The four-engined, propeller-driven Douglas DC-6 crashed at 1:41 pm Eastern Daylight Time on June 17, 1948, outside Aristes, Pennsylvania, resulting in the deaths of all 4 crew members and 39 passengers on board.
The crew had deployed the aircraft's CO2-based fire extinguisher system, without opening the pressure relief valves designed to ventilate CO2 out of the cabin.
[1]: 3 The aircraft had a CO2-based fire extinguisher system, which deployed CO2 into a compartment below the cabin and cockpit when activated by the flight crew.
This meant that when the fire extinguisher system was deployed, CO2 could escape from the under-floor compartment into the cabin and cockpit, which could cause passenger and crew incapacitation.
These pressure relief valves had to be manually operated, and this was included in the procedure for the fire extinguisher system, in the step before activating the CO2 selectors.
Some elements of the transmission were unintelligible, but investigators concluded that this was likely from Flight 624 and also included mentions of the fire extinguisher system being discharged.
As the aircraft encountered higher terrain, it increased its climb until it struck a hillside three miles from Mount Carmel, with an explosion occurring after impact.
[1]: 1–2 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 and co-founder of Esquire.
It stated that in general, emergency procedures should take into account the possibility of human error as a result of a higher workload.
[8] It was found that pilots conducting test flights for Douglas following the November 1947 groundings reported adverse effects caused by the CO2-based fire extinguisher system, despite having activated the pressure relief valves.
After these test flights, Douglas fitted an additional relief valve to provide improved ventilation; they also commissioned an independent report, which was finalized in February 1948 with several recommendations.