Flight 624 from San Diego had just completed a routine initial descent as part of its approach into the New York area, when the forward cargo hold fire indicator light illuminated, leading the flight crew to believe there was a fire in that cargo hold. Although this later turned out to be a false alarm, the crew decided to discharge Carbon dioxide| bottles into the forward cargo hold, to try to extinguish the possible fire. While proper operating procedure called for opening the cabin pressure relief valves prior to discharging the bottles, to allow for venting of the gas buildup in the cabin and cockpit, there was no evidence the crew opened the relief valves. Consequently, the released gas seeped back into the cockpit from the front cargo hold and apparently partially incapacitated the flight crew. The crew then put the aircraft into an emergency descent, and as it descended lower it hit a high voltage power line, bursting into flames, then smashing through the trees of a wooded hillside.
Notable victims
Among the passengers were Broadway theatre impresario Earl Carroll and his girlfriend, actress Beryl Wallace, plus Henry L. Jackson, men's fashion editor of Collier's Weekly magazine and co-founder of Esquire Magazine. An interesting item discovered in the wreckage was a script of Garry Moore's Take It or Leave It show broadcast at the time each Sunday night over NBC, but an NBC spokesman in New York said he was "certain" Moore was not on the plane.
The airplane, named Mainliner Utah, arrived in Chicago at 09:52 en route from Los Angeles to New York. After a 52-minute turnaround, the DC-6 departed for New York. The airplane climbed en route to its planned altitude of 17,000 feet. At 12:23, and at 12:27 the crew made a routine acknowledgment of a clearance to descend en route to an altitude between 13,000 and 11,000 feet. A little later a fire warning led the crew to believe that a fire had erupted in the forward cargo hold. They then discharged at least one bank of the fire extinguisher bottles in the forward cargo hold. Because they did not follow the correct procedure, the cabin pressure relief valves were closed. This caused hazardous concentrations of the gas to enter into the cockpit. These concentrations reduced the pilots to a state of confused consciousness probably resulting in loss of consciousness. An emergency descent was initiated until it described a shallow left turn, heading towards constantly rising terrain. Five miles east of Shamokin the airplane, flying only 200 feet above the ground, entered a right climbing turn. As it passed to the north of Mt. Carmel, the climbing turning attitude increased sharply. The airplane then crashed in a power line clearing on wooded hillside at an elevation of 1,649 feet. The airplane struck a 66,000 volt transformer, severed power lines and burst into flames. Investigation revealed that the fire warning in the cargo compartment had been false. — CAB File No. 1-0075-48
The CAB concluded with the following probable cause for the accident: "The Board determines that the probable cause of this accident was the incapacitation of the crew by a concentration of gas in the cockpit."
