On December 5, 2001, at 1343 eastern standard time, a homebuilt Baby Ace, N916MG, was substantially damaged during approach to a private airstrip in Spencerport, New York. The certificated private pilot was seriously injured. Visual meteorological conditions prevailed and no flight plan was filed for the local personal flight conducted under 14 CFR Part 91. A student pilot, who had approximately 200 hours of flight experience, was at his place of business and witnessed the accident. The student pilot stated that he heard the airplane fly overhead, and recognized the sound of the engine. He then saw the airplane turn from base to final leg for runway 35, and it was about 200 feet above the ground. The airplane was in a steep bank, possibly in excess of 60 degrees. The witness told his wife, "he's going to stall," and then the airplane flew directly into the ground. According to the pilot's written statement, he did not recall the accident. He reported a total flight experience of 310 hours, of which, 40 hours were in the same make and model as the accident airplane. Examination of the wreckage by a Federal Aviation Administration inspector revealed that the right wing aileron cable was not secured in a pulley groove. Rather, the cable was resting between the side of the pulley and an attachment bracket. The inspector added that emergency personnel had severed the wings during the rescue effort, and he did not know if the cable dislodged before the accident or during the rescue effort.