United Airlines Flight 608 - Cause of The Crash

Cause of The Crash

Just over three weeks later, on November 11, 1947, a similar in-flight incident almost claimed a second commercial DC-6 airliner.

An American Airlines DC-6 (NC90741), on a flight from San Francisco to Chicago with 25 crew and passengers aboard, reported an on-board fire over Arizona and managed to make an emergency landing in flames at the airport at Gallup, New Mexico. All 25 occupants escaped the burning plane, and the fire was extinguished. But unlike the Bryce Canyon crash a month earlier, investigators now had a damaged but intact aircraft to examine and study.

The cause of both the Bryce Canyon crash and the near-fatal Gallup incident was eventually traced to a design flaw. A cabin heater intake scoop was positioned too close to the number 3 alternate tank air vent. If flightcrews allowed a tank to be overfilled during a routine fuel transfer between wing tanks, it could lead to several gallons of excess fuel flowing out of the tank vent and then being sucked into the cabin heater system, which then ignited the fuel. This caused the fire which destroyed the United aircraft at Bryce Canyon and severely damaged the American aircraft that landed in flames at Gallup.

In the Bryce Canyon crash, the Civil Aeronautics Board found the causes to be the design flaw, inadequate training of the crew about the danger, and the failure of the crew to halt the fuel transfer before the tank overflowed.

The UAL 608 crash, and the second incident in Gallup, New Mexico three weeks later, led to the temporary grounding of all DC-6 aircraft while corrective modifications were made on the fuel tank and heater air intake venting systems to prevent any similar incidents from occurring.