N1848RCessna A185F · 1974

On March 4, 2021, about 1500 Alaska standard time, a Cessna A185F, N1848R, was substantially damaged when it was involved in an accident near Skwentna, Alaska. The pilot and three passengers were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight.

The pilot reported that, after takeoff from the remote frozen snow-covered lake, with the hydraulic-wheel-ski-equipped airplane, the right rear check cable “came loose” from the fuselage attach fitting. The right ski then rotated to a nose high position as he maintained control of the airplane. His initial decision was to fly to the Lake Hood Airport (LHD), Anchorage, Alaska; however, a “very serious” vibration was noted in the right horizontal stabilizer, and he decided to return to the departure lake to make a precautionary landing.

While landing on loose, untracked snow, as the heel of the right ski touched the snow, the tip of the ski lowered, and the ski pivoted “into its normal landing position.” The airplane veered hard to the right, the right main landing gear separated from the fuselage, and the right wing impacted the snow. The fuselage sustained substantial damage.

A post-accident examination of the right wheel-ski assembly revealed that the retainer clip that secured the aft check cable to the fuselage fitting became detached and was missing. The pilot had installed the retainer 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. The pilot reported that the skis equipped on the airplane were manufactured in either 2010 or 2011.

The manufacturer issued a service bulletin (SB) on the skis in March 2018. The SB states in part:

Airglas has received 4 reports of the LH4000 yoke (PN LH3600-5) of the welds shearing off the outboard arm of the yoke. The common thread between all occurrences is a heavy side load is applied to the skis. This sideways load appears to happen in a turn. In three cases, pilot reported to Airglas that they hit an obstruction in a turn.

The yokes Airglas manufactured up until 2014 were manufactured differently than current yoke. The older yokes were welded and then post heat-treated, followed by a mag-particle inspection. We have also seen some evidence that yokes may have been partially cracked, prior to final heavy side load occurrence. The yokes are powder coated, and this may be masking partial cracks.

Compliance: Mandatory: This service document must be accomplished at the next 50-hours. Note: After the initial inspections, this service letter inspection must be completed every 50 hours.

The pilot stated that he and his mechanic were unaware of the SB. Compliance with the information in the SB was also not mandated by the Federal Aviation Administration.

Examination

The pilot reported that he had just replaced the ski check cables several months prior to the accident, due to aging wear. He added that he had the mechanic then sign the installation off.

The retainer clip used to secure the right rear check cable hardware was not recovered. Examination of the components indicated that the rear check cables on the skis were attached with unapproved hardware. The cables are supposed to be attached with a cotter key and instead were secured using a retainer clip.

A National Transportation Safety Board Materials Engineer examined the damaged ski components 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 the main yoke body were undamaged. Examination of the joints revealed thin weld metal deposits that did not penetrate the base metal of the parts and appeared to bond only to the tube surface. The ends of the outboard members were smooth and consistent with machined metal, and there were wear patterns consistent with contact marks from the mating outboard arm members on the surface of the main yoke body at the joints.