NTSB Identification: ANC02LA034.
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Accident occurred Thursday, May 16, 2002 in North Pole, AK
Probable Cause Approval Date: 11/25/2003
Aircraft: Cessna 170B, registration: N4669C
Injuries: 1 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The right main landing gear leg separated from the airplane while landing on a gravel airstrip, which resulted in substantial damage to the airframe. The main landing gear had recently been rebuilt, and this was the second landing since the work was completed. The gear leg had been secured to the airplane with an AN7-20A bolt and an MS21044C stainless steel nut. The bolt, which remained attached to the separated gear leg was bent and the threads were damaged. The MS21044C nut had fractured, and was found in the right main landing gear bulkhead assembly. Examination of the nut determined it fractured due to circumferential overstress tension. The airplane manufacturer's parts catalog specifies a AN365-720C steel nut to secure the main landing gear legs. The military standard, under which the original AN365 nut was manufactured, requires any substitute nut to be of like material, plating, and thread size, and specifically excludes the substitution of the MS21044C nut in lieu of the AN365-720C nut. The proper substitute for the AN365-720C is the MS21044N, which is an all steel nut. However, the rated tensile strength of the AN365, MS21044C and MS21044N series nuts are all 125 ksi. 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 a side load was present during the failure. This side load could have resulted from a hard landing with the airplane moving sideways or improper reinstallation of the landing gear may have allowed normal landings to introduce side loads.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

The separation of the right main landing gear from the airplane during landing. A factor associated with the accident is the unapproved hardware used to attach the main landing gear.

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