Accident Details
On December 16, 2016, at about 1045 eastern standard time, a Stinson model 108-3 single-engine airplane, registration N812C, equipped with snow skis, sustained substantial damage during landing at Ionia County Airport (Y70), Ionia, Michigan. The private pilot, who was the sole occupant, was not injured. The flight had departed Lowell City Airport (24C), Lowell, Michigan, about 1030 under day visual meteorological conditions.
According to the pilot, shortly after takeoff from a snow-covered runway, he heard a thump and observed that the left main landing ski tip had rotated upward past vertical and was contacting the left wing strut. He decided to land at Y70 because he anticipated needing assistance upon landing, and the departure airport was unmanned. Y70 had a snow-covered turf runway south of the paved runway 9/27, typically used for glider operations. During landing, the pilot was unable to reposition the left ski, and the ski separated from the axle. The left gear leg dug into the snow, causing the airplane to rapidly decelerate and nose over. The right wing, left wing strut, and vertical stabilizer were substantially damaged.
Investigation Findings
The main landing skis were designed to be supported by two 5/32-inch braided steel cables and bungee/shock cords. Each end of the cables terminated with a thimble-eye and a compressed plain copper oval nicopress sleeve (No. 18-4-P). On the accident airplane, the forward and aft support cables had pulled through their respective nicopress sleeves where the cables attached to the left ski's tip and tail. These nicopress sleeves were not located during the investigation.
Examination of the remaining nicopress sleeves, located where the support cables attached to the left landing gear leg, revealed inadequate compression. Similarly, a visual inspection of the right main landing ski cables also showed inadequately compressed nicopress sleeves. The width of these remaining sleeves measured between 0.440 and 0.446 inch using a dial caliper. According to manufacturer specifications, a properly formed 5/32-inch nicopress sleeve should have a width of less than 0.395 inch.
A test cable eye was fabricated using 5/32-inch braided steel cable and two plain copper oval nicopress sleeves. One sleeve was compressed with a proper 5/32-inch swage tool, resulting in a width of 0.383–0.386 inch. The other sleeve was compressed using a larger 3/16-inch swage tool, yielding a width of 0.442–0.452 inch. This indicated that the sleeves on the accident airplane were likely compressed with an improperly sized tool.
Ski Installation and Maintenance
The pilot reported that the airplane was typically equipped with snow skis during winter snow seasons. In February 2009, he purchased the main landing skis, Federal Aircraft Works model A-2500A, from a private individual. The skis were used, with an undocumented service history, and included all cables, bungees, brackets, and hardware appropriate for a Stinson model 108-3 installation. The forward and aft support cables were already fabricated and installed when purchased. The pilot stated that the support cables had not been repaired or replaced since he owned the skis.
On December 14, 2016, the pilot, who was also an aviation mechanic, installed the main landing skis for the 2016/17 winter season. An airframe logbook entry noted that the installation was performed per manufacturer's drawing No. 11R955/AB. The accident occurred during the first flight following installation.