NTSB Factual Report

On July 9, 2014, about 1500 central daylight time, the pilot of a Grumman-Schweizer G-164B, N6648Q, made a forced landing in a field 8 miles east of Garrison (D05), North Dakota, after the airplane's engine lost power. The pilot was not injured. The airplane was substantially damaged. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 137 as an aerial application flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight had originated from D05 approximately 1400.

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. He verified engine control continuity. He said there were large punctures in the fuselage, and both wings were wrinkled but contained fuel. Photographs of the airplane at the accident site showed the main landing gear was broken aft and the forward fuselage structure bent aft and crushed upward.

According to the maintenance records furnished by the pilot-operator, the last airframe annual inspection was accomplished on May 17, 2014, and the last engine 100-hour inspection was accomplished 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. General Electric (GE) Corporation purchased Walter Engines in 2008, and the engine was shipped to their facility in Prague, Czech Republic, for disassembly and examination. This took place on November 10-14, 2014, under the auspices of a National Transportation Safety Board (NTSB) aerospace engineer. Also in attendance was a representative from GE Aviation. 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's aerospace engineer said there had been five similar events in the history of the M601 engine. He said the spline was not lubricated and was located in a relatively hot area of the engine. The spline was copper clad to reduce fretting. Newer splines are coated with a black iron oxide-like coating.

The NTSB's aerospace engineer noted that maintenance done on the engine just two days before the accident appeared to have been done improperly. He found a considerable amount of extremely fine black particle in the oil filter that was consistent with excessive spline wear. The wear appeared to have been occurring for some time. Maintenance performed just prior to the accident should have detected the contamination.

As a result of these findings, GE Aviation issued Service Bulletin (SB) M601D/44 that provided instructions for inspecting 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 (EASA is the agent for member states of the European Community, and Czechoslovakia is a member of EASA). On April 10, 2015, FAA issued a Notice of Proposed Rulemaking (NPRM), incorporating the same requirements.

NTSB 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.