On April 24, 2008, at 1710 Mountain Daylight Time, a twin-engine Cessna 421C, N513FS, sustained substantial damage after it veered off the runway during landing roll-out at Sterling Municipal Airport (STK), Sterling, Colorado. The airline transport pilot and the two passengers were not injured. The airplane was registered to and operated by Alpine Air LLC, Telluride, Colorado. A visual flight rules flight plan was filed for the flight that departed the City of Colorado Springs Municipal Airport (COS), Colorado Springs, Colorado, at 1630, and was destined for Sterling, Colorado. Visual meteorological conditions prevailed for the on-demand, nonscheduled flight conducted under 14 Code of Federal Regulations Part 135.

In a written statement, the pilot said that he continuously monitored the weather conditions prior to landing on Runway 33 at Sterling Municipal Airport. He made a normal landing and the wind felt like it was blowing right down the runway. The pilot said, "As I was decelerating through what I estimate to be around 50-60 knots, an extreme gust of wind estimated to be at least 50 knots hit the aircraft from the right side. The aircraft weathervaned into the wind, at which time I applied full left rudder to compensate. The input was not effective." The pilot attempted to maintain control of the airplane as it skipped off the right side of the runway onto the grass. Subsequently, the left main gear collapsed due to the side load and the aircraft came to an immediate stop. The pilot immediately secured the airplane and evacuated the two passengers.

Several Federal Aviation Administration (FAA) safety inspectors performed an on-scene examination of the airplane. According to an inspector, the nose gear had collapsed and the left main gear had separated from the airframe. The right main landing gear remained attached to the airframe, but the upper and lower torque links had separated, and the wheel was rotated to the right. Examination of the upper and lower link assembly's securing bolt, bushing, two AN washers, castellated nut, and cotter pin, revealed it had pulled through the lower torque link boss, which allowed the main wheel, brake, and strut assembly to rotate inside the strut housing. In addition, both propellers were damaged and the left wing sustained substantial damage.

The airplane was last inspected on March 13, 2008, in accordance with the Cessna 421C progressive care operation 3 &4 checklists at an airframe total time of 6,791.9 hours. The logged entry stated, "Adjusted main gear camber and toe." The accident occurred approximately 39.1 hours after this last inspection.

On November 23, 1983, Cessna issued Service Kit SK 421-118, which called for the replacement of washers and for inspection of the main gear torque links. The Service Kit specified washer P/N 504518-2 to be installed under the bolt head and nut, with replacement washer P/N S1450-5H12-063 to be incorporated into the next revision to the Service Kit. Cessna issued a Service Information Letter in conjunction with the Service Kit for the purpose of "improved service life and main gear alignment retention." The washers listed in the Service Kit were not installed on the accident airplane. Compliance with the Service Kit and Service Information Letter is not mandatory.

The pilot held an airline transport pilot rating for airplane multi-engine land, and instrument airplane. He reported a total of 8,720 flight hours, of which, 557 hours were in a Cessna 421C airplane.

Weather reported at Sterling Municipal Airport at 1653 included variable wind at 5 knots gusting to 14 knots, visibility 10 miles, scattered clouds 6,000 feet, overcast clouds at 10,000 feet, temperature 63 degrees Fahrenheit, dewpoint 33 degree Fahrenheit, and a barometric pressure setting of 29.71. The remarks section indicated there was a wind shift at 1614, and a peak wind from 310 degrees at 32 knots was recorded at 1625. In addition, there was lightning east of the airport.

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