Primary finding
Probable cause
The loss of engine power as the result of the fatigue fracture and separation of two compressor turbine blades, which induced a catastrophic failure of the compressor turbine section.
Investigator assessment
Analysis narrative
The engine lost power about 20 to 30 feet above ground level in the takeoff initial climb. The pilot landed in a pasture, with the airplane coming to rest upright after colliding with several fences. A post accident examination of the engine revealed that the engine failure was the result of the fatigue fracture and separation of two compressor turbine blades. Once liberated, the two blades impacted the other compressor turbine blades, which resulted in the airfoil portions of those remaining blades to fracture in overstress. The fatigue crack in one blade occupied about 40 percent of the fracture surface, while the fatigue crack in the second blade occupied about 60 percent of the fracture surface.
Source record
Factual narrative
On June 30, 2007, about 0800 Pacific daylight time, an Air Tractor Inc., AT-502A, N6176L, operated by Thiel Air Care, Inc., of Chowchilla, California, sustained substantial damage following a loss of engine power and subsequent forced landing about 9 nautical miles southwest of Chowchilla. The certificated commercial pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed at the time of the accident, and a flight plan was not filed. The aerial application flight was being operated in accordance with 14 Code of Federal Regulations Part 137, and was originating from a private airstrip at the time of the accident. In a written statement submitted to the National Transportation Safety Board investigator-in-charge (IIC), the pilot reported the after taking off and about 20 to 30 feet [above ground level], the engine failed. The pilot stated that he dumped the load and added full flaps before assuming a three-point attitude and landing in a pasture. The airplane subsequently went through two fences before coming to rest in an upright position. The pilot reported that most of the damage to the airplane was caused by the left wing impacting a corner post. A post accident damage assessment of the airplane revealed that both ailerons, both flaps, and the underside of both wings sustained substantial damage. It was also revealed that the right wing's rear spar was bent. At the direction of the IIC, the engine's compressor turbine disc and 59 associated turbine blades were sent to the NTSB Materials Laboratory in Washington, D.C. for examination. A Senior Metallurgist reported that the blades are manufactured with serrations at the root portion, and that when viewed from the pressure or non-pressure side, a blade root contains three serrations. The metallurgist further reported that a visual examination revealed that all of the compressor turbine blades were fractured in the airfoil portion, with the exception of blades "10" and "13"; these two blades were fractured through the first serration adjacent to the root platform. Bench binocular microscope examination of the fracture faces on blades "10" and "13" revealed crack arrest marks, a smoothly curving boundary, and a more reflective surface, features typical of fatigue cracking. It was also revealed that blades "10" and "13" fractured through the serration portion and the remaining blades fractured at the airfoil portion. Additionally, the fracture faces of blades "10" and "13" in the areas located outside of the fatigue crack regions showed ductile dimple features typical of overstress separation. A representative selection of the other blades were also examined with the scanning electron microscope, which revealed that these fractures also contained ductile dimple features typical of overstress. Maintenance records revealed that the total time on the compressor turbine blades since new was 420.2 hours, and the total time on the compressor turbine disc since new was 5605.9 hours.