On July 9, 2001, at 0930 central daylight time, a Clark CGS HAWK, experimental amateur built airplane, N6323Q, sustained substantial damage when it struck power lines near Bullard, Texas. The airplane was owned and operated by the pilot under 14 Code of Federal Regulations Part 91. The commercial pilot received serious injuries. Visual meteorological conditions prevailed for the local flight, and a flight plan was not filed. The flight originated from the pilot's private airstrip at 0845. During a personal interview, conducted by the FAA inspector, and on written statements to the Texas Department of Agriculture and the NTSB investigator-in-charge, the pilot reported the following information. On July 4, 2001, the pilot was hired to spray 560 acres of pasture land with herbicide for broad leaf plants. On July 8th, the pilot began the aerial application flights and dispensed herbicide on 140 acres. On the morning of July 9th, the applications resumed and approximately 114 acres were treated with the herbicide. While "practicing low level maneuvers," the airplane "came into contact with a secondary power line consisting of 2 wires." Subsequently, the airplane struck the ground on the main landing gear and fuselage. The pilot walked 1/2 mile to a farm house and was transported to the hospital. The FAA inspector reported that the aircraft was originally a two seat tandem ultralight. The aircraft had been modified for aerial application flights by replacing the aft seat with a 20 gallon hopper and adding a spray boom to the airframe. The fuselage, wing, main landing gear, and engine mounts were bent. The pilot/operator did not hold a 14 Code of Federal Regulations Part 137 Operating Certificate issued by the FAA.