On June 6, 1996, at 0850 mountain daylight time, an Ayres S2R Turbo Thrush, N5686X, operated by Driscoll Aviation, Inc. (d/b/a Agrijet Inc.) of American Falls, Idaho, sustained substantial damage in a collision with terrain after the airline transport pilot became incapacitated in flight 4 miles west of the Aberdeen, Idaho municipal airport. The single-seat agricultural aircraft was being operated under 14 CFR 91 on an instructional flight. The pilot was seriously injured. The local flight originated from Aberdeen and no flight plan had been filed. Visual meteorological conditions existed at the time of the accident. The pilot reported that he had been hired for a seasonal agricultural pilot job with Driscoll Aviation and that he had driven up to Aberdeen from his home in Prescott, Arizona, during the three days immediately preceding the accident flight. His report indicated that the drive from Arizona to Idaho took 22 1/2 hours of driving time. The trip history, as reported by him, consisted of departure from Prescott at 1900 on June 3rd; rest from 0200 to 0600 in the early morning of June 4; arrival at Provo, Utah, at 1500 on June 4th; rest in Provo from 2300 to 0600 on the night of June 4-5; departure from Provo at 0630 on the 5th with arrival at Aberdeen at 1400; and retiring at 2330 on the night of the 5th, arising at 0445 MDT (0345 Arizona time) on the morning of the accident flight. He stated: "I am not aware of my hydration condition upon arriving at Aberdeen and it may have been low. I really was not paying too much attention to my liquid and solid intake on the days prior to the accident." He reported that he was "wound up with anticipation" before starting work on the morning of the accident. He also reported in a telephone conversation with the NTSB investigator-in-charge (IIC) on January 9, 1997, that he had flown only one to two hours in the 90 days prior to the accident, consisting of one 30-to-45 minute flight in the accident aircraft doing basic airwork and touch-and-go landings about one month before the accident, and "a couple of flights in a Cessna 172." The pilot reported that he got into the airplane at 0800, and first performed 10 to 12 touch-and-go landings using "70-90 degree bank turns" in the pattern without feeling any nausea or discomfort. He stated that he then flew west of the airport to simulate spraying a field and familiarize himself with a Global Positioning System (GPS) swath guidance system which was installed in the airplane. He stated that after about 15 to 20 swaths, he began to feel nausea and stomach discomfort and also began feeling hot and sweaty. He said that at this point he stopped swathing the field, climbed to 500 to 800 feet above ground level, shallowed his turns, opened the aircraft air vents, and unzipped his Nomex flight suit (which he said he wore over a long-sleeve cotton turtleneck shirt and blue jeans.) He reported: "After a few minutes of cooling off and flying straight and level I [felt] better, so I decided to go back [to] swathing the field." The pilot stated that after two to three more swath passes, "the stomach discomfort returned with a vengeance." He reported that at this point, he decided to return to the airport "since I was feeling like throwing up." He stated that he also unfastened his 4-point harness at this point because it was aggravating his stomach discomfort. The pilot stated: "I have over 300 hours of aerobatic instructional time...in the last 5 years and yet I had never experienced this intensity of stomach discomfort....As I flew back towards the airport I resisted the urge to throw up. All of a sudden I started to feel clammy and sweaty and a little light headed. The next thing I remember was waking up in the cockpit in an unusual sprawl on the floor ...." The pilot also stated the following regarding the accident sequence: "When I passed out the airplane was about 350-400 feet altitude and cruising at 120 MPH IAS in a [southeasterly] direction pointed towards the airport. The airplane made contact with the ground in a flat, level attitude....There is a heavy impact mark on the windshield support post in front of the pilot[']s seat. Luckily my helmet saved my life." The pilot held an FAA first-class medical certificate, dated August 2, 1995, at the time of the accident. A certified copy of his FAA medical records did not disclose any significant medical history. The pilot, who characterized himself as "one of the most health conscious pilots around", stated that subsequent medical tests (which included magnetic resonance imaging [MRI], computerized axial tomography [CAT] scanning, an electroencephalogram [EEG], an electrocardiogram [EKG], a neurological examination, and laboratory tests) did not disclose any previously undetected disqualifying medical conditions. A report of the neurological examination, conducted by Neurology Group Ltd. of Phoenix, Arizona on July 12, 1996, stated the evaluating physician's impression that the pilot had suffered a "vasovagal response while flying high G aeronautical maneuvers." The American Medical Association Encyclopedia of Medicine (Random House, 1989) states that a "vasovagal attack" is a "temporary loss of consciousness due to sudden slowing of the heart beat", stating that it is "a common cause of fainting in healthy people....usually brought on by severe pain, stress, shock, or fear....This cause of fainting is often attributed to instances where no other cause can be found." The Textbook of Medical Physiology, by Arthur C. Guyton, M.D. (7th ed., W.B. Saunders Co., 1986), describes "vasovagal syncope" as "fainting that results from an emotional disturbance." FAA Advisory Circular (AC) 67-2, Medical Handbook for Pilots, states: Although motion sickness is uncommon among experienced pilots, it does occur once in a while....This is probably a result of combining...anxiety, unfamiliarity, and...bumping received from the airplane and is quickly overcome with experience. Motion sickness is caused by continued stimulation of the tiny portion of the inner ear which controls your sense of balance. The symptoms are progressive....you begin to perspire freely. Eventually, you become nauseated and disoriented. Your head aches and you may have to vomit. If the air sickness becomes severe enough, you may become completely incapacitated....If you suffer from airsickness while piloting your aircraft, open up the air vents, loosen your clothing...and keep your eyes on a point outside the airplane. Avoid unnecessary head movements. Then cancel your flight plan and land as soon as possible. By contrast, the text Basic Flight Physiology, by Richard O. Reinhart, M.D. (TAB Books, 1992) characterizes G-induced loss of consciousness as occurring within a few seconds of being subjected to a high G load, resulting from the heart's inability to supply adequate blood (and hence oxygen) to the brain against the G force.