WPR10LA407 · Clark RV-6A

The pilot was performing a series of high-speed taxi test runs and planned to complete his first flight in the recently completed amateur-built experimental aircraft later in the day. During the first test run, the fully-castoring nose wheel began to shimmy, and the pilot appeared to slightly raise and lower the airplane's nose, in what was assumed by witnesses to be an attempt to eliminate the shimmy. Upon reaching the end of the runway, the pilot reversed course and made another test run in the opposite direction. During the second test run, the nose wheel began a significant shimmy, followed soon thereafter by the nose of the airplane beginning to rise. Almost immediately after the nose began to rise, the airplane, in what was most likely an unintended consequence, lifted off the runway. Soon after it became airborne, the airplane's nose lowered, in what appeared to be the pilot's attempt to get it back onto the runway surface. The nose wheel then contacted the runway, and the airplane entered into a porpoising sequence that ultimately resulted in the nose gear strut collapsing. The airplane then slid off the side of the runway, and, after encountering soft terrain, it nosed over onto its back. The reason for the occurrence of the nose wheel shimmy could not be determined. The pilot had a reported history of sinus and migraine headaches, which had previously occurred during flight, requiring the pilot to turn over the aircraft controls to a different pilot. Results of post mortem toxicology testing were consistent with the relatively recent use of two different impairing antihistamines, which are often used to treat sinus symptoms. It is possible that the pilot was impaired by his recent use of the antihistamines or by the condition for which the medication was taken, though the possible role of any such impairment in the accident sequence could not be established.

Verbatim NTSB analysis and probable cause from the NTSB dataset