CEN23LA317

While conducting an aerial application flight, the pilot observed a loss of engine power. Because the airplane was at a low altitude and its airspeed was decreasing, the pilot performed a forced landing in a corn field, which resulted in the airplane sustaining substantial damage to both wings, the fuselage, and empennage. An examination of the airplane’s turboprop engine showed rotational scoring, hard body impact damage, and missing material throughout the compressor turbine (CT) and power turbine (PT) sections of the engine, indicating that the engine was producing some level of power at the time of impact. Multiple CT blades were fractured above the blade platform and about 75% of the PT blades were fractured at various lengths above the blade platform. Metallurgical examination of two of the fractured CT blades showed material necking near the fracture surfaces, microstructural rafting, creep voids, and localized alteration of the microstructure that is consistent with heat-related material degradation. Examination of the fuel control unit showed the maximum Ng speed adjustment screw was adjusted to the maximum fuel flow position and lock-wired in place. Based on the condition of the blades, the loss of engine power was likely due to the failure of two CT blades as a result of the engine being operated near or above the engine’s operating limits for an undetermined amount of time beyond the recommended engine’s time between overhaul (TBO) limit; this led to material creep and the overload failure of the CT blades.

On July 22, 2023, about 1113 central daylight time, a Grumman G-164A, N9892, was substantially damaged when it was involved in an accident near Patoka, Illinois. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 agricultural flight. The pilot reported that during the 4th pass of the chemical spray load, the the engine lost power. Based on the airplane’s low altitude and decreasing airspeed, the pilot elected to make a forced landing into the corn field. Both wings, the fuselage, and the empennage sustained substantial damage. Flight control continuity was confirmed. The airplane was retained for further examination. The airplane’s turboprop engine was examined at the manufacturer’s facility. The examination showed rotational scoring, hard body impact damage, and missing material throughout the CT and PT sections of the engine. Multiple CT blades were fractured above the blade platform and about 75% of the PT blades were fractured at various lengths above the blade platform. Metallurgical examination of two blades from the CT showed material necking near the fracture surfaces, microstructural rafting, creep voids, and localized alteration of the microstructure consistent with heat related material degradation. Examination of the fuel control unit (FCU) showed the maximum Ng speed adjustment screw was found adjusted to the maximum fuel flow position and was lock wired in place. Additionally, the FCU housing was missing a bolt and showed a fatigue fracture which created a gap between the FCU housings. Movement of the power control linkage was observed to cause a reactive open and close movement of the gap between the two housings. The operator reported the engine’s time since new (TSN) was 14,092 hours. The time since overhaul (TBO was 6,738.5 hours and the time since inspection was 226 hours. A review of the engine logbooks showed the airplane underwent an annual inspection about two months before the accident. The recorded (TSN) at the annual was 14,217 hours. A corresponding airplane logbook entry showed the airplane TSN as 12,475 hours. The engine manufacturer’s recommended TBO is 3,600 hours.