Accident
On June 24, 2011, about 1943 mountain daylight time, an Air Tractor AT-502, registration N1022N, sustained substantial damage during a forced landing near Lewistown, Montana. The aircraft was registered to and operated by SRB Applications of Charleston, Mississippi, under Title 14 Code of Federal Regulations Part 137. The commercial pilot, the sole occupant, 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.
Pilot Report
In a telephone interview with the National Transportation Safety Board (NTSB) investigator-in-charge, the pilot reported that after completing an aerial application on a nearby field, he was returning to the staging area to refill the aircraft's hopper. While cruising at about 800 feet above ground level, the engine lost power. The pilot initiated a forced landing to an area of open, uneven terrain. During the landing roll, the left wing struck the ground, causing the aircraft to ground loop.
Examination Findings
An examination by a Federal Aviation Administration (FAA) inspector revealed structural damage to the outboard left wing and buckling 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 disclosed that the fuel bypass line, part number 3011849, was fractured and separated from the fuel control unit. The fractured line was removed and sent to the NTSB Materials Laboratory in Washington, DC, for further examination.
Metallurgical 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 indicated 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 identified the crack initiation region and direction of propagation, consistent with fatigue fracture. A scanning electron microscope examination revealed striations consistent with fatigue crack propagation. The orientation of the striations aligned with the fracture initiating on the outside surface and propagating toward the inside surface and circumferentially. No maintenance records for the aircraft were recovered during the investigation.