1948 aviation accident
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 been responding to a false signal of a fire in the front cargo hold by releasing carbon dioxide (CO2), apparently without opening the pressure relief valves designed to ventilate CO2 out of the cabin. The part-incapacitated crew began an emergency descent and hit a high-voltage power line.
Background.
The aircraft, a Douglas DC-6, 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 1948.3
Fire extinguisher system.
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.5, 7
However, pilots conducting test flights following the groundings reported adverse effects caused by the CO2-based fire extinguisher system, despite having activated the pressure relief valves. A chief test pilot later stated that he was "almost completely out" during one such test. 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.8 This report was not sent to United before the accident, but Douglas stated that it had been discussed with United's flight safety engineer; though this was denied by the engineer. According to court testimony, any such discussion may have only related to a small part of the report and not its entire contents. Separately, Douglas' chief pilot said, "I probably did not spend more than, oh, an hour on the report; a report of that nature cannot be properly evaluated in an hour". Some other key personnel within Douglas had no knowledge of either the report, or the underlying CO2 issue identified during the test flights.
Flight.
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. 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", which investigators later concluded were likely from Flight 624.2
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.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".
Investigation and final report.
The forward cargo hold fire indicator light had illuminated, leading the flight crew to believe a fire was in that cargo hold. Although this later turned out to be a false alarm, the crew decided to discharge CO2 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 CO2 bottles, to allow for venting of the CO2 gas buildup in the cabin and cockpit, no evidence was found of the crew opening the relief valves. Consequently, the released CO2 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.
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Ed Darlington of radio station WCNR at nearby Bloomsburg said, "there was no sign of life and apparently everyone was killed." The scene of the wreck was in a sparsely wooded area about five miles from Mount Carmel, Pennsylvania, a small town 135 miles from Philadelphia, where delegates are gathering for the Republican National Convention. News of the crash brought excited whispering from the delegates. No one knew for certain whether any high-ranking Republican officials were on the plane.
Ira F. Roadarmel of Mount Carmel, one of the first persons on the scene, said, "everything was scattered. The largest piece of the plane left was an engine. The rest of the plane was in small parts — so small they could be carried."
George Minnich, an employee of Midvalley Colliery No. 2, which the plane missed by only 100 yards in its descent, said that he saw the plane bank. "Suddenly, there was a horrible crash," he said. "All you could see was a mass of flames. It sounded as though the end of the world was coming."
The plane's logbook, found near the scene of the crash in a thickly wooded area, identified the plane's pilot as Captain George Warner.
The Civil Aeronautics Board investigated the accident and published a narrative describing the following sequence of events in its final report: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 CO2 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 CO2 gas in the cockpit."
References.
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