Amateur-built SMYTH SIDEWINDER N3786A accident near Boulder, Colorado

Probable cause

The pilot’s abrupt maneuver, which exceeded the structural limits of the left wing and resulted in its in-flight separation.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN14FA072 for the full record.