On August 26, 2000, about 1600 Pacific daylight time, a Cable-Breigler BG-12BD experimental homebuilt glider, N4458, collided with terrain when a wing separated in-flight while returning to land at the Mountain Valley Airport, Tehachapi, California. The owner was operating the glider under the provisions of 14 CFR Part 91. The private pilot sustained fatal injuries; the glider was destroyed. The personal flight departed Mountain Valley about 1545. Visual meteorological conditions prevailed and no flight plan had been filed. Witnesses told the Federal Aviation Administration (FAA) accident coordinator that the glider was returning to the airport. It was about 1 mile north of the airport and about 2,000 feet above the ground. It appeared to be preparing to enter downwind for the landing pattern for runway 27. The witnesses thought the altitude was higher than normal for this approach, and they thought the glider's speed was very fast. The witnesses heard a loud snap, and the left wing and flap separated from the glider. Witnesses familiar with this type of glider said it did not have speed brakes; they said the flaps were used to slow it down. They said the flaps appeared to be activated when the left flap and wing separated. The wing was located about 0.25 miles from the main wreckage and the flap was about 300 feet from the wing. The accident coordinator examined the wing. Three hinges, attached by three screws on each hinge, connected the flap to the wing. The outboard hinge remained attached to the wing. One screw on the outboard hinge pulled out of the flap; two other screws sheared through the hinge material. The flap control assembly ripped out of the left wing. A section of the rear spar fractured and separated from the wing. The left wing spar fracture surface exhibited twisting characteristics. The flap and remaining two hinges were recovered the next day. The accident coordinator had the owner of a soaring facility examine the flap and summarized the observations. The inboard hinge remained connected to the flap and the screws pulled out of the aft wing spar. The center hinge screws pulled through the hinge holes.