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 fuel tank air vent. If flightcrews allowed a fuel 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.