ANC99LA042

The pilot was landing the wheel/ski equipped airplane on a frozen lake to wait for improving weather. During the landing roll, the pilot heard a 'whump' and the airplane settled onto the left wing. Inspection revealed the main landing gear attaching bolt had fractured in overload across a threaded area. The installed NAS-147R bolt was not the one specified for installation, and had a shorter smooth shank section (1.3 inches) than the specified AN bolt (1.43 inches). When an undamaged NAS-147R bolt is installed in the landing gear, a threaded area remains within the bolt hole. The bolt had been installed three weeks prior to the accident during the last annual inspection.

On April 4, 1999, about 1430 Alaska daylight time, a wheel/ski equipped, Cessna 180 airplane, N420SB, sustained substantial damage during landing on a frozen lake, about 27 miles south-southwest of Iliamna, Alaska. The pilot and sole passenger were not injured. The airplane was being operated under 14 CFR Part 91 as a personal flight. The flight had departed from a frozen lake about 10 miles southwest of the accident site about 1400. Visual meteorological condition prevailed at the time of the accident, and no flight plan was filed.

The pilot told the NTSB investigator-in-charge during a telephone interview on April 9 that he was returning to Palmer, Alaska, at the completion of a hunting trip. He indicated that after takeoff, the ceilings and visibility deteriorated, with low ceilings and visibility in snow showers. He decided to land on another frozen lake and wait for the weather to improve. During the landing roll, the left main gear leg separated from its mount, damaging the main gear supporting structure. The pilot and passenger were picked up by a passing helicopter.

Research revealed that the main landing gear legs had been installed three weeks prior to the accident, during the last annual inspection. Inspection revealed a fracture of the main landing gear attaching bolt. The bolt which fractured was identified as a NAS-147R bolt, with a smooth shank length of 1.3 inches. Examination at the NTSB metallurgical laboratory revealed an overstress failure through a threaded area of the bolt. The bolt which is specified in the Cessna 180 parts manual is an AN bolt, which has a smooth shank length of 1.43 inches. When an undamaged NAS-147R bolt is installed in the landing gear, a threaded area remains within the bolt hole.