NTSB Identification: ANC05LA051.
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Accident occurred Saturday, March 19, 2005 in Anchorage, AK
Probable Cause Approval Date: 12/20/2005
Aircraft: Arctic Aircraft Company S-1B2, registration: N67AT
Injuries: 2 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 private certificated pilot was conducting short-field landings with the wheels extended on a frozen lake in a wheel/ski-equipped airplane. On the ninth landing, the main landing gear collapsed, and the airplane settled onto its belly. The pilot said both landing gear A-frame type gear assemblies rotated outward and upward, as the airplane settled onto the ice. He said that the touchdown was not hard, and the common point of failure was at the center, fuselage-mounted spring shock assembly. Each landing gear assembly has an extended arm that attaches in the center of the fuselage to the bottom end of a vertically mounted oleo shock. The left gear arm attaches to the oleo lug by a clevis bolt that is installed front-to-back. The bolt also retains the top end of two oval-shaped metal link plates that are positioned vertically at the front and aft side of the oleo attach lug. A clevis bolt, through the bottom hole of each link, retains a spacer and the right landing gear extended attaching arm. Each clevis bolt uses a tension castle nut, with a cotter pin for security. The aft link plate had extensive elongation of the upper bolt hole radius. The forward link plate upper bolt hole was fractured, and had opening and elongation of a segment of the bolt hole radius. The upper clevis bolt was found at the scene, but its castle nut and cotter pin were not located. The clevis bolt was bent along its span. The threaded portion of the bolt had flattening of the threads about 1/3 of its circumference, and partial threads on the remaining 2/3 of its circumference, with slight necking of the bolt end. An FAA aerospace engineer examined the recovered parts and reported, in part, that: "...the cotter pin failed in service, or was never installed, and the shear nut loosened and fell off the aft end of the upper bolt, or was never installed." The airplane had accrued 11 hours since the most recent annual inspection.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: A main landing gear collapse during the landing touch down due to a non-secured clevis bolt at the common attach point on the landing gear oleo shock. Full narrative available
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