On October 1, 2006, about 1320 Pacific daylight time, an Oneil Glassair II-S experimental amateur-built airplane, N204BL, sustained substantial damage following a loss of engine power and impact with terrain near the Richland Airport, Richland, Washington. The airplane was registered to and operated by a private individual. The commercial pilot was not injured, and his sole passenger received minor injuries. Visual meteorological conditions existed for the personal flight, which was conducted in accordance with 14 CFR Part 91, and no flight plan was filed. The local flight was originating at the time of the accident. According to the Pilot/Operator Aircraft Accident/Incident Report (NTSB Form 6120.1), and in telephone conversations with the NTSB investigator-in-charge (IIC), the pilot reported that after taking off and maintaining climb power until reaching 1,000 feet above ground level (agl), "...I reached for the throttle lever to reduce power to cruise climb, but the engine power surged down 20 per cent." The pilot stated that he then started a shallow left turn back to the airport, and after making a 180-degree turn noted that his airspeed had slowed to 100 miles per hour. The pilot reported that when he was about 1 mile from the runway at an altitude of 600 feet agl, he made the decision that he could not make it back to the runway. The pilot stated that he continued to turn to the 270-degree point in an attempt to land in a farm field, which was about one-quarter mile away. The pilot further stated that at about 100 feet agl he looked at the surface, and believing it was flat enough, lowered the landing gear. The pilot reported that he cleared the trees bordering the field on the east, landed, and then rolled approximately 1,000 feet before maneuvering to avoid a tree line on the west side of the field. The pilot further stated that after impacting two trees the airplane came to rest in the middle of a gravel road. The pilot reported that damage to the airplane included the nose gear and right main landing gear collapsing, aft crushing to the right wing spar, and the fuselage being bent and buckled. There was no post-crash fire. The pilot reported to the IIC that there were no anomalies noted with the airplane prior to the flight. During a post-accident examination of the engine by a certified Federal Aviation Administration airframe and powerplant mechanic, the mechanic reported that no anomalies were noted which would have precluded normal operations. After the removal of the original propeller and the installation of an exemplar propeller, the engine was started and operated at 1,000, 1,500, and 1,800 revolutions per minute, with all indications reported as normal. The reason for the partial loss of engine power could not be determined.