On July 26, 2004, about 1650 eastern daylight time, an amateur built Stephens Akro, N45KY, was substantially damaged during collision with terrain, while maneuvering near Westport, New York. The foreign certificated pilot was fatally injured. Visual meteorological conditions prevailed for the flight that departed Westport Airport (N25), Westport, New York, about 1630. No flight plan was filed for the local personal flight conducted under 14 CFR Part 91. According to a Federal Aviation Administration (FAA) inspector, witnesses observed the accident airplane performing aerobatics over N25 throughout the day. One witness stated, "...I would hear the engine turn off, the plane would appear to go into a nose dive and at the last possible second I would hear the engine come back on..." The airplane had completed about 10 minutes of an approximate 15-minute aerobatic routine; and the first 10 minutes seemed normal. The airplane then performed an unusual maneuver, never seen before, and descended in a spiraling nose down attitude with no recovery. The witnesses further stated that the pilot performed aerobatics often. The airplane impacted a field approximately 1/4-mile northwest of the airport. The airplane came to rest in a flat upright attitude, and the engine was imbedded in mud at an approximate 45-degree angle. The FAA inspector observed fuel in the fuel tank. He did not observe any evidence of forward travel at the accident site, or rotational signatures on the propeller. The witnesses reported a lack of engine noise during the spiral. Due to the impact damage to the engine, the inspector did not attempt to rotate the propeller. The inspector further stated that the pilot had been "belted in," and there was no evidence that the pilot attempted to jettison the canopy. The pilot's autopsy report stated: "....found was 90% stenosis of the left anterior descending coronary artery. There was cardiac hypertrophy and dilatation, in addition to minimal interstitial fibrosis. There was no evidence of acute myocardial damage, or recent or old myocardial infarction....Due to the changes in the heart, it is possible that the patient may have suffered some type of cardiac seizure or momentary blackout...However, given the amount of blood at the scene and the degree of hemorrhage encountered, it is apparent that heart action was continuing when the patient struck the ground. Findings indicate that the death the due to the accident, per se. The role played by the cardiac changes remain in the realm of speculation." Review of the Airman's Information Manual revealed: "Physiologically, humans progressively adapt to imposed strains and stress, and with practice, any maneuver will have decreasing effect. Tolerance to G forces is dependent on human physiology and the individual pilot. These factors include the skeletal anatomy, the cardiovascular architecture, the nervous system, the quality of the blood, the general physical state, and experience and recency of exposure. The pilot should consult an Aviation Medical Examiner prior to aerobatic training and be aware that poor physical condition can reduce tolerance to accelerative forces." The pilot had accumulated about 900 hours of total flight experience. Toxicological testing, conducted on the pilot at the FAA Toxicology Accident Research Laboratory, Oklahoma City, Oklahoma, was negative.