On September 3, 1999, about 1130 Pacific daylight time, a Schwarzler Kolb Mark 3, N8979, registered to a private owner, and operated by the pilot as a 14 CFR Part 91 personal/pleasure flight, was substantially damaged while landing at Independence State Airport, Independence, Oregon. Visual meteorological conditions prevailed and no flight plan was filed for the local flight. The private pilot, the sole occupant, was uninjured. The flight originated from Independence approximately 30 minutes prior to the accident. There was no fire, and no report of ELT activation. The pilot stated that no unusual difficulties were encountered during the aircraft's first test flight, which took place two days prior to the accident. On the second test flight, the pilot elected to stay in the pattern and check the aircraft's flight characteristics and control responses. During a landing, the aircraft crossed the runway threshold at an altitude approximately 20 feet above the ground (agl), with an airspeed approximately 50 mph. According to the pilot, after crossing the threshold, the aircraft's left wing dropped suddenly, and the aircraft subsequently contacted the terrain. During a telephone interview, the pilot stated that he must have "stalled" the aircraft.