Accident Details
Upon landing, the pilot heard a bang and the right wing dropped. The airplane incurred substantial damage during a ground loop maneuver immediately after the separation of the right main wheel and lower strut end.
Examination Findings
Post-crash examination revealed that the lower end of the right main landing gear spring strut had broken off. Examination of the fracture surfaces showed features characteristic of fatigue cracking in multiple locations. The fatigue cracks originated at the surface of the holes, at the edge between the wall of the hole and the flat face of the leg. Separation of the spring strut was caused by fatigue cracks that originated from the surface of the two upper holes. No chamfer was found at the surfaces of the two upper holes. For the most part, the surface of these holes contained a corner edge that acts as a stress raiser, which sensitized the hole to cracking. The two upper holes most likely were drilled oversize to remove anomalies such as corrosion or fretting damage. The diameter of the oversize holes was sufficient to have removed evidence of the chamfer. No airframe log information was available identifying any maintenance on the spring strut.
Cause
The separation of the spring strut was caused by fatigue cracks that originated from the surface of the two upper holes. The absence of a chamfer at the surfaces of these holes created a stress raiser, contributing to the initiation of fatigue cracking.