On September 5, 2012, about 1000 mountain daylight time, an experimental amateur built Ellefson T-Bird II ultralight, N8106C, experienced a loss of flight control during landing, and impacted the ground about 1 mile south of the Richfield Municipal Airport (RIF), Richfield, Utah. The commercial pilot operated the ultralight under the provisions of 14 Code of Federal Regulations Part 91 as a test flight. The pilot, the sole occupant, received serious injuries. The ultralight sustained substantial damage. Visual meteorological conditions prevailed for the local area flight, and no flight plan was filed.According to the pilot, he had been contacted by the owner for assistance with his ultralight as it was oscillating in level flight. The owner had told the pilot that he had purchased the ultralight in New Mexico, and that he and the former owner disassembled it for transportation to RIF. The new owner then reassembled the ultralight, and noted that according to the plans, there were saddle washers and spacers missing on the horizontal stabilizer. The pilot told the owner that he had those washers and spacers, and the owner authorized the pilot to "fix whatever was wrong with it." A couple of days before the accident, the pilot met with the owner to get a better understanding of the oscillations, and at that time, the pilot added the washers and spacers that were missing on the horizontal stabilizer. On the day of the accident he performed a preflight, with no mechanical abnormalities noted. On the first flight, immediately after takeoff, the ultralight started to oscillate randomly up and down without any corresponding elevator input from the pilot. He remained in the pattern and performed a low flyby. The oscillations continued at different altitudes and airspeed. The ultralight touched down between 40 and 45 knots, with oscillations in an upward motion, and the yoke was full forward. The pilot decided to take off again to experiment with different trim settings. Upon liftoff, the oscillations began immediately with no throttle or elevator input. He decided to depart the pattern as he wanted to experiment with the trim and power settings; there was no effect on the oscillations. He reentered the pattern for landing. The pilot stated that the oscillations were random and he did not know if the oscillations were going to be up or down. He could only react to the direction and not anticipate the direction, and input flight control authority accordingly. About 5 feet above the ground and 40 knots, the oscillations began again. He pushed the yoke forward, which stopped the oscillation, but the rear main landing gear touched down on the runway and the yoke slammed into his stomach. The ultralight's nose picked up and the ultralight moved to the right. At this point the ultralight's nose was pointed to the ground and the pilot recalled pushing full left rudder before the ultralight impacted the ground. The fuselage, nose, wings, and tail sections were substantially damaged. The pilot stated that after the accident, he found out that the owner had reassembled the ultralight without assistance from a mechanic. The pilot also found out that if the pitch control cable was loose, it would move randomly. The owner of the ultralight did not respond to requests to provide additional information about the ultralight, nor was it made available for additional inspection.