NTSB Factual Report

On March 19, 2005, about 1615 Alaska standard time, a wheel/ski-equipped Arctic Aircraft Company, S-1B2 airplane, N67AT, sustained substantial damage when the main landing gear collapsed during touchdown on a frozen lake, about 18 miles west of Anchorage, Alaska. The airplane was being operated as a visual flight rules (VFR) local area personal flight under Title 14, CFR Part 91, when the accident occurred. The airplane was operated by the pilot. The private certificated pilot, and the sole passenger, were not injured. Visual meteorological conditions prevailed. The flight originated at Merrill Field, Anchorage, Alaska, about 1555, and no flight plan was filed, nor was one required.

During a telephone conversation with the National Transportation Safety Board (NTSB) investigator-in-charge (IIC), on March 22, the pilot reported that he was conducting touch and go landings toward the north on Figure-Eight Lake. He said the weather conditions were clear and calm. He decided to do short-field landings with the wheels extended. On the ninth landing, as the airplane touched down, 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. The pilot also stated that the touchdown was not hard, and the common point of failure appeared at the center, fuselage-mounted spring/oleo shock assembly. Inspection of the airplane revealed the left front landing gear attach point was torn from its fuselage mounting lug.

The main landing gear assembly (left and right) is an A-frame design with two attaching/pivot mounts on the fuselage tubing. The aft side of each A-frame 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, and a cotter pin for security.

After the airplane was recovered, a Federal Aviation Administration (FAA) airworthiness inspector retrieved the landing gear link plates, which were retained by the lower clevis bolt attached to the right landing gear arm. The link plates and clevis bolts were examined on July 26, 2005, by a aerospace engineer with the FAA's Alaska Region Aircraft Certification Office, Anchorage, Alaska, which has engineering certification responsibility for the accident airplane. The engineer reported that each link plate was bent, and each lower bolt hole in the links were elongated. 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.

On July 29, the FAA engineer 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 an annual inspection on August 20, 2004, and had accrued about 11 hours since the inspection.

NTSB Probable Cause

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.