had flowed from the outside diameter toward the inside during an attempt to braze repair the sheet metal on the castings outside diameter. The seal support showed evidence of operation at a temperature a few hundred degrees hotter than normal. Allied Signal reported that this overtemp condition will cause the seal support to crack after a few hundred hours of operation. Review of the Allied Signal overhaul documents for the second stage stator assembly disclosed that a braze repair is only authorized for the rear air seal attachment to the casting, and not in the area of the cooling passages. The historical engine maintenance records disclosed that the second stage stator was overhauled on August 23, 1995, and installed in the engine on October 23, 1995, 207 hours and 230 cycles since overhaul.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: The pilot's failure to maintain directional control during a rejected takeoff, and the catastrophic failure of the engine's turbine section due to an improper overhaul procedure. A factor in the accident was the close proximity of the drainage ditch to the edge of the private airstrip's runway. Full narrative available
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