On November 1, 2010, about 1415 central daylight time, an amateur-built McFarland Challenger II airplane, N448WM, was substantially damaged when it impacted the ground following an in-flight separation of the left wing during cruise flight. The pilot, who was the sole occupant, was fatally injured. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which was not on a flight plan. The flight originated about 1400 from the Sauk Prairie Airport, Prairie Du Sac, Wisconsin, and was en route to the Merrill Municipal Airport, Merrill, Wisconsin.

Witnesses reported seeing the airplane traveling north when one wing suddenly separated from the airplane. One of the witnesses stated that the other wing separated shortly after the first one. The witnesses estimated that the airplane was 300 to 500 feet above the ground prior to the first wing separation. The airplane subsequently impacted a corn field.

The pilot held a commercial pilot certificate with helicopter and instrument helicopter ratings. The pilot’s most recent second class airman medical certificate was issued on February 17, 2010. On the pilot’s application for the medical certificate, he reported 6,000 total flight hours. The pilot did not have a rating for fixed wing single engine airplanes. A friend of the pilot reported that the pilot had taken several hours of flight instruction in the accident airplane. The pilot’s flight logbook was not available for review during the course of the investigation.

The airplane was a high wing, tricycle gear kit aircraft with a frame structure built from aluminum alloy tubing fastened with bolts and rivets and covered with fabric. The airplane was fitted with amphibious floats and could carry 2 occupants in a tandem seat arrangement. The 65-horsepower Rotax 582 engine was mounted in pusher configuration. According to Federal Aviation Administration records, the airplane was initially certified in the experimental amateur-built category on July 14, 2004. The airplane was sold to the pilot by the widow of the original builder on August 20, 2010.

A review of recorded data from the Baraboo Wisconsin Dells Airport automated weather observation station, elevation 979 feet, revealed that at 1415 the weather conditions were: calm wind; 10 miles visibility; scattered clouds at 3,200 feet above ground level; temperature 8 degrees Celsius; Dew Point 0 degrees Celsius; altimeter setting 30.42 inches of mercury.

The airplane impacted a harvested cornfield. The components came to rest in 3 major groupings; the left wing, right wing, and fuselage. The fuselage structure was upright with the lower fuselage crushed upward. The rudder and vertical stabilizer remained attached to the aft fuselage of the airplane. The right horizontal stabilizer and elevator had separated from the airplane and were found next to the left wing. The left horizontal stabilizer and elevator were separated from the airplane but were located next to the aft fuselage at approximately their normal installed position.

Both wings were completely separated from the airplane with the exception of the right wing aft spar and aileron. The internal structure of the right wing remained attached to the forward spar. The right wing aft spar and control surfaces remained attached to the fuselage. The forward right wing attach fitting was fractured. The left wing was separated from the fuselage and both wing attachment fittings were fractured. The fabric covering was torn and a large portion of the lower wing fabric covering was found a short distance away. The left rear spar and control surfaces exhibited uniform bending along the entire span that resulted in an upward curvature.

Components retained for further examination included 9 wing attachment fittings that exhibited deformation or fracture at the accident scene. Examination of the wing fittings by a specialist in the National Transportation Safety Board's Materials Laboratory revealed signatures consistent with overload failure on all parts exhibiting fractures. The parts that were not fractured exhibited evidence of plastic deformation, with the exception of one fitting that had no perceptible damage due to deformation or fracture. That fitting was the left-rear-lower wing strut attachment fitting that connected the wing strut to the fuselage. The attachment bolt for this fitting was not recovered.

A piece of fabric covering material from the left wing was also retained for testing. A portion of the fabric was sent to a commercial laboratory to perform the ASTM D 5035-08 Breaking Strength of Textile Fabrics (1" Cut Strip Method). The results of this test indicated an average breaking strength of 67.75 pounds.

Federal Aviation Administration minimum performance standards for new intermediate grade fabric are specified in TSO-C14B “Aircraft Fabric Intermediate Grade” and indicate a minimum of 65 pounds as the requirement when applicable.

No preaccident defects were found with regard to the components examined.

An autopsy of the pilot was performed on behalf of the Sauk County Coroner by the University of Wisconsin School of Medical and Public Health, on November 2, 2010. The pilot’s death was attributed to injuries received in the accident.

Toxicology testing was performed by the FAA Civil Aerospace Medical Institute. Testing results were negative for all substances in the screening profile.

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