Incident Overview
During a landing on a frozen lake, the pilot of a wheel/ski equipped airplane heard a 'whump' sound as the aircraft settled onto its left wing. Post-incident inspection revealed that the main landing gear attaching bolt had fractured in overload across a threaded area.
Bolt Analysis
The installed bolt was identified as an NAS-147R, which was not the specified type for the landing gear assembly. The NAS-147R bolt had a smooth shank section measuring 1.3 inches, shorter than the specified AN bolt's smooth shank length of 1.43 inches. When an undamaged NAS-147R bolt is installed in the landing gear, a threaded area remains within the bolt hole, which can concentrate stress.
Maintenance History
The bolt had been installed three weeks prior to the incident during the last annual inspection. The fracture occurred in overload across the threaded region, consistent with the use of a non-specification bolt.
Conclusion
The incident resulted from the failure of the landing gear attaching bolt due to overload. The bolt's shorter smooth shank and threading within the bolt hole likely contributed to the failure, though no official causal determination has been made.