United Air Lines Flight 624

United Air Lines Flight 624 was a scheduled domestic passenger flight from San Diego, California, to New York City, with stopovers in Los Angeles and Chicago. 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. The part-incapacitated crew then began an emergency descent and subsequently crashed into a hillside.

Background

The aircraft, a Douglas DC-6 registered as NC-37506, had been purchased brand new from Douglas by United Air Lines in March 1947. In November 1947, all DC-6 aircraft were grounded following two in-flight fires: one involving United Air Lines Flight 608, which resulted in 52 fatalities, and another involving an American Airlines flight that landed safely. The accident aircraft returned to service on 3 June 1948, following authorization by the Civil Aeronautics Administration (CAA).

Smoke detectors in operation around the time of the accident were found to be unreliable. This had resulted in some emergency landings, where on later inspection, no fire was actually present. In April 1948, the CAA allowed operators to disconnect faulty detectors that were indicating too many false positives. United Air Lines decided not to disconnect the detectors, but had experienced 44 false positives from the start of January 1948 to the start of May 1948.

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 compartment was pressurized like the cabin, and air could move between pressurized compartments. 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. To prevent this, the DC-6 was equipped with cabin pressure relief valves to ventilate the cabin. 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.

The pilot oxygen masks fitted to the aircraft were of the rebreather type, which mixes oxygen from storage tanks with the air already present in the cockpit. However, the fire extinguisher procedure also instructed the crew to turn off the oxygen supply when deploying the CO2 system, so using the oxygen masks would not have increased the amount of oxygen available to the pilots.

Flight

United Air Lines Flight 624 was a domestic flight from San Diego to New York City, with stopovers in Los Angeles and Chicago. A change of crew occurred at Chicago, and the flight departed at 10:44 for LaGuardia Airport, with 39 passengers and 4 crew onboard. It then climbed normally up to its cruising altitude of 17,000 feet. The crew's last routine air traffic control communication occurred at 12:27, when they acknowledged a descent clearance. Four minutes later, crew from another United flight heard unidentified shouts of "New York" and "This is an emergency descent". 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.

The aircraft was observed to be in a shallow descent by witnesses, until around 200 feet above ground, when it started to climb and turn to the right. 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. All 43 occupants on board were killed. 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.