On March 5, 2006, approximately 1415 mountain standard time, an Aviat Pitts S-2B, N221RS, piloted by a commercial pilot, was substantially damaged when it nosed over during a forced landing near Severance, Colorado. Visual meteorological conditions prevailed at the time of the accident. The local personal flight was being conducted under the provisions of Title 14 CFR Part 91 without a flight plan. The pilot was not injured but the passenger reported a minor injury. The flight originated at the Fort Collins-Loveland Municipal Airport (FNL) approximately 1345. In a telephone conversation with the pilot shortly after the accident, he stated that he last refueled the airplane two days prior to the accident, adding 8.0 gallons. This filled the main 23-gallon fuel tank to capacity. On the afternoon of the accident, he flew one passenger for about 30 to 45 minutes. He returned to the airport, offloaded the passenger, boarded a second passenger, and took off again. They had been flying for about the same period time when the engine suddenly began to run rough, then lost power. He made a forced landing in an open field. During the landing roll, the airplane nosed over, crushing the vertical stabilizer and breaking a rib in the upper left wing. The pilot said fuel was leaking from the fuel tank and it looked and smelled like kerosene. The fixed base operator (FBO) was contacted and the fuel samples that had been sumped that day from the fuel trucks and fuel farm were quarantined. The salvage crew that responded to the accident site up-righted the airplane and retrieved a fuel sample as it dripped from the right rear exhaust manifold. Their first impression was that the fuel was possibly contaminated. After recovering the airplane the next day, the salvage company re-examined the fuel sample and said they didn't believe the fuel was contaminated, but that it may have mixed with the smoke oil, giving it the appearance and smell of kerosene. The airplane was equipped with a smoke-generating system. On March 15, NTSB examined the airplane at Beegles Aircraft Service, Inc., Greeley, Colorado. Neither the fuel nor smoke oil tanks had been compromised. All supply lines were intact and securely fastened. Less than a quart of fuel was recovered from the fuel tank. The minute, suspicious-looking fuel sample that was collected by the salvage crew was cloudy in appearance and had an oily feel to it. The thirteen jars of fuel that had been sumped from the FBO's fuel trucks and from the fuel farm were retrieved and examined. They were all clear, blue (similar to 100-LL), and appeared to be free of contaminants. It was decided to functionally test the engine, using the sumped fuel samples. The pilot submitted a written statement of the events that led up to the accident. He indicated each flight leg lasted 30 minutes. The digital fuel flow gauge indicated 15.5 gallons of fuel had been consumed, and 7.5 gallons remained. When NTSB examined the airplane, these fuel flow gauge figures were confirmed. However, in the lower right portion of the rear cockpit was a fuel gauge, comprised of a clear plastic tube with graduated markings. When asked if he had checked the fuel gauge, the pilot said he couldn't see it due to the cramped confines of the cockpit, and his right leg and knee blocked his view. Textron Lycoming was contacted and asked to provide fuel consumption data. According to the Fuel Consumption Curve Chart, the IO-540-D engine will burn 17.4 gallons per hour (gph) at 75 percent power, which is 195 horsepower (hp) --- or 2,450 revolutions per minute (rpm) --- when leaned for best power. At full power --- or 260 hp and 2,700 rpm --- the engine will burn 21.5 gph. On March 20, asked that the airplane be released to him for repairs. He was told that it would be released to him after the engine had been functionally tested. The pilot said that this would be unnecessary because he had "run out of gas." He subsequently submitted a written statement to that effect.