On November 28, 1999, about 1120 hours mountain standard time, an amateur-built Lewis Stewart S-51, N151L, owned and operated by the pilot, experienced an in-flight landing gear extension malfunction while approaching the Eagle Roost Airpark, Aguila, Arizona. Unable to fully extend one main landing gear, the private pilot retracted the other gear and elected to land his airplane in a plowed field near the airpark. The airplane was substantially damaged while sliding to a stop in the rough terrain, and the pilot was seriously injured. The flight was performed under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed at the time of the personal flight that originated from the airpark between 1030 and 1040. The Maricopa County Sheriff's Department reported that during the pilot's flight in his conventional gear airplane, the pilot broadcast that he was experiencing difficulty extending one of the main landing gear. The pilot circled the area burning off fuel. The pilot landed on soft dirt terrain about 1 mile east of the uncontrolled airstrip. In the pilot's "Aircraft Accident Report" he wrote, in pertinent part, that when he attempted to extend the landing gear only two green lights illuminated. The right landing gear was stuck in the wheel well. The pilot used the emergency hydraulic landing gear extension system, but despite all efforts the gear remained jammed in the retracted position.