On June 24, 2011, about 1943 mountain daylight time, an Air Tractor AT-502, N1022N, sustained substantial damage during a forced landing near Lewistown, Montana. The airplane was registered to and operated by SRB Applications, Charleston, Mississippi, under the provisions of Title 14 Code of Federal Regulations Part 137. The commercial pilot, sole occupant of the airplane, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the local aerial application flight. The flight originated from a staging area near Lewistown about 45 minutes prior to the accident.

During a telephone interview with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), the pilot reported that upon completion of his aerial application on a nearby field, he was returning to the staging area to refill the airplaneā€™s hopper. While returning, in cruise flight about 800 feet above ground level, the engine lost power and the pilot initiated a forced landing to an area of open, uneven terrain. During the landing roll, the left wing struck the ground and the airplane ground looped.

Examination of the airplane by a Federal Aviation Administration (FAA) inspector revealed that the outboard left wing was structurally damaged and buckling was observed on the fuselage between the cockpit and empennage. Local law enforcement reported that fuel was present in both wing fuel tanks. Examination of the engine revealed that the fuel bypass line, part number 3011849, was fractured and separated from the fuel control unit.

The fractured fuel bypass line was removed from the engine and shipped to the NTSB Materials Laboratory, Washington, DC, for further examination.

An NTSB senior metallurgist reported that the tube portion of the fuel bypass line fractured near a ferrule at the edge of the braze filler metal fillet. Fractographic evaluation revealed that the fracture initiated on the outside surface of the tube and propagated inward and circumferentially around the tube wall. Ratchet marks on the fracture surface revealed the region where the crack initiation and direction of crack propagation. The presence of ratchet marks is consistent with fatigue fracture. A scanning electron microscope was used to examine the fracture initiation region, which revealed striations consistent with fatigue crack propagation. The orientation of the fatigue striations is consistent with the fracture initiating on the outside surface of the tube and propagating towards the inside surface and circumferentially around the tube wall.

No maintenance records for the airplane were recovered during the course of the investigation.

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