Accident Overview
On August 27, 2015, about 1200 central daylight time, an Air Tractor AT-502B, registration N61267, experienced a partial loss of engine power during low-altitude maneuvering flight and impacted terrain near Monroe, Nebraska. The pilot sustained minor injuries, and the airplane sustained substantial damage to both wings, the horizontal stabilizer, and the elevator. The airplane was registered to and operated by Red Willow Aviation Spraying, Inc., of McCook, Nebraska, as a 14 Code of Federal Regulations Part 137 aerial application flight. Day visual meteorological conditions prevailed, and no flight plan was filed for the local flight, which had departed about 1130 from Genoa Municipal Airport (97Y), Genoa, Nebraska.
Flight Details
The pilot was seeding a cover crop into corn when he experienced a partial loss of engine power. Engine instruments indicated the engine was operating but producing less than idle power. The pilot advanced the throttle to full power with no effect and decided to land. He turned the airplane into the wind and lowered the flaps. As the pilot heard corn contacting the bottom of the airplane, he pulled the control stick full aft to stall the airplane. The left wing dropped, the airplane turned 90 degrees, and impacted the ground sideways. The right wing subsequently struck the ground, and the airplane came to rest upright. The engine continued running until the pilot shut it down and exited the airplane.
Wreckage Examination
Examination of the wreckage revealed adequate fuel on board, no fuel spill, and no postimpact fire. The airplane's engine and propeller governor were examined and no anomalies were found. The fuel control unit and fuel pump were removed and sent to the manufacturer in Prague, Czech Republic, for examination; no anomalies were found. A Turbine Start Limit and Monitor (TSLM) and a USB thumb drive containing GPS data from a GPS unit on board were sent to the NTSB Vehicle Recorder's Laboratory for examination. The GPS data on the USB thumb drive did not include the accident flight.
Data Analysis
The TSLM, which monitors internal turbine temperature, N1, N2, torque, oil pressure, and bus voltage, recorded 74 events. Event 74 was triggered by the N1 parameter exceeding the user-set upper limit. The exceedance ended 1.5 seconds into the event. Following the exceedance, internal turbine temperature, N1, and N2 (propeller rpm) decreased over time, consistent with lower engine output. The TSLM data was reviewed by the engine manufacturer, who concluded that the propeller went into feather either due to an incorrect pilot command or a governor problem. The manufacturer also noted that the engine was operated above the manual temperature limits at the time of the event and that the internal turbine temperature and core speed did not correlate correctly, which could be attributed to scaling of the data or improper engine operation.