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A United Air Lines, Inc., Vickers Vicsount 745D, N7405, Flight 823, crashed 2-1/4 miles northeast of Parrotsville, Tennessee, at 1815 e.s.t., July 9, 1964. Thirty-four passengers and the four crewmembers died in the crash. One passenger died of injuries following a free fall from the aircraft before the crash. The aircraft was destroyed by fire and impact damage.

Flight 823 was a regularly scheduled operation from Philadelphia, Pennsylvania, to Huntsville, Alabama, with en route stops at Washington, D. C., and Knoxville, Tennessee. The flight operated without any reported discrepancies or difficulties until approximately 1810 e.s.t., when it was observed flying at low altitude trailing smoke. The flight continued in a southwesterly direction and at a point approximately 1.6 nautical miles before the impact site, a passenger was seen falling from the aircraft, and a short time later a cabin window was seen falling. 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.

The Board determines that the probable cause of this accident was an uncontrollable inflight fire of undetermined origin, in the fuselage, which resulted in a loss of control of the aircraft.

1.1 History of Flight

The aircraft, N4705, operated as United Air Lines (UAL) Flight 609 from Raleigh-Durham, North Carolina, to Philadelphia with intermediate stops at Washington, D. C., Buffalo and Elmira, New York, and Williamsport, Pennsylvania. The aircraft departed Raleigh-Durham at 0705^[1] and arrived at Philadelphia at 1245. The captain and senior stewardess of the flight from Buffalo to Philadelphia stated that the flight was routine and the aircraft operated normally.

​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,^[2] the flight operated without any reported difficulties to the Holston Mountain VOR.^[3] 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^[4] 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 ​northeast of the crash site was the first to report seeing anything unusual. She noted a violet red light burning on the fuselage. She could offer no further clarification as to the location of this light. While she could read the company name on the side of the aircraft, she did not see any smoke. The time was about 1810 and the aircraft was estimated to be at an altitude of 500 feet.

The first witness to report smoke from the aircraft was approximately five miles from the crash scene. He stated that "...smoke was coming out of the tail part..." and "...there were brown spots like the paint was off of it about half way back on the body...." Witnesses about one mile farther along the flightpath did not observe anything unusual, except the low altitude, until the aircraft had passed them at which time they observed smoke coming from the aircraft.

A number of witnesses about two miles from the crash site, near the flightpath, did observe black smoke coming from aircraft fuselage near the wings. A large black object, later identified as a passenger, was observed to fall away from the aircraft, followed by dense black smoke. The witnesses stated that the object did not strike the tail of the aircraft after coming out the left side over the wing. Farther along the flightpath a bright object, later identified as the left No. 9 emergency cabin window, fell from the aircraft. Heavy smoke was seen continuing to come from the aircraft.

While the majority of the witnesses did not report seeing any fire, some witnesses did report signs of fire in or on the aircraft.

Shortly after the passenger and the window fell from the plane the aircraft nosed up, the left wing went down, the aircraft nosed down, and crashed into a rocky wooded hillside.

The crash occurred approximately 41 nautical miles east-northeast of the Knoxville VORTAC and about 2-1/4 nautical miles northeast of Parrottsville, Tennessee,^[5]