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N812C accident description

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Crash location 42.938055°N, 85.060556°W
Nearest city Ionia, MI
42.987255°N, 85.071117°W
3.4 miles away
Tail number N812C
Accident date 16 Dec 2016
Aircraft type Stinson 108-3
Additional details: None
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NTSB Factual Report

On December 16, 2016, about 1045 eastern standard time, a Stinson model 108-3 single-engine airplane, N812C, equipped with snow skis, was substantially damaged while landing at Ionia County Airport (Y70), Ionia, Michigan. The private pilot, the sole occupant, was not injured. The airplane was registered to and operated by the pilot under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Day visual meteorological conditions prevailed for the flight that departed Lowell City Airport (24C), Lowell, Michigan, about 1030.

The pilot reported that shortly after takeoff from a snow covered runway 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 decided to land at nearby Y70 because he anticipated requiring assistance upon landing and the departure airport was unmanned. Additionally, Y70 had a snow covered turf runway, south of the paved runway 9/27, that was typically used for glider operations. The pilot reported that he was unable to reposition the left ski into a normal position and upon landing the ski separated from the axle, the left gear leg dug into the snow, and the airplane rapidly decelerated before it nosed over. The right wing, left wing strut, and vertical stabilizer were substantially damaged during the accident.

As designed, the main landing skis are supported by two 5/32 inch braided steel cables and bungee/shock cords. Both ends of the 5/32 inch steel cables terminate with a thimble-eye and a compressed/swaged 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. The nicopress sleeves for the left ski tip and tail attachments were not located during the investigation. However, a visual examination of the remaining nicopress sleeves, located where the support cables attached to the left landing gear leg, appeared to be inadequately compressed. Additionally, a visual inspection of the right main landing ski cables also revealed inadequately compressed nicopress sleeves. The width of the remaining nicopress sleeves, as measured with a dial-caliper, were between 0.440 and 0.446 inch.

According to manufacturer specifications, when properly formed, the width of a 5/32 inch nicopress sleeve should be 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 (No. 18-4-P). The first nicopress sleeve was compressed using a proper 5/32 inch swage tool. The other test sleeve was compressed using a larger 3/16 inch swage tool. The width of the properly formed test sleeve, using a 5/32 swage tool, measured 0.383-0.386 inch. The width of the other test sleeve, using a 3/16 swage tool, measured 0.442-0.452 inch.

The pilot reported that the airplane was typically equipped with snow skis during the 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 purchased used, with an undocumented service history. The skis were acquired as complete assemblies, which 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 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.

On December 14, 2016, the pilot, who was also an aviation mechanic, installed the main landing skis for the 2016/17 winter snow season. According to an airframe logbook entry, the main landing skis were installed per manufacturer's drawing No. 11R955/AB. The accident occurred during the first flight since the skis were installed for the season.

NTSB 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.

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