On June 5, 2004, about 1030 mountain daylight time, an experimental D'Amico Kitfox Series 7, N1852N, registered to and flown by the pilot as a 14 CFR Part 91 personal flight, experienced a hard landing at Caldwell Industrial, Caldwell, Idaho. Visual meteorological conditions prevailed at the time and no flight plan was filed for the local flight. The aircraft was substantially damaged and the private pilot, the sole occupant, was not injured. During a telephone interview and subsequent written statement, the pilot reported that he accomplished his normal run-up and control checks prior to takeoff from runway 12. The pilot stated that as the aircraft was accelerating during the takeoff ground roll, he suddenly could not feel the usual back-pressure on the elevator control. The pilot checked the manual trim and confirmed that it was in the takeoff position and as he was about to reduce power to abort the takeoff, the "...plane catapulted up in the air at a high angle of attack, without pitch control input from me." The pilot stated that he immediately put the manual pitch trim control in a nose-down range and the airplane responded "violently." The pilot continued to manipulate the elevator trim as the aircraft oscillated three to four times before gradually attaining a more level attitude. The aircraft landed hard and flat on the remaining runway, subsequently breaking the nose gear wheel, and damaging the propeller and main gear. Further damage included buckled floor boards and structural damage to the fuselage. After the aircraft was pushed off the runway and returned to the pilot's hangar, the seats were removed to view the pitch push rod assembly. The pilot reported that he found that the connecting bolt between the pitch control assembly and the pitch rod (to the elevator) had come off. The pilot discontinued the inspection and notified the Federal Aviation Administration of the accident and his findings. An Inspector from the Boise, Idaho, Flight Standards District Office inspected the aircraft and confirmed the findings. The Inspector reported that the bolt used to secure the elevator control push rod to the push rod end was found laying on the bottom of the elevator control surface. The nut which was used to secure the bolt was not found. The pilot reported that the nut used to secure the bolt was a self-locking castle nut with a nylon insert. The pilot also reported that the controls had been previously inspected by a Designated Airworthiness Representative (DAR) and signed off. The DAR reported that he was sure that he confirmed the presence of the nut during his inspection and sign-off of the aircraft on April 24, 2004. During a follow-up conversation with the pilot on July 16, 2004, he reported that after an extensive search of the aircraft, the missing nut was not located.