Accident Overview
On March 4, 2021, about 1500 Alaska standard time, a Cessna A185F, registration N1848R, was substantially damaged during a precautionary landing on a frozen snow-covered lake near Skwentna, Alaska. The pilot and three passengers were not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Sequence of Events
According to the pilot, after takeoff from a remote frozen lake, the right rear check cable of the hydraulic-wheel-ski-equipped airplane came loose from the fuselage attach fitting. The right ski rotated to a nose high position, but the pilot maintained control. Initially intending to fly to Lake Hood Airport in Anchorage, he noted a severe vibration in the right horizontal stabilizer and decided to return to the departure lake for a precautionary landing.
During the landing on loose, untracked snow, the heel of the right ski touched the snow, causing the ski tip to lower and pivot into its normal landing position. The airplane then veered sharply to the right, the right main landing gear separated from the fuselage, and the right wing struck the snow. The fuselage sustained substantial damage.
Post-Accident Examination
Examination of the right wheel-ski assembly revealed that the retainer clip securing the aft check cable to the fuselage fitting had become detached and was missing. The pilot had installed this clip, which was not approved by the ski manufacturer. Additionally, the pilot reported that the ski yoke had fractured at a weld site. The airplane was equipped with Airglas LH4000 hydraulic wheel-skis, manufactured in either 2010 or 2011.
A National Transportation Safety Board materials engineer examined the fractured yoke and noted that the outboard arm separated from the yoke through the weld metal at the attaching joints. The separations were only through the weld filler metal; the outboard arm members and main yoke body were undamaged. The weld deposits were thin and did not penetrate the base metal, appearing to bond only to the tube surface. Wear patterns consistent with contact marks were observed on the main yoke body.
Manufacturer Service Bulletin
The manufacturer, Airglas, issued a service bulletin (SB) for the LH4000 skis in March 2018. The SB reported four instances of the yoke (part number LH3600-5) welds shearing off the outboard arm, with a common thread of heavy side loads during turns. The bulletin stated that yokes manufactured before 2014 were welded, post-heat-treated, and mag-particle inspected, but noted that powder coating might mask partial cracks. Compliance was mandatory at the next 50 hours, with subsequent inspections every 50 hours. The pilot stated that he and his mechanic were unaware of the SB, and compliance was not mandated by the Federal Aviation Administration.