Grumman Acft Eng Cor-Schweizer G-164B N6648Q accident near Garrison, North Dakota

Probable cause

Excessive wear between the power turbine shaft and quill shaft spline, which led to the disconnection of the power turbine wheel, an overspeed of the power turbine, and the subsequent loss of engine power. Contributing to the accident was maintenance personnel’s failure to detect contaminants in the oil filter during the engine’s recent 100-hour inspection.

NTSB analysis

The pilot reported that he was spraying fungicide on a field when the engine lost power and that he then observed white smoke coming from the exhaust stacks. He feathered the propeller, shut the fuel off, and landed the airplane in a wheat field. The airplane struck a barbed wire fence before coming to a stop, which resulted in substantial damage to the forward fuselage.

The examination of the turboprop engine revealed that wear between the power turbine shaft and the quill shaft had led to the disconnection of the power turbine wheel, an overspeed of the power turbine, and a subsequent loss of engine power. During postaccident examination, a considerable amount of extremely fine black particles were found in the oil filter, consistent with excessive spline wear, which appeared to have been occurring for some time. Maintenance records indicated that the last 100-hour inspection of the engine was accomplished 2 days before the accident. It is likely that maintenance personnel did not examine the oil filter during that inspection. If they had examined the oil filter, they would have detected the contaminants, which would have indicated an impending failure of an internal engine component. There have been five similar events involving the same make and model engine, and the manufacturer issued a service bulletin with instructions for inspecting the engine; the Federal Aviation Administration subsequently issued a notice of proposed rulemaking to require the inspection.

Text from the NTSB. Open the NTSB docket for CEN14LA358 for the full record.