On July 7, 2000 at 1800 central daylight time, a Staudacher S-300D2, N112KG, piloted by an airline transport pilot, sustained substantial damage while performing aerobatics near Kenosha, Wisconsin. The 14 CFR Part 91 flight was not on a flight plan. Visual meteorological conditions prevailed at the time of the accident. The pilot, who was the sole occupant, was not injured. The flight originated from the Kenosha Regional Airport, Kenosha, Wisconsin, at an undetermined time, for a local flight. According to the pilot, he had been practicing aerobatic maneuvers and was performing a 1/2 snap roll when the left aileron departed the airframe. The pilot said that the aircraft was still controllable and an uneventful landing was made. A postaccident examination of the airframe revealed that the spherical rod end bearing that was used for the center hinge had broken. The inboard 1-foot of the aileron and the inboard hinge remained attached to the wing. The outboard 6-inches of aileron spar as well as the outboard hinge remained attached to the wing. No other aileron pieces have been recovered. A metallurgical examination of the remaining portion of the spherical rod end bearing that was used for the center hinge was performed. Energy dispersive spectroscopy (EDS) examination revealed "... only iron and manganese consistent with plain carbon steel. There were no appreciable amounts of chromium, nickel, or molybdenum as might be expected with an AISI 4130 or AISI 8420 material." The report also states that the "... microstructure and hardness is typical of a normalized or annealed, medium carbon steel." The metallurgical report states that the rod end fracture was due to "... reverse bending fatigue (from cycling about its lateral axis) initiating at the root of the shank threads." Telephone interviews with the aircraft designer revealed that the accident aircraft had a different style rod bearing installed than the original design. The designer stated that he believed that the new rod end bearing shank was made of the same material and heat treat as the original. The specification for the original rod end bearing shows that the shank is to be made from AISI 4130 or AISI 8420 steel.