About 5 min after takeoff, the pilot initiated a high speed low pass over the airport. When the aircraft was about 1000 ft from the airport at about 100 ft AGL, the right winglet separated. A witness estimated that the aircraft's speed was about 200 mph. Subsequently, the plane rolled inverted and crashed. An examination revealed that the winglet was not installed in accordance to the designer's plans. Specifically, the plans called for an 8-ply fiberglass build-up on the outside radius from the lower wing to the outside surface of the winglet. Fiberglass build-up was not incorporated. The designer estimated that it had less than 1/20 of the design strength. At high speed, with zero side slip, there is an inward aerodynamic (bending) load on the winglet that is equivalent to a 15 deg side slip at low speed. This winglet had failed inward. Also, a number of other deviations from the designers plans were noted.