On July 18, 2001, at 0843 hours mountain standard time, a Piper PA-34-200, N15120, made an off-airport emergency landing following onset of an engine nacelle fire in flight near Chandler, Arizona. The commercial certificated pilot received minor injuries and the sole passenger was not injured. The airplane was substantially damaged. The local area, post maintenance checkflight was operated by Garrison Aviation, Inc., under 14 CFR Part 91. The airplane departed from Chandler at 0745, and was returning to land when the fire commenced. Visual meteorological conditions prevailed and no flight plan was filed. The pilot told the Safety Board investigator that the purpose of the flight was to functionally test the aircraft following repairs performed after a landing incident in which the nosewheel collapsed. The repairs to the aircraft included replacement of both engines and propellers. His passenger was a mechanic. The early portion of the flight was routine. The fire occurred after about 1 hour of flying as they were returning to land. Both engines were operating normally. The pilot had reduced the power during the descent and lowered the landing gear while on a 3- or 4-mile straight-in approach. After lowering the landing gear, he advanced the throttles and sensed a loss of power in the right engine by yawing of the aircraft. He did not recall any roughness or other abnormality in the right engine up to that point; however, when he looked at the right engine he saw black smoke coming from under the top cowl seam at the firewall and black streaking on the top of the nacelle. He secured the engine by pulling the throttle to idle, the prop to feather, and the mixture to idle cutoff. When he looked back at the engine, flames were visible on the outboard side of the nacelle and the cabin rapidly filled with black smoke. The visibility in the cockpit dropped to about 1 foot and he couldn't see outside, or the instrument panel, or any controls. He declared an emergency to Chandler air traffic control tower and opened the pilot's window. He instructed his passenger to crack open the main door; however, his passenger couldn't find the door handles in the dense smoke. After a period of time he got the door cracked open and the smoke cleared. At that point they were down to 300 or 400 feet agl, and he identified an open field as an emergency landing site. He reduced power on the left engine, pulled up to fly over power lines, and landed in the field. The airplane encountered a berm during the landing roll, which collapsed the landing gear, and the airplane slid to a stop. As they exited through the door on the right-hand side, flames were still visible from the right engine, accompanied by black smoke. The fire expanded to envelop the right engine nacelle and wing. Fire department trucks arrived soon and extinguished the fire. The pilot said that, because of the rapidness with which the cabin filled with smoke and the rapidness of subsequent landing events, he never completed the engine shutdown checklist. He never turned off the fuel selector valve or the electric fuel boost pump. He said the fresh air vent had been open at the time of the fire and it draws air in through an inlet in the leading edge of the wing outboard of the right engine. He believed that the smoke entered the cockpit by means of the fresh air vent system. He said that after the event was over his clothes smelled of avgas. The airplane was further examined at the facilities of Air Transport Company in Phoenix on July 24, 2001. The right-hand engine was mechanically intact; however, a hose-end "B-nut" at the inlet to the engine driven fuel pump exhibited an unusual number of exposed threads. The B-nut was loose and required 1 1/8 turns to finger-tighten and 1 1/4 turns to snug tighten with a wrench. When the B-nut was unscrewed from the fitting, the threads of the fitting and the B-nut itself did not exhibit any cracks or stripped threads. No other mechanical anomalies were found.