On July 9, 2004, approximately 2010 Pacific daylight time, a Hughes 269C helicopter, N56PH, impacted the terrain during an attempted autorotational landing about 12 miles north of Arlington, Washington. The commercial pilot and his passenger were not injured, but the aircraft, which is owned and operated by the pilot, sustained substantial damage. The 14 CFR Part 91 personal pleasure flight, which departed Bellingham, Washington, about 55 minutes prior to the accident, was being operated in visual meteorological conditions. The flight was en route to Arlington, Washington. No flight plan had been filed. According to the pilot, while cruising at about 2,500 feet above the ground (agl), he heard a "pop" and felt the helicopter yaw to the right. Almost immediately he noticed that the engine was starting to lose power, and he therefore lowered the collective and initiated a partial autorotation "...to take demands off the engine." He then attempted to get the engine to accelerate, but he continued to hear "sputtering and popping," and the engine continued to gradually lose power. The pilot therefore elected to perform a full autorotational landing in a nearby opening in the heavily wooded area he was flying over. Although he was able to touch down with only a very slow forward speed, the terrain he was landing on turned out to be a wet and muddy bog or swamp, and as the helicopter touched down, one of the skids sunk into the mud, and the aircraft rolled over onto its side. A post-accident inspection of the helicopter's engine and fuel system by an FAA Airworthiness Inspector and maintenance personnel from Precision Helicopters did not find any evidence of an anomaly that would have contributed to a loss of power in the engine. After the initial inspection was completed, the engine was mounted on an engine stand and run at various speeds for a period of about eight minutes. During the test run, the engine started easily and ran smooth at all tested speeds up to and including a maximum of 3,100 rpm.