Accident Summary
On July 9, 2014, about 1500 central daylight time, a Grumman-Schweizer G-164B (registration N6648Q) made a forced landing in a field 8 miles east of Garrison (D05), North Dakota, after the engine lost power. The pilot, who was the sole occupant and operator, was not injured. The airplane was substantially damaged. The flight was conducted under 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed, and no flight plan was filed. The local flight had originated from D05 approximately 1400.
Pilot Account and Initial Examination
According to the pilot, he had been spraying fungicide on a field. As he pulled up at the end of the third pass, the engine lost power and he observed white smoke coming from the exhaust stacks. He feathered the propeller, shut the fuel off, and landed in a wheat field. The airplane struck a barbed wire fence before coming to a stop.
A Federal Aviation Administration (FAA) aviation safety inspector examined the airplane at the accident site. Engine control continuity was verified. The inspector noted large punctures in the fuselage, and both wings were wrinkled but contained fuel. Photographs showed the main landing gear broken aft and the forward fuselage structure bent aft and crushed upward.
Maintenance and Engine History
According to the maintenance records, the last airframe annual inspection was accomplished on May 17, 2014, and the last engine 100-hour inspection on July 7, 2014. At the time of the accident, airframe total time was 8,690.9 hours and engine total time was 1,675.3 hours. The airplane was powered by a Walter M601 turboprop engine (serial number 901048), manufactured on March 28, 1990.
Engine Examination and Findings
General Electric (GE) Corporation, which purchased Walter Engines in 2008, disassembled and examined the engine at their facility in Prague, Czech Republic, from November 10-14, 2014. An NTSB aerospace engineer and a GE Aviation representative were present. According to GE's "Metallurgical Investigation Report," a power-turbine-shaft-to-quill-shaft spline failure had caused a loss of connection with the power turbine wheel, resulting in an over-speed of the power turbine and subsequent loss of engine power.
In an e-mail dated November 18, 2014, the NTSB aerospace engineer stated there had been five similar events in the history of the M601 engine. The spline was not lubricated and was located in a relatively hot area of the engine. It was copper clad to reduce fretting; newer splines are coated with a black iron oxide-like coating.
Improper Maintenance Indicated
The NTSB aerospace engineer noted that maintenance done on the engine just two days before the accident appeared to have been done improperly. A considerable amount of extremely fine black particles was found in the oil filter, consistent with excessive spline wear. The wear appeared to have been occurring for some time, and the maintenance performed just prior to the accident should have detected the contamination.
Subsequent Service Actions
As a result of these findings, GE Aviation issued Service Bulletin (SB) M601D/44 providing inspection instructions for the M601 engine. This was followed by the European Aviation Safety Agency (EASA) issuing Airworthiness Directive (AD) 2015-0014 on January 30, 2015, mandating a one-time inspection of the reduction gear box and supporting cone for cracks or wear exceeding 0.12 mm on the quill shaft, power turbine shaft, or supporting cone. On April 10, 2015, the FAA issued a Notice of Proposed Rulemaking (NPRM) incorporating the same requirements.