On February 24, 1998, about 1230 central standard time, a Collie Cumulus motorglider, N1171U, registered to a private individual, operating as a 14 CFR Part 91 personal flight, experienced a wing separation while in powered flight near Hillsboro, Tennessee. Visual meteorological conditions prevailed and no flight plan was filed. The commercial-rated pilot sustained fatal injuries, and the airplane was destroyed. The flight originated about 15 minutes before the accident. An eyewitness stated he heard the aircraft takeoff from a nearby private airstrip, and shortly thereafter, it came into his view at about 400 to 500 feet altitude in straight-and-level flight. He then heard a loud "pop", looked up again, and saw a wing separated from the fuselage and the fuselage plunge earthward. The airplane impacted the terrain beyond the eyewitness' field of vision. He did not see a parachute deploy. The fuselage crashed in a farm field, in a nose-down attitude. Both wings were found 200 to 300 yards from the main wreckage. A ballistic recovery system, BRS, was installed and had been fired, but the shroud lines had fouled in the airplane's pusher-type propeller resulting in nondeployment of the parachute's canopy. On-site examination of the wreckage by FAA personnel revealed that the left wing spar failed. Subsequent inspection by representatives of the kit manufacturer, U.S. Aviation, and the FAA, revealed that the initial failure was the left wing spar due to improper bonding techniques. Specifically, the glue-bonded joint between the D-cell leading edge and the upper spar capstrip of the left wing failed. The findings were unanimous. ( See enclosed letter indicating inspection team's composition and findings.) Postmortem examination of the pilot and toxicology testing were not accomplished. The protocol guidance given the FAA investigators on the scene, by the FAA Flight Surgeon's Office/ Southern Region, was to treat the accident as an ultralight accident, and therefore the testing was not ordered.