CEN14FA072

Radar data showed that, during the first 15 minutes of the cross-country flight, the experimental, amateur-built, airplane had a normal departure and climbout. However, the airplane then suddenly turned and rapidly descended. A witness reported that he heard a loud sound, looked up, and saw the airplane falling out of the sky. Another witness reported seeing flocks of migrating birds in the area about the time of the accident. No radio calls or distress transmissions were received from the pilot. The airplane's left wing was found about 1,156 ft from the main wreckage. Examination of the engine and flight controls did not reveal any mechanical anomalies that might have contributed to the accident. Examination of the left wing revealed no evidence of preexisting conditions that would have contributed to its separation. The wing's left lower spar cap was fractured about 8 inches outboard of the attachment point through the two rivet holes immediately outboard of the wing-to-fuselage fittings. There was no obvious deformation of the lower spar cap members in the fracture area. The upper spar cap members were fractured between 10 and 12 inches outboard of the attachment point with significant S-type bending of the members in the fracture area. The upper shear bolt remained intact and installed, but the lug portion of the fuselage fitting was fractured. The fracture faces on the upper and lower spar cap members all had a dull, angled, grainy appearance consistent with overstress separation. The lack of deformation of the lower spar cap members, the bending deformation of the upper spar cap members, and the appearance of the fracture faces on the individual members are consistent with a tension overload failure of the lower spar cap and compression overload failure of the upper spar cap. The evidence is consistent with the separation of the left wing due to a positive overload condition. Although flocks of migrating birds were seen in the area about the time of the accident, no evidence was found indicating that an in-flight bird strike occurred. However, it is likely that the pilot made an abrupt maneuver, possibly to avoid birds, which resulted in excessive g forces on the wing structure and led to its separation. An autopsy of the pilot did not reveal any medical conditions that might have contributed to the accident.

The airplane wreckage was released to the owner's representative. The pilot had built and owned the airplane since 1981. The airplane was issued its Experimental Amateur-Built airworthiness certificate on April 7, 1982. Build records that were provided exhibited detail and all maintenance logs were current and up to date. The two seat (side-by-side) airplane was powered by a Lycoming O-360 series reciprocating engine. Documentation found with the maintenance logs showed that the Sidewinder design was conceived in the mid 1960s by a formal Naval fighter pilot. Contemporaries of the Sidewinder would be airplanes such as the Tailwind, T-18, Cavalier, and Mustang II. Its attributes included; fighter-like control response, a smooth aerodynamic look, and easy handling for low time pilots. Its design could accommodate limited aerobatics at plus-6 and minus-3 Gs. It's maximum cruise speed was 150 miles per hour. The wings are all metal with built-up, riveted spars, formed aluminum ribs, and internal fiberglass wing fuel tanks. The wings are similar in design to the Mustang II, T-18, or RV-6. The airfoil is a NACA 64-212 laminar flow. There are 11 ribs in each wing, formed over wood blocks, and each rib is unique because the wing is tapered. On November 27, 2013, about 1150 central daylight time, a Biesemeier manufactured Smyth Sidwinder experimental homebuilt airplane, N3786A, registered to the pilot, was destroyed when it impacted terrain after an in-flight separation of it's left wing while maneuvering over a rural area near Boulder, Colorado. The private pilot, who was the sole occupant, sustained fatal injuries. The personal flight was being conducted under the provisions of Code of Federal Regulations Part 91. Visual meteorological conditions prevailed throughout the area and a flight plan was not filed. The flight originated about 1130 from the Rocky Mountain Metropolitan Airport (BJC), and it's intended destination was Loveland, Colorado. During examination on site, the airplane's fuselage was found in a creek bed. The left wing of the airplane was located about 1/3 of a mile from the main wreckage. The wreckage was recovered to a secure facility in Greeley, Colorado for further examination. Radar data showed a normal departure and climbout from Rocky Mountain Airport during the first 15 minutes of flight, until a sudden turn and rapid descent. A witness in the area reported that he heard a loud sound, looked up, and saw the airplane falling out of the sky. Another witness reported that there were flocks of