On August 14, 2005, at 1217 Pacific daylight time, an experimental Prescott Aeronautical Pusher, N43PP, lost its propeller during flight and the pilot made a forced landing in a field 1/2-mile south of Redding Municipal Airport (KRDD), Redding, California. During the forced landing, the airplane impacted trees and sustained substantial damage. The pilot was operating the privately registered airplane under the provisions of 14 CFR Part 91. The pilot sustained serious injuries. Visual meteorological conditions prevailed for the cross-country flight. The flight departed from Pearson Field Airport (KVUO), Vancouver, Washington, at 0920, and was destined for Willows-Glenn County Airport (KWLW), Willows, California. No flight plan had been filed. According to the pilot, he was going to stop at Willows for fuel, and then continue to Whiteman Airport, Los Angeles, California. He was established in cruise flight at 7,500 feet mean sea level when he felt a vibration. The vibration then became "severe" and he heard a series of clinking noises. He radioed the Redding Air Traffic Control Tower and advised that he needed to land immediately because he knew that his propeller had separated from the pusher airplane. The pilot force-landed the airplane in a vacant field. The pilot stated that about 2 weeks prior to the accident he and a friend had removed the wooden American Propeller and installed a different propeller. The new propeller was too long for ground clearance so the pilot and his friend reinstalled the original wooden American Propeller. The original hardware was used to install the propeller and the bolts were torqued to the recommended amount. The bolts were also safety wired. After its installation, the propeller was operationally tested. Prior to its removal, the propeller had been installed on the airplane for about 1 year with no operational or mechanical problems noted. The pilot did not have any reported vibrations on flights leading up to the accident flight. The responding airport personnel sent photos of the accident site to the National Transportation Safety Board investigator-in-charge. The photos showed that the propeller and the attachment bolts were not on the propeller flange. Three of the bolt holes showed evidence of rubbing and partial elongation of the holes. The propeller and its attachment hardware were not recovered.