On May 31, 1997, at 0715 hours mountain standard time, a Staudacher S300D, N540DH, had the right aileron separate during an in-flight aerobatic maneuver 5 miles south of Chandler, Arizona. The aircraft, registered to and operated by the owner/builder, sustained substantial damage when the aileron struck the horizontal stabilizer and bent it's spar. The certificated commercial pilot was not injured. Visual meteorological conditions prevailed and a flight plan was not filed. The flight originated in Chandler at 0700. The Safety Board was notified of the extent of damage on June 24, 1997. An FAA airworthiness inspector determined that the aileron attach points were fabricated with "automotive hardware welded to plates, receiving no further heat treatment, and then embedded in the aileron spar." Additionally, the aileron had been modified six flights previously with an addition of an aluminum plate to the trailing edge without rebalancing the flight controls. The inspector reported that the owner commonly attempted to improve the aircraft's flight characteristics by moving the aileron fore and aft with shim plates under the attach brackets. According to the FAA inspector, the parts showed signs of slippage and wear at the right center attach point where the shim plates were inserted. According to the kit manufacturer, there are approximately 20 airplanes in service with this type of aileron attach fittings. The kit manufacturer has initiated a modification to the aileron attach fittings and the ailerons to remove the subject fabricated plate and bolt fittings. The kit manufacturer stated that all 20 airplanes in service will receive the modified ailerons and fittings. The owner reported to the kit manufacturer that the "two nuts that hold the aileron hinge to the aileron at the hinge that failed were found loose on several occasions."