On February 27, 2005, about 0951 mountain standard time, a Piper PA-32-301, N8339P, registered to a private individual, impacted a trailer at the H. A. Clark Memorial Field, Williams, Arizona. Visual meteorological conditions prevailed. The runaway airplane was substantially damaged during the collision. The pilot had been standing in front of the airplane when the accident sequence began. He had inadvertently started the airplane's engine by partially rotating the propeller with his hand. The two occupants/passengers inside the airplane were positioned in the most rearward (fifth and sixth) seats. The pilot-operator held a commercial pilot certificate. He received minor injuries upon falling to the ground as the airplane commenced moving under power. The passengers were not injured. The intended personal flight to Rialto, California, was performed under the provisions of 14 CFR Part 91. No flight plan was filed. The flight was originating at the time of the accident. According to the airport manager, the pilot had boarded the airplane with his passengers for their planned flight. When the pilot was unable to start the engine, he exited the airplane and commenced hand propping the propeller. The engine started, revved up to what sounded like near full power, and the airplane accelerated across the tarmac. After rolling about 300 feet and nearly colliding with two commuter-type airplanes and dozens of passengers/bystanders, the airplane impacted the side of an unoccupied horse trailer on the airport. The airport manager also stated that the transient parking area where the accident airplane had been parked was equipped with functional tie down chains. Evidently, the chains had not been used to secure the airplane. The pilot reported to the National Transportation Safety Board investigator that, when he initially tried to start the airplane, the starter did not properly engage. The airplane's battery was not dead. Therefore, the pilot decided to rotate the propeller by hand in an effort at resolving the apparent starter engagement problem. He did not intend to hand prop the propeller to start the engine. However, when he moved the propeller the engine suddenly started.