United Airlines Flight 823 was a scheduled flight from Philadelphia International Airport, Pennsylvania to Huntsville International Airport, Alabama with 39 on board. On July 9, 1964 at approximately 18:15 EST, the aircraft, a Vickers Viscount, registration N7405, crashed 2 miles northeast of Parrottsville, Tennessee after experiencing an uncontrollable fire on board, killing all 39. The fire of unknown origin initiated below the passenger floor and eventually involved the passenger cabin. One passenger attempted to abandon the aircraft through the No.4 escape window prior to impact but did not survive the free-fall. The ignition source was never determined, but it is thought the plane's battery overheated or something in a passenger's luggage caused the fire. According to NTSB investigators, lethal amounts of CO2 were present in the cockpit, explaining witnesses' reports of the aircraft seen flying erratically. The fire eventually burned through the cockpit and it is likely the crew was unconscious by that time.
United used the Flight 823 designation on its Washington-Dulles to Mexico City, Mexico route.