On October 12, 1996, at 1755 hours Pacific daylight time, an experimental Rotorway Exec helicopter, N18WC, crashed at Mather Airport, Mather, California. The pilot was conducting a local visual flight rules personal flight. The helicopter, registered to and operated by the pilot, was destroyed by impact forces and the resulting postimpact fire. The certificated private pilot, the sole occupant, sustained fatal injuries. Visual meteorological conditions prevailed with the winds estimated by witnesses to be at 10 to 15 miles per hour with gusts. The flight departed Mather Airport at an undetermined time. Ground witnesses reported that they observed the helicopter execute a hard right turn followed by a hard left turn with some oscillations. During this maneuver, the main rotor blades struck the tailboom. The helicopter entered an uncontrolled descent, crashed and burned. Examination of the wreckage revealed that the main rotor blades were of a composite material and not the aluminum blades authorized by the manufacturer. There was evidence of some mast bumping next to the teeter stops behind the thrust stops. There was no evidence found of a mechanical malfunction. According to a manufacturer representative at the accident site, he was informed by other pilots that the accident pilot was making a bomb run over a ground target using a bag of flour. He was told that a bag of flour was somewhere in the cabin area and the helicopter was at the correct location over the target area when the accident sequence began.