On August 1, 1998, at 1215 hours Pacific daylight time, a Yakolev YAK-55M, N910RT, collided with a Cessna 172M, N4451R, while both aircraft were on short final approach to runway 15 at the Lincoln, California, airport. Both aircraft sustained substantial damage. Neither of the respective private pilots, the sole occupants of their aircraft, were injured. Visual meteorological conditions prevailed and flight plans were not filed for either operation. The Yakolev, owned and operated by its pilot, was returning to the airport at the conclusion of a local area personal flight, which originated about 1130. The Cessna, rented by its pilot from the operator, Atkin Air at the Lincoln airport, was in the traffic pattern performing takeoff and landing practice. Federal Aviation Administration (FAA) inspectors from the Sacramento, California, Flight Standards District Office responded to the accident site, examined the two aircraft, and interviewed both pilots and a witness to the accident. The pilot of the Yak-55M stated that he was returning to the field after practicing aerobatic maneuvers and entered left traffic for runway 15 by crossing over the runway at midfield. As he overflew the runway he heard an aircraft call on final approach for runway 15 over the Common Traffic Advisory Frequency (CTAF). He observed an aircraft on a 1/2-mile final and followed that aircraft on a short turning base to final approach. As he rolled out on final and prepared to flare, he felt the collision with an unseen aircraft. The pilot of the Cessna 172 stated that he was in the traffic pattern practicing takeoffs and landings. He stated that he called on the CTAF number three for landing behind a Cessna 150 and extended his pattern slightly to allow time for that aircraft to clear the runway. The pilot also reported that he announced his position while on base and final approach. Just as he was flaring the aircraft, he felt a jolt and the aircraft was forced down onto the runway. The pilot of a Cessna 150 in the pattern behind the Cessna 172 stated that he was on downwind when the Yak-55M crossed over the field and cut him off while turning on downwind. The witness reported that the Yak-55M nearly hit his aircraft.