Aero Commander 690A N115AB accident near Tacoma, Washington

Probable cause

The collapse of the right main landing gear for undetermined reasons.

NTSB analysis

After landing and during the roll out the right main landing gear collapsed. An examination of the landing gear revealed that the right main landing gear inboard clevis, which serves as the connection between the inboard hydraulic actuating cylinder to the main landing gear body, had failed. A metallurgical examination of the clevis fracture region revealed that it had failed in overstress. No preexisting cracks or corrosion were noted at the fracture and the laboratory could not determine an underlying reason for the overstress fracture. The main landing gear is retained in the down and locked position by hydraulic pressure being applied to the inboard main landing gear actuating cylinder, as well as being assisted by the outboard main landing gear (hydraulic-pneumatic) actuating cylinder. Additionally, bungee cords attached to the main landing gear drag braces assures that the main landing gear is locked and retained in the down position. As a result of these redundant features that are designed to ensure that the main landing gear is locked in the down position, the gear collapse cannot be positively attributed to the failure of the clevis. During a post accident examination of the right main landing gear a replacement clevis was installed and multiple landing gear extension and retraction cycles were performed. The landing gear system, including the proper sequencing of all components, operated correctly and no anomalies were noted. The reason for the landing gear collapse was undetermined.

Text from the NTSB. Open the NTSB docket for SEA08LA147 for the full record.