Grumman G-164A N9892 accident near Patoka, Illinois

Probable cause

A partial loss of engine power due to the failure of two compressor turbine blades as a result of the engine operating near or above the engine’s operating limits for an undetermined amount of time beyond the recommended engine’s TBO limit, which led to material creep and overload failure.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN23LA317 for the full record.