On April 7, 1999, about 1215 eastern daylight time, a David C. Bock Challenger II, N929DB, registered to an individual, began to flutter and went out of control near Astatula, Florida, while on a Title 14 CFR Part 91 personal flight. Visual meteorological conditions prevailed at the time and no flight plan was filed. The aircraft received substantial damage and the private-rated pilot was not injured. The flight originated from Zellwood, Florida, the same day, about 1130. The pilot stated he was performing Phase I flight testing of the homebuilt aircraft. While investigating the upper flight envelope, he was descending through 4,000 feet, at approximately 115-118 mph. The aircraft began to flutter and vibrate, and the control stick became hard to hold. He reduced engine power and raised the aircraft's nose. He heard a loud "pop" and the aircraft rolled to an inverted position. The aircraft was uncontrollable and he deployed the aircraft's ballistic recovery parachute (BRS). The parachute deployed properly and he and the aircraft floated to the ground, landing in an orange grove, between two trees. The kit manufacturer estimated the never exceed speed (Vne) to be 120 mph. In earlier tests the aircraft was fully controllable to 110 mph. Postcrash examination of the aircraft showed that one of two main, 2-inch diameter longerons in the tail had fractured as well as a 1-inch diameter longeron.