What happened
While taxiing at a slow speed in the run-up area, the Cessna aircraft was executing a right-hand turn when the left main landing gear collapsed.
The investigation
Investigators examined the landing gear assembly and found that the side brace for the main landing gear is attached to a casting, which is bolted to the inboard wing rib within the landing gear wheel well. The examination revealed that the wing rib web had cracked around this casting. This structural failure allowed the side brace to move, leading to the collapse of the landing gear. Portions of the crack showed discoloration, suggesting the crack was old.
Maintenance records for the aircraft showed no previous history of hard landings or damage related to the landing gear. The most recent annual inspection had been completed 7 months and 60 flight hours prior to the event.
Further investigation into manufacturer recommendations found that Cessna Aircraft Company had issued Service Letter ME76-2 on January 5, 1976. This letter was issued in response to cracks discovered on the inboard wing rib web around the side brace fitting and detailed the installation of a steel reinforcement/doubler plate on the wing rib web behind the fitting. Inspection of the aircraft confirmed that Service Letter ME76-2 had not been accomplished.