What happened
On November 27, 2013, a Biesemeier Smyth Sidewinder experimental amateur-built airplane, N3786A, crashed near Boulder, Colorado. The flight departed from Rocky Mountain Metropolitan Airport (BJC) at approximately 11:30 and was en route to Loveland, Colorado.
Radar data indicated that the aircraft maintained a normal departure and climbout for the first 15 minutes of the flight. This was followed by a sudden turn and a rapid descent. A witness reported hearing a loud sound and seeing the aircraft falling from the sky. While witnesses noted the presence of migrating bird flocks in the area at the time of the accident, no radio calls or distress signals were received from the pilot. The pilot, who was the sole occupant, sustained 1 fatal injury.
The investigation
The aircraft's fuselage was located in a creek bed, while the left wing was found approximately 1,156 feet from the main wreckage. Investigators examined the engine and flight controls, finding no mechanical anomalies. The wooden propeller blades showed fractures consistent with the engine being under power upon impact.
Examination of the left wing revealed that the lower left spar cap had fractured about 8 inches outboard of the wing-to-fuselage attachment point. The upper spar cap members exhibited significant S-type bending and fractures between 10 and 12 inches outboard of the attachment point. The fracture faces on both the upper and lower spar cap members had a dull, angled, and grainy appearance. The investigation found no evidence of pre-existing cracks or an in-flight bird strike.
Findings
Structural analysis of the wing components indicated a positive overload condition. Specifically, the lack of deformation in the lower spar cap was consistent with a tension overload failure, while the bending of the upper spar cap was consistent with a compression overload failure. These findings suggest the wing separated due to excessive forces applied to the structure.