GRUMMAN ACFT ENG COR-SCHWEIZER G-164B

Full Narrative

On July 19, 2021, about 1410 central daylight time, a Grumman G-164B agricultural airplane, N8422K, was substantially damaged when it was involved in an accident near Amana, Iowa. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 137 aerial application flight.
The airplane operator reported that the airplane’s engine lost power shortly after departure from runway 26 at the Amana Airport (C11). The accident takeoff was the 6th takeoff of the day after the engine was started about 0630. Before takeoff, the airplane was loaded with 420 gallons of fungicide and 110 gallons of fuel. The pretakeoff engine indications were normal and the airplane accelerated normally, lifted off at an airspeed of about 65 mph, and then accelerated in ground effect to about 78 mph when the engine lost power. The airplane then settled back into a cornfield about 300 ft from the end of the runway. The airplane nosed over and came to rest inverted and sustained substantial damage to the fuselage and wings.
During a postaccident examination, all three propeller blades were bent aft and opposite the direction of rotation. The blades were twisted toward low pitch and chordwise/rotational scoring was visible predominately on the cambered side of the blades. Additionally, the propeller examination found no visible preimpact discrepancies that would have prevented normal propeller operation.
The engine was a Honeywell model TPE331-6-252M turbo-propeller engine. It was a single-shaft engine with a two-stage centrifugal compressor, an annular combustion chamber, and a three-stage axial turbine that drives the compressor and the reduction/accessory gearbox. An attempted engine run at the manufacturer’s facility indicated low torque indicative of a discontinuity in the drive system. Subsequent disassembly of the engine confirmed that the aft end of the torsion shaft was found free and resting against the splined end of the main shaft. Further examination of the engine revealed that burnt fibrous material was found throughout the engine. No pre-impact damage was found in the engine that would have precluded normal operation.
The propeller governor and fuel control unit were tested at the manufacturer’s facility. Except for slight deviations attributed to possible field adjustments, both the governor and fuel control unit operated within the manufacturer’s specifications. No anomalies were detected that would have prevented normal operation.