migrating birds in the area about the time of the accident. No radio calls or distress transmissions were reported from the pilot. When the NTSB investigator arrived on-scene later in the day, he observed flocks of migrating birds in the area. Recovery of the airplane wreckage was accomplished on the day after the accident. Flocks of migrating birds were observed on that day as well. An autopsy of the pilot was performed by the Office of theBoulder County Coroner on February 1, 2014, and the cause of death was listed as "Multiple Blunt Trauma Injuries." Toxicology tests performed by the FAA Civil Aeromedical Institute were negative for drugs, alcohol, and carbon monoxide. The pilot held a valid FAA Private Pilot certificate with an Airplane Single Engine Land rating. He also held a valid FAA Mechanic Airframe & Powerplant certificate. His total flight experience, as reported on his most recent FAA medical application, was about 1,977 hours. The FAA issued the pilot a Third Class Medical Certificate on June 4, 2013. The certificate had an annotation that the pilot "must wear corrective lenses." No pilot logbooks were located during the investigation. According to an associate of the pilot, he had about 900 hours in the accident airplane and flew it about once a week. The main spar at the inboard end was a simple built up structure consisting of 8 aluminum straps on both the upper and lower spar caps with a thinner web installed between. Four straps are installed on the forward side of the web and four are installed on the aft side. Steel fittings are installed on the forward and aft sides of the spar caps at the inboard end to produce a clevis fitting for installation of the wing on the airplane. The clevis fittings mate with lugs at the airplane side-of-body with a shear bolt installed to attach the main spar to the fuselage. The left lower spar cap fractured about 8 inches outboard of the attach point through the two rivet holes immediately outboard of the fittings. The lower shear bolt remained intact and installed in the fuselage with the section of lower spar cap attached. There was no obvious deformation of the lower spar cap members in the area of fracture. The upper spar cap members were fractured between 10 and 12 inches outboard of the attach point with significant S-type bending of the members in the area of fracture. The upper shear bolt remained intact and installed but the lug portion of the fuselage fitting was fractured. The fracture faces on the upper spar cap members all had a dull, angled, grainy appearance consistent with over stress separation. There were some small areas of smearing on the fracture faces consistent with fracture face re-contact. There was no evidence of pre-existing cracks on any of the members. The fracture faces on the lower spar cap members all had a dull, angled, grainy appearance consistent with over stress separation. There was no evidence of preexisting cracks on any of the members. The lack of deformation at the lower spar cap fracture and the appearance of the fracture faces of the individual members is consistent with a tension overload failure of the lower spar cap. The bending deformation of the upper spar cap members and the appearance of the fracture faces of the individual members is consistent with a compression overload failure of the upper spar cap. Taken together the evidence is consistent with the separation of the left wingdue to a positive overload condition. There was no evidence of pre-existing conditions that would contribute to the separation. The main wreckage was located along the side of a creek at grid coordinates: N40 03.740, W105 05.731. The separated left wing was located about 1,156 feet from the main wreckage on an 152 degree azimuth. The entire main wreckage was completely destroyed from impact forces. Control cable continuity was established from the flight control surfaces in the empennage and right wing forward to the cockpit controls. The breaks noted in the flight control cables were consistent with stretching and overload breaks upon impact. After on scene documentation of the main wreckage in-situ, the left wing was moved next to the fuselage to accomplish flight control continuity examination. The breaks on the flight control cables outboard to the left wing were consistent with stretching and overload breaks upon wing separation. Examination of the engine did not reveal any mechanical anomalies. Examination of the propeller assembly showed that the wooden propeller blades were severely fractured consistent with rotation/power upon impact. Examination of the separated left wing revealed that the lower left spar cap was found fractured about 8 inches outboard of the wing-to-fuselage attach point. The fracture appeared to exhibit overload evidence and the components were retained for further examination. There was no evidence found of an in-flight bird strike.