The private pilot reported that, shortly after takeoff from a snow-covered runway in Ionia, Michigan, he heard a “thump” and saw that the left main landing ski tip had rotated up, past vertical, and was in contact with the left wing strut. He was unable to reposition the left ski into a normal position.
Upon landing, the ski separated from the axle, the left gear leg dug into the snow, and the Stinson 108-3 rapidly decelerated before it nosed over.
As designed, the main landing skis are supported by two 5/32″ braided steel cables and bungee/shock cords. Both ends of the steel cables terminate with a thimble-eye and a compressed/swaged nicopress sleeve.
On the Stinson, 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. The nicopress sleeves for the left ski tip and tail attachments were not located during the investigation.
However, a post-accident examination of the remaining nicopress sleeves established that they were likely improperly formed with a 3/16″ swage tool instead of a properly-sized 5/32″ tool. As a result, the steel support cables were able to pull through the inadequately-formed nicopress sleeves during the accident flight.
It is likely that the aft support cable pulled through its nicopress sleeve during takeoff, which allowed the ski to rotate into a vertical position. The forward support cable likely pulled through its nicopress sleeve when the left ski separated from the axle during the subsequent landing.
According to the pilot, the plane was typically equipped with snow skis during the winter. He purchased the main landing skis in used condition, with an undocumented service history, from an individual about eight years before the accident. The forward and aft support cables were already fabricated and installed on the skis when they were purchased.
Additionally, the pilot reported that the support cables had not been repaired or replaced since he owned the skis.
The pilot, who was also an aviation mechanic, installed the main landing skis for the winter snow season two days before the accident. The accident occurred during the first flight since the skis were installed for the season.
Probable cause: A failure of the aft support cable on the left main landing ski due to an inadequately formed nicopress sleeve, which allowed the ski to rotate into a vertical position shortly after liftoff, and its unavoidable separation during the subsequent landing.
NTSB Identification: CEN17LA057
This December 2016 accident report is provided by the National Transportation Safety Board. Published as an educational tool, it is intended to help pilots learn from the misfortunes of others.