Investigation Summary
The accident highlighted a critical design flaw in the DC-6's fire suppression system and its associated emergency procedures. At the time, the DC-6 used a CO2-based system that required pilots to manually open cabin pressure relief valves before discharge to prevent the gas from entering the pressurized cabin and cockpit. During the high-stress environment of a perceived fire, the crew of Flight 624 missed this step. Furthermore, the oxygen masks provided were rebreathers that mixed cockpit air with oxygen; because the procedure also dictated shutting off the oxygen supply during CO2 discharge, the masks were ineffective against the gas concentration. This crash occurred only months after the DC-6 fleet had been grounded due to in-flight fires, and it was later revealed that Douglas test pilots had experienced near-incapacitation during tests but the findings were not fully shared with operators or the CAA.
Final Conclusions
The Civil Aeronautics Board (CAB) determined the probable cause was the incapacitation of the crew by a concentration of CO2 gas in the cockpit. The board noted that the emergency procedure was overly complex for a high-workload environment and failed to account for human error. The fire warning that triggered the sequence was found to be false, caused by unreliable smoke detectors that were prone to false positives.
Video Analysis
Toxic Leak | United Airlines Flight 624 — A detailed look at how a false fire alarm and a poorly designed fire suppression system led to the incapacitation of the crew and the loss of all 43 people on board a Douglas DC-6 in 1948.
If playback is blocked, the owner has disabled embedding — use the link above.
Airframe & Maintenance
Systems & Failure Modes
CO2 Fire Extinguishing System
A system designed to flood cargo compartments with CO2; it lacked an automatic ventilation interlock, requiring manual valve operation.
Cabin Pressure Relief Valves
Manual valves intended to vent the cabin during CO2 discharge to prevent passenger and crew suffocation.
Rebreather Oxygen Masks
Pilot masks that mixed ambient air with oxygen, making them useless if the ambient air was saturated with CO2.
Interesting Facts
- 01The aircraft, 'Mainliner Utah', had returned to service only two weeks prior following a fleet-wide grounding.
- 02The fire warning was false; post-crash inspection found no evidence of fire in the cargo compartments.
- 03United Air Lines had recorded 44 false smoke detector alarms in the four months preceding the crash.
- 04Douglas test pilots had reported being 'almost completely out' during CO2 system tests in early 1948.
- 05Notable fatalities included Broadway impresario Earl Carroll, actress Beryl Wallace, and Esquire co-founder Henry L. Jackson.
Safety Actions & Advisories
Fire System Redesign
Douglas redesigned the DC-6 fire extinguisher system to automatically open pressure relief valves when the CO2 was discharged.
Ventilation Improvements
An additional relief valve was added to the DC-6 design to further reduce CO2 concentrations in the cabin after system activation.
Known Controversies & Unanswered Questions
Withholding of Test Data
Reference ↗During litigation, it was revealed that Douglas test pilots had documented CO2 hazards in early 1948, but a chief test pilot reportedly marked the findings 'Do not discuss with CAA', and the full report was not shared with United before the accident.