Accident Overview
The right main landing gear leg separated from the aircraft during a landing on a gravel airstrip, causing substantial damage to the airframe. This event occurred on the second landing after the main landing gear had been recently rebuilt.
Examination Findings
The separated gear leg remained attached to an AN7-20A bolt, which was bent and exhibited damaged threads. The corresponding nut, an MS21044C stainless steel nut, had fractured and was located in the right main landing gear bulkhead assembly. Analysis of the nut revealed that the fracture was due to circumferential overstress tension.
Specification Compliance
The aircraft manufacturer's parts catalog specifies an AN365-720C steel nut for securing the main landing gear legs. The military standard under which the original AN365 nut was manufactured requires that any substitute nut be of like material, plating, and thread size. It explicitly excludes the substitution of the MS21044C nut in lieu of the AN365-720C nut. The correct substitute is the MS21044N, an all-steel nut. However, the rated tensile strength of the AN365, MS21044C, and MS21044N series nuts are all 125 ksi.
Load Analysis
Examination of the bolt revealed sheared threads adjacent to the bolt grip, indicating that the nut was intact on the bolt when the bolt/nut combination was loaded to failure. The bending deformation of the bolt indicated that a side load was present during the failure. This side load could have resulted from a hard landing with the aircraft moving sideways, or improper reinstallation of the landing gear may have allowed normal landings to introduce side loads.