What happened
A pilot was conducting a personal cross-country flight when he attempted to land at an off-airport, sod-covered site. During the landing sequence, the left main landing gear collapsed. According to reports from a witness, the aircraft initially bounced approximately 12 to 18 inches upon contact before settling onto the landing area a second time.
The pilot reported no known mechanical anomalies with the aircraft prior to the accident. As a result of the collapse, the aircraft sustained structural damage to the gear box, left wing, aileron, and horizontal stabilizer. There were no injuries reported in the event.
The investigation
An inspection of the aircraft revealed that the left main landing gear strut attachment bolt had broken. A metallurgist at the NTSB materials laboratory in Washington, D.C., examined the failed bolt alongside new, exemplar bolts. The laboratory analysis determined that the bolt was made of the correct material and possessed the proper dimensions. However, the examination revealed that the bolt had failed due to a ductile overstress separation in both bending and shear mode.