On September 20, 2006, at 1934 eastern daylight time, an unregistered amateur built Air Trikes Tourist, was destroyed when it impacted trees and terrain while maneuvering near Middlebury, Vermont. The certificated private pilot/owner and the certificated sport pilot were fatally injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local personal flight, which departed from a private airport in Middlebury, Vermont. The flight was conducted under 14 CFR Part 91. Two witnesses observed the accident aircraft, as it maneuvered and performed several takeoffs and landings over the period of about an hour. The witnesses described that while maneuvering, the aircraft was "flying up to high altitudes and cutting the engine." The aircraft would then glide to a lower altitude before restarting the engine and repeating the process. Additionally, the aircraft performed several steep bank turns at a "lower than normal" altitude. Just prior to the accident, one witness called out to the other and told her to look at the aircraft, because it was "doing all sorts of crazy maneuvers." The witness looked up, saw the aircraft "spinning" to the left, and descending vertically to the ground. The witness described the spinning motion as "corkscrewing," and that the aircraft was rotating around the tip of one wing. The aircraft then disappeared from view behind a tree line, but "at the last second" both witnesses heard the engine briefly increase power, followed by silence. Another witness observed the aircraft as it spiraled towards the ground. He described the spiral as "tight," and that the aircraft was turning around itself while falling vertically. According to a report prepared by a Vermont Agency of Transportation (VAOT) inspector, the second pilot was providing flight instruction to the pilot/owner during the accident flight, and just prior to departing, the "instructor" was seen giving a preflight briefing to the pilot. Examination of the accident scene by the VAOT inspector revealed signatures consistent with a near-vertical descent, with the entire debris field contained within a 20-foot radius of the main wreckage. After impacting the ground, the aircraft caught fire. Portions of the wreckage, including the main mast and the forward angle brace, were forwarded to the Safety Board Materials Laboratory for further examination. Optical and microscopic examinations of the mast revealed fracture features consistent with overstress. The deformation associated with the separation was indicative of rearward bending loads at the top of the mast that compression buckled the aft side of the mast during separation. Both sides of the fracture were sooted, and the paint on the upper half was missing consistent with exposure to a post-separation fire. The rivets holding the four side plates to the forward angle brace were fractured at the interface between the pieces. Magnified examinations established that all rivet fractures were consistent with shearing overstress. No preexisting cracking was evident and no indications of preexisting looseness, wear, or damage were found at the separated connections. Based on the individual rivet shearing directions, the relative motions of the upper plates was consistent with the lower end of the plates rotating forward about the bolt location, and was consistent with a tension load of the front strut. The rivet fractures and the faying surfaces of the plates and brace were sooted and charred consistent with exposure to a post-separation fire. The individual shearing directions of the lower plate rivets were consistent with forward movement at the lower end of the angle brace and an aft movement at the upper end. The rivet fractures and faying surfaces of the plates exhibited attributes similar to those of the upper rivets and plates. The pilot/owner held a private pilot certificate with a rating for airplane single engine land, which was issued on September 29, 2005. A search of the Federal Aviation Administration (FAA) airman database revealed that there was no medical certificate information associated with the pilot, nor were any flight hours recorded. The second pilot held a sport pilot certificate with a rating for weight shift control land, which was issued on August 6, 2006. A search of the FAA airman database revealed no medical certificate information associated with the second pilot, nor were any flight hours recorded. The accident aircraft was an Air Trikes Tourist, a two-seat, weight-shift control aircraft produced in the Ukraine. According to the aircraft's North American distributor, who was based in Canada, it had an empty weight of 550 pounds when equipped with the G13BB engine, and a maximum gross takeoff weight of 1,100 pounds. The distributor also stated that after assembling the aircraft and installing the engine, he performed a test flight and a demonstration flight with the pilot. The pilot took delivery of the aircraft on August 4, 2006, and at that time the aircraft had accumulated about 3 flight hours. In subsequent communications to the distributor, the pilot stated that he had flown the aircraft for about 45 minutes and planned to get flight instruction from the second pilot. A search of the FAA aircraft registry revealed no entries associated with the pilot/owner. Autopsies were performed on both pilots by the Vermont State Department of Health, Office of the Chief Medical Examiner. The weather conditions reported at Edward F Knapp State Airport (MPV), Barre, Vermont, about 30 nautical miles northeast of the accident site, at 1951, included winds from 260 degrees at 6 knots, 10 statute miles visibility, an overcast ceiling at 6,500 feet, temperature 55 degrees Fahrenheit, dewpoint 46 degrees Fahrenheit, and an altimeter setting of 29.71 inches of mercury.