Schlater, MS, USA
N9185F
Air Tractor AT-502
The pilot stated he was on takeoff roll to conduct aerial application flight. Just before rotation he heard a loud pop and observed sparks in the vicinity of the engine compartment. There was insufficient runway remaining to stop the airplane. The airplane went off the end of the runway and collided with a ditch. Examination of the engine assembly revealed a total failure of several compressor turbine blades. Metallurgical examination of the compressor turbine blades revealed voiding and grain separation were observed toward the tip grain boundary along a reaction zone which is indicative of creep propagation. A comparison of the microstructure at the airfoil tip revealed a significant difference in the structure with the tip appearing to be solutioned. According to the original equipment maintenance manual blade tips that are solutioned is an indication of over temperature. if the inter-turbine temperature exceeds 875-degrees Celsius for 2 seconds the engine is required to be returned to an overhaul facility. It could not be determined when or who exceeded the over temperature limit.
On March 5, 2002, about 1230 central standard time, an Air Tractor AT-502, N9185F, registered to Schlater Flying Inc., operating as a 14 CFR Part 137 aerial application flight experienced a loss of engine power on takeoff from a private airfield in the vicinity of Schlater, Mississippi. Visual meteorological conditions prevailed and no flight plan was filed. The airplane sustained substantial damage. The commercial pilot reported no injuries. The flight was originating from Schlater, Mississippi, at the time of the accident. The pilot stated he was taking off to the north. Just before rotation he heard a loud pop and observed sparks. He stated, there was insufficient runway remaining to decelerate and stop the airplane. The airplane went off the end of the runway collided with a ditch separating the landing gear. The engine assembly and accessories were transported to Pratt and Whitney Engine Services, Inc., Atlanta, Georgia, for further analysis. The engine displayed minimal impact damage. Disassembly of the engine revealed the compressor 1st stage blades and shroud exhibited circumferential rubbing. The compressor turbine guide vane ring airfoil leading edges displayed no distress. The trailing edges were heavily battered and gouged due to contact with separated blade debris. The vane ring inner drum forward lip was fractured from the guide vane assembly. Under unaided visual and macroscopic inspection the fracture surface displayed coarse granular features. The compressor turbine shroud was heavily battered, gouged, and circumferentially scored due to contact with the compressor turbine blades. The compressor turbine blades were numbered 1-58 counterclockwise in reference to the master spline for reference. Blades No. 9, 11 through 19, 21, 44, and 45 were fractured at their roots. Under unaided visual macroscopic inspection the root fracture surfaces displayed coarse granular features. The remaining blades were battered and gouged characteristic of contact with the separated blade debris. The platform upstream side of blades No. 33 through 13 displayed heavy circumferential rubbing due to contact with the compressor turbine guide vane ring inner drum. The blade part number being prefixed with a "T" denotes the blades to be non-Pratt and Whitney Canada manufactured units. The interstage turbine temperature probes were fractured due to contact with separated debris, and the harness was intact. The power turbine guide vane ring and interstage baffle vane airfoils were battered and gouged due to contact with debris. The power turbine shroud was fractured from its seat and located within the exhaust duct. The shroud was heavily battered, gouged, and circumferentially scored due to contact with separated blade debris. The power turbine blades were fractured at varying heights due to contact with event generated debris. The compressor turbine disc and compressor turbine blades were forwarded to the NTSB Materials Laboratory for further examination. The compressor turbine disk was not examined by the NTSB Materials Laboratory due to lack of damage. The metallurgical examination of the turbine blades by the NTSB laboratory revealed cracking on the convex root of the airfoil. A cross-sectional metallographic mount through the root crack confirmed that the crack propagated intergranulary. Toward the crack tip grain boundary voiding and grain separation were observed along a reaction zone indicative of creep propagation. A comparison of the microstructure at the airfoil tip revealed a significant difference in the structure with the tip appearing solutioned. According to original equipment maintenance manual the blade tips being solutioned is an indication of over temperature and is cause for rejection. The solutioning temperature for the alloy is somewhere in the 2,000 degrees Fahrenheit range. Review of Pratt and Whitney Canada Maintenance Manual figure 501 (Sheet 2) states on Page 507, If the inter-turbine temperature exceeds 865-degrees celcius for 2 seconds return the engine to an overhaul facility. The compressor and power turbine blades must be discarded and both turbine fisks subjected to stretch check and flourescent penetrant inspection. The components examined by the NTSB Materials Laboratory were returned to Ray E. Ladd and Associates Inc., on January 3, 2003.
an unknown person at an unknown time exceeded the engine temperature limitations that resulted in the failure of compressor turbine and the subsequent loss of engine power during takeoff.
Source: NTSB Aviation Accident Database
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