A United Air Lines Vickers Viscount 745D, N7405, Flight 823, crashed near Parrottsville, Tennessee. Thirty-five passengers and the four crewmembers died in the crash. Flight 823 was a regularly scheduled operation from Philadelphia, PA, to Huntsville, AL, with en route stops at Washington, D.C., and Knoxville, TN. The flight operated without any reported discrepancies or difficulties. Flight 823 departed Washington-National Airport, DC (DCA), at 16:36 with an estimated arrival time of 18:13 at Knoxville-McGhee Tyson Airport, TN (TYS). The flight proceeded on an IFR flight plan at FL140 to the Holston Mountain V0R. The crew reported to the Atlanta ARTCC over that fix at 17:58:35 and estimated their arrival at Knoxville at 18:21. Approximately one minute after having reported passing Holston Mountain, the crew requested a clearance to descend to the lowest available altitude. They were cleared to descend to and maintain 8,000 feet. Three minutes later the crew cancelled their IFR clearance. The controller offered to pass control of the flight to Knoxville Approach Control when they were closer in and advised they could stay on the Center frequency. At 1802:55 the crew responded to this transmission with “OK.” This was the last known transmission from the aircraft. It is believed that the crew discovered a fire sometime during the period between cancelling their IFR and before being observed in a descent about 4,000 feet above the ground. The aircraft deviated to the south of Airway V16 but was proceeding in a descent approximately parallel to the airway. At approximately 18:10, the aircraft was observed about 500 feet above the ground, trailing smoke. The aircraft continued to operate at very low altitudes and well to the left of the airway from this point on to the crash. At a point approximately 3 km before the impact site, a passenger opened the left hand overwing exit and exited the plane. He fell straight down and did not survived the jump. The aircraft was then observed going into a nose-high attitude, the left wing and the nose went down, and the aircraft dived into the ground, exploded, and burned. PROBABLE CAUSE: “An uncontrollable in-flight fire of undetermined origin, in the fuselage, which resulted in a loss of control of the aircraft.”