On May 20, 2011, at 1513 central daylight time, N8957D, a Piper PA-22-160 airplane, was substantially damaged from an in-flight fire. The pilot made a forced landing to a field and the airplane was consumed by fire. The private pilot was seriously injured and the passenger was not injured. The airplane was owned and operated by the pilot. Visual meteorological conditions prevailed and no flight plan was filed for the local flight conducted under 14 CFR Part 91. According to the pilot, about 20 minutes after departure he was maneuvering at an altitude of 600 feet above ground level (agl) when he began to feel his feet getting hot. He thought it might have been the cabin heat, but the heat was getting more intense and the cockpit began to fill with a gray/bluish colored smoke. The pilot then observed black smoke and flames near his left foot, which he attempted to stomp out. The pilot made a forced landing to a muddy field where he and his passenger were able to exit the airplane before it was rapidly consumed by fire. The airplane was examined by an inspector with the Federal Aviation Administration(FAA)the day after the accident. He stated that the entire fuselage, tail section, right wing and the inboard section of the left wing were consumed by fire. The engine compartment also sustained extensive fire damage. Several photographs of the engine and airplane were sent to a National Transportation Safety Board Fire and Explosion Specialist. The specialist reviewed the photographs and was unable to determine the origin or cause of the fire due to the extensive damage and limited investigative materials.