On July 11, 1998, at 1150 hours Pacific daylight time, a Pitts S1-S, N71HR, impacted a gravel pile near the Tracy, California, airport. The aircraft sustained substantial damage, and the private pilot/owner, the sole occupant, suffered serious injuries. The local flight was conducted under 14 CFR Part 91 and originated at an unspecified time. Visual meteorological conditions prevailed and no flight plan was filed. There were three ground witnesses to the accident who were working approximately 50 yards from the point of impact. None of them is a rated pilot. One witness reported that he had just finished some construction work when he heard a airplane close by. He reported that he saw the aircraft maneuver under a series of telephone lines and around construction equipment about 15 feet agl or less. He stated that the aircraft appeared to be attempting to pull up as it impacted the gravel pile. The witness further reported that the aircraft appeared to have power throughout the maneuvering and he did not notice any decrease in engine noise. The co-owner of the airplane reported that he and the pilot have co-owned the aircraft since 1995 and use it to compete in advanced category aerobatic competitions. The co-owner is also the chapter president of the International Aerobatic Club (IAC) and is the Federal Aviation Administration (FAA) designated administrator for the local aerobatic practice area. The gravel pile is in the middle of a charted aerobatics practice box. The floor of the aerobatic practice area is 500 feet agl and the ceiling is 4,000 feet agl. The co-owner reported that he and the pilot usually watch each other's aerobatic routine and maintain radio communication to critique certain maneuvers. On the day of the accident, the co-owner was on the ground watching the pilot run through his routine. The pilot flew for approximately 25 minutes, then reported that he was finished. The co-owner started to head for the airport when he noticed the airplane disappear behind a ridge. The pilot suffered serious injuries and has no recollection of the accident. The co-owner reported that the pilot did not report experiencing any mechanical problems during the accident flight. The aircraft recoverer reported that the fuel lines were leaking at the accident site. The aircraft was cut into three pieces in order to extricate the pilot, and flight control continuity was not established. Both the pilot and aircraft co-owner were sent the pilot/operator report, NTSB form 6120.1/2, several times. Neither contacted the Safety Board investigator nor returned the completed form.