WILLIAMS, CA, USA
N447AT
Ayres S2R-G6
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.
On May 11, 1996, about 0800 Pacific daylight time, a Ayers S2R-G6, N447AT, collided with ground obstructions during a rejected takeoff at a private agricultural airstrip near Williams, California. The aborted takeoff was precipitated by a catastrophic failure of the airplane's Garrett TPE-331 turboprop engine at the rotation point in the takeoff ground roll. The aircraft was owned and operated by Caldwell Flying Service, Inc., of Williams, and was beginning a local area aerial application flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft sustained substantial damage. The certificated commercial pilot, the sole occupant, was not injured. 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. The Garrett TPE-331-6 engine, serial number 20025C, was removed from the airframe and shipped to the facilities of Allied Signal Aerospace (Garrett) for disassembly and investigation under the supervision of an FAA airworthiness inspector from the Scottsdale, Arizona, Flight Standards District Office. The disassembly revealed that the second stage turbine wheel rim separated over almost its entire circumference. The wheel hub remained in the engine. Detailed examination of the fracture area on the hub revealed machining type scoring to a depth of about 0.1 inches on the forward face of the wheel. According to a materials laboratory report from Allied Signal, the scoring reduced the strength in the wheel web until overload failure occurred. Inspection of the second stage turbine stator assembly revealed that the air seal between the stator and second stage turbine wheel was destroyed. The sheet metal seal support is attached to the inner diameter of the second stage stator and was noted to be separated from the stator. Allied Signal reported that a separation of the aft side of the seal support can allow the sheet metal support to deform and contact the second stage turbine wheel. Further detailed examination of the stator disclosed that 22 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 examining metallurgist opined that the material flowed from the outside diameter toward the inside during an attempt to braze repair the sheet metal on the castings outside diameter. The metallurgist also noted that examination of the remaining seal support material showed evidence of operation at a temperature a few hundred degrees hotter than normal. Allied Signal reported that their past experience is that "depending on the number of cooling holes plugged the seal support will 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. Examination of 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. The overhaul was accomplished by AIM, Inc., of Durant, Oklahoma. The engine was installed in the airframe on November 2, 1995. At the time of the accident the stator had accumulated 207 hours and 230 cycles since overhaul.
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.
Source: NTSB Aviation Accident Database
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