On March 8, 2003, at 1045 Pacific standard time, an experimental Kolb Firestar, N670JW, made an emergency landing in an open field approximately 6 miles west of Brown Field Municipal Airport, San Diego, California. The emergency landing was precipitated by a sudden onset of "abnormal vibrations." The airplane was operated by the pilot/owner under the provisions of 14 CFR Part 91, and sustained substantial damage. The private pilot and one passenger were not injured. Visual meteorological conditions prevailed for the local area flight and no flight plan had been filed. The flight originated at Brown Field at 1035. The pilot stated in the Pilot/Operator Accident Report (NTSB Form 6120.1/2), that while flying westward towards the coastline, at 1,500 feet mean sea level, he heard a "bang," followed by vibration. He elected to perform an emergency landing in an open field. Upon landing in soft ground the airplane pitched nose down and came to rest inverted. The pilot's initial inspection revealed a "splintered" propeller, vertical stabilizer damage, and damage to the right "aileron tube." He did not recall any object flying into the propeller arc during the flight or any object striking the propeller at any time prior to the accident flight. The two carbon fiber composite propeller blades separated from the hub and were retained by a Federal Aviation Administration (FAA) Inspector. He sent them to be inspected by the manufacturer under the supervision of the FAA and National Transportation Safety Board. After being examined by the manufacturer, the FAA sent the propeller to the Safety Board investigator-in-charge (IIC). Inspection of the propeller revealed two separate blades with dissimilar damage. The first blade (blade 1) was split in half along the leading edge thru the center to the trailing edge and fit together like a sandwich. Foam core material was attached to the inside of each blade half. Seven inches from the blade tip was a distinct 1.875-inch-wide abrasion mark that ran parallel to the blade chord on the uncambered side for the width of the blade. The 1.875-inch-wide mark exhibited a pattern of narrow parallel lines embedded into the erosion strip on the leading edge. When the two sides of the blade are fitted together a single 1.875-inch indentation can be identified on the leading edge at the 1.875-inch-wide abrasion mark location. The second blade (blade 2) fractured completely in two pieces with the fracture location approximately 6 inches from the blade-hub attachment base location. The fracture line ran roughly parallel to the chord line of the blade. On the blade-hub attachment barrel location there was a score line scribed on the trailing edge side. The score line was 2.25 inches in length along the circumference of the barrel and 2.25 inches from the propeller blade base. Approximately 16 inches from the blade tip there was a 0.5-inch indentation on the leading edge. The pilot-owner said that the propeller had a total operational time of approximately 12 hours on it at the time of the accident. The manufacturer's examination of the carbon fiber composite propeller revealed numerous indentations on the leading edges and patterns consistent with a foreign object strike prior to blade delamination and subsequent vibration. The manufacturer identified a 1.9-inch-wide abrasion mark along the entire blade width that resembled patterns that are commonly associated with nylon safety-belt webbing. The New Kolb Aircraft Company told the IIC that the standard harness webbing provided by them is 1.75 inches wide. The pilot said that the rear seat of the airplane had the standard Kolb 3-point harness webbing and the pilot seat had an after market 4-point harness. The airplane is a high wing design, with the wing mounted above and behind the open cockpit. The engine and pusher propeller are mounted on top of the wing.