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The captain, first officer, and one stewardess served aboard UAL Flight 662 from Washington, D. C., to Philadelphia, Pennsylvania, arriving at Philadelphia at approximately 1215.
United Air Lines Flight 823, a Vickers Viscount 745D, N7405, departed Philadelphia at 1513 arriving at Washington, D. C., at 1554. No discrepancies were reported by the crew at Washington and no maintenance, other than servicing, was required or performed prior to departure for Knoxville.
Flight 823 was a regularly scheduled domestic public transportation flight from Philadelphia, Pennsylvania, to Huntsville, Alabama, with intermediate stops at Washington, D. C., and Knoxville, Tennessee. Flight 823 departed Washington, D. C., at 1636 with an estimated arrival time of 1813 at Knoxville, Tennessee.
An Instrument Flight Rule (IFR) flight plan, filed for Flight 823 from Washington to Knoxville, requested routing via Victor Airways 143, 140, and 16 at an altitude of 14,000 feet m.s.l. Following departure from Washington with 35 passengers and a crew of four,[1] the flight operated without any reported difficulties to the Holston Mountain VOR.[2] The crew reported to the Atlanta Air Route Traffic Control Center (ARTCC) over that fix at 1758:35 and estimated their arrival at Knoxville at 1821.
Approximately one minute after having reported passing Holston Mountain, the crew[3] 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 canceled 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.
The aircraft's radar target disappeared from the controller's scope at 1813:30. At 1814, after waiting four sweeps of the radar antenna, the Atlanta controller called the aircraft to advise them he had lost radar contact but he received no reply.
Numerous ground witnesses observed the aircraft flying at what they considered to be a very low altitude. Witnesses estimated the altitude of the aircraft to be from 200–500 feet above the ground along a line generally parallel to, but south of, V16. The last 10 to 12 miles of the flightpath were observed by a number of ground witnesses, several of whom stated they saw smoke of varying density apparently coming from the fuselage of the aircraft.
Flight 823 was first observed approximately 36 miles southwest of Holston Mountain VOR at an estimated altitude of 5,000 feet descending. There was no visible difficulty at that time. A witness who observed the aircraft from a position 11 miles
