DFW07LA138 · Air Tractor AT-401

The 5,475-hour commercial pilot reported that after performing an aerial application pass in his radial-engine-powered airplane, he initiated a turn to reverse direction to start another pass. At the top of the turn, he heard a loud bang from the radial engine, followed by the sound of the engine winding down. He also noticed white smoke from the exhaust, a loss of power, and a drop in manifold pressure. The pilot made an forced landing to a field, where the airplane rolled about 250 yards before it flipped over. The airplane was released to the owner after the accident, who later sold it to another operator. Due to the damage sustained in the accident, the engine was disassembled and examined before the new owner could return the airplane back to service. The new owner provided an engine disassembly report. According to the report, "undetermined causes resulted in distress to the phenolic separator bracket of the rear impeller support bearing resulted in its separation and fragmentation." The reports adds that "this initiated a chain reaction of events throughout the rest of the engine as clearances, lack of lubrication and subsequent wearing of engine components took place." This damage ultimately resulted in destruction of the blower labyrinth seal and the impeller gear train, which reduced power output. The engine reports concludes that "the destroyed impeller drive gear train and the distress caused by the displaced blower spring drive mechanism, coupled with reduced power output, resulted in seizure of the propeller shaft/crankshaft and complete loss of power."

Verbatim NTSB analysis and probable cause from the NTSB dataset