On July 30, 1993, about 1932 hours Pacific daylight time, a homebuilt P.D. Nall Varieze airplane, N75PD, collided with a barbed wire fence while executing an emergency landing to an open field at Newark, California. The emergency landing was precipitated by a severe vibration, a partial loss of a propeller blade, and an ultimate loss of engine power. The pilot was conducting a visual flight rules personal flight to Palo Alto Airport, Palo Alto, California. The airplane, registered to and operated by the pilot, was destroyed and there was no post-impact fire. The certificated private pilot, the sole occupant, sustained minor injuries. Visual meteorological conditions prevailed. The flight originated at Cameron Airpark, Cameron Park, California, at about 1845 hours. The pilot reported in a telephone interview conducted on August 3, 1993, that when the flight was about 6 miles north of Palo Alto Airport, the airplane suddenly began to vibrate violently and the engine power simultaneously reduced to about 50 percent of its available power. The pilot immediately selected a road to execute a precautionary landing. During the approach to the road, the engine sustained a total loss of power. The airplane's altitude was insufficient to land on the road due to trees crossing the airplane's flight path. The pilot selected a corn field, but collided with a barbed wire fence during the approach. The pilot reported that part of one propeller blade was not found. He also stated that extensive engine maintenance was performed about 3 flight hours preceding the accident. Mr. Roy Hutto, Aviation Safety Inspector [Airworthiness], examined the airplane's engine on August 19, 1993, at the registered owner's facility. He reported that he established continuity of the engine gear and valve train assembly and that compression appeared to be normal. He noted, however, that one bolt/screw was missing on the right side of the engine. This bolt/screw attached the exhaust bracket to the valve cover. The remaining screws in the valve covers had holes for safety wire, but no safety wire was present. Inspector Hutto stated that "... these screws had star-type lock washers that were so old and reused, they did not appear to be locking. Most of the rocker arm cover screws were tight, but three or four were somewhat loose." Both magnetos produced spark during hand rotation of their respective drive shafts. The impulse couplings operated normally. The wooden propeller, a Serba 60 X 68, had one blade that was splintered. The splintered blade exhibited at least one screw indentation on its leading edge. The splinter, however, appeared to originate from the trailing edge. Inspector Hutto, at the request of the Safety Board, sent the propeller to Sensenich Propeller, Inc., Lititz, Pennsylvania, for examination. Mr. Kenneth C. DeGraff, Chief Propeller Engineer, Sensenich, reported that: 1) Several indentations were found on the leading edge and flat face of both blades. After further examination of the indentations, it was determined to have been caused by machine screws and/or bolts which had entered the propeller plane during rotation. 2) One (1) failed edge glued joint was found on the #1 [the splintered propeller blade]. This was an adhesive failure as indicated by a smooth surface at the failed joint. Mr. DeGraff concluded: Although a failed edge glue joint was found in the failed propeller, it is our opinion that this did not cause a propeller blade failure. Based on the extensive cracking from tip to hub, it is our opinion that the damage was a result of an impact with the ground.