At approximately 2040 Mountain time, the airplane impacted the terrain during an off-field forced landing about one and one-half miles northwest of the approach end of Runway 12 at Gallatin Field, Belgrade, Mon. The Commercial pilot and two passengers were not injured, but the aircraft was destroyed by the post-crash fire. According to the pilot, when the aircraft was about three miles from the approach end of the runway, the engine suddenly developed a loud howling sound, the torque became very erratic and the exhaust gas temperature (EGT) exceeded its normal limits. Soon thereafter the engine lost all power, and the pilot executed a forced landing. Although the touchdown was successful, after the airplane came to a stop a small fire developed near the engine exhaust. All occupants exited the aircraft without injury, but they were unable to extinguish the fire, which consumed the aircraft.