Chicago

The Board determines that the probable cause of this accident was the loss of the outer panel of the left wing which separated from the aircraft as a result of a fatigue crack in the left front outer panel attachment fitting which had been induced by a faulty design of that fitting, the fatigue crack having been aggravated by severe turbulence encountered in the thunderstorm.

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.
The following factors were considered as contributing:
- A fire warning caused the crew to discharge at least one bank of the CO2 fire extinguisher bottles in the forward cargo pit (the forward underfloor baggage compartment),
- Six 15-pound CO2 bottles and six discharge valves were found in the wreckage, however, both the bottles and the valves (which had become separated from their respective bottles upon impact) were so damaged that no conclusions could be drawn as to how many of such bottles had been discharged prior to impact,
- At the time of impact, the emergency cabin pressure relief valves were closed, and the control mechanism for such valves was in the closed position,
- Except for the apparent failure of the fire detection instrument referred to in finding No. 5, supra, the investigation revealed no mechanical failure of the aircraft or fire in flight,
- The emergency procedure for the operation of the DC-6 fire extinguisher system was established after flight tests were conducted in a descent configuration of 300 miles per hour, with landing gear and flaps up, no flight tests were conducted prior to the accident in a descent configuration of 160 miles per hour with gear and flaps down, which configuration was also approved for DC-6 operations,
- At the time of impact the landing gear was in the “up" position, thus indicating that the aircraft had descended in the configuration of 300 miles per hour. The extensive breakage of the aircraft precluded any positive determination as to the position of the flaps,
- After the release of CO2 gas hazardous concentrations of the gas entered into the cockpit,
- Due to the physiological and toxic effects of high concentrations of CO2 gas in the cockpit, which would probably not have occurred had the cabin pressure relief valves been open, the members of the flight crew of the aircraft were rendered physically and mentally incapable of performing their duties.
The following comment was added to the conclusion:
A fire in flight permits little opportunity for the exercise of detached and thoughtful consideration of emergency procedure. Immediate action is required if a fire is to be controlled. Too little consideration has been given to the psychological and physical limitations of crew members in time of stress and danger as related to the complexity of emergency fire procedure. It is not safe to assume that the pilot and co-pilot, under emergency pressure, will always adhere rigidly to the sequence of steps outlined in the CAA Approved Airplane Operating Manual. The possibility of human error under great mental stress is well documented in air transport experience and the design of aircraft controls, especially those of an emergency character, should take into consideration the natural limitations of human nature. These limitations argue against involved procedures applicable in emergencies. In harmony with this objective, the Douglas Aircraft Company has designed and is testing a modified fire extinguishing system which will permit all necessary steps to be executed by the movement of one control. An additional vent is also being designed to reduce CO2 concentration in the cockpit. Seven days after the Mt Carmel accident, the Director of Aviation Safety of the CAA directed telegrams to all CAA regional administrators calling attention to his telegram of June 10, 1948, referred to above, and advising that further investigation had disclosed the existence of the CO2 concentration condition found in Constellation aircraft by the Chillicothe tests in other makes of aircraft. The telegram concluded "Hence, flight crews of all aircraft should be advised to wear oxygen masks and utilize emergency cockpit smoke clearance procedures when carbon dioxide is released into any fuselage compartment from other than portable extinguishers.” All scheduled U S air carriers operating DC-6s have equipped the airplanes with demand type full face oxygen masks for the use of the crew.

The Board determines that the probable cause of this accident was the in-flight failure of the fuselage carry-in structure beneath the fin, due to extreme turbulence.
The following factors were considered as contributory:
- The aircraft was flown through severe turbulence approximately one-half hour before the accident,
- The rivets and fuselage skin immediately beneath the vertical fin failed during flight,
- The separation of the fuselage skin allowed the vertical fin to rock across its longitudinal axis until the fuselage carry-in structure beneath the fin failed completely,
- The failure of the vertical fin caused loss of control of the aircraft, and it crashed.

The Board determines that the probable cause of these accidents was the combustion of gasoline which had entered the cabin heater combustion air intake scoop from the No 3 alternate tank vent due to inadvertent overflow during the transfer of fuel from the No 4 alternate tank.
The failure of the manufacturer and the Civil Aeronautics Administration to exercise full caution in the analysis of the fuel system of the DC-6 relative to proper location of fuel tank vents to provide non-hazardous location for fuel drainage, as required by existing regulations, and the insufficient attentiveness on the part of the manufacturer, the Civil Aeronautics Administration, and the air carriers to the procedures of fuel management employed by pilots operating DC-6 aircraft, were contributing factors.
This case was categorized as similar as the United Airlines' DC-6 that crashed in Bryce Canyon on October 24, 1947, killing all 52 occupants.

The probable cause of this accident was the accumulation of carburetor ice following the loss of power in both engines as a result of fuel starvation. The reason for fuel starvation has not been determined.

On the basis of the foregoing, the Board determines that the probable cause of this accident was the error of the pilot in establishing an approach toward an area not cleared for landing as a result of having mistaken the end markers of Runway 23 for those of Runway 31. A contributing factor was the failure of the pilot to abandon his attempt to land at Cleveland Airport when confronted with weather conditions below the minimums prescribed for that airport.

On the basis of the foregoing, the Board determines that the probable cause of the accident was the faulty execution of an emergency single engine approach following failure of the left engine in flight. A contributing factor was the failure of the pilot to exercise sufficient caution by having, a qualified mechanic determine the reasons for oil leakage in the left engine prior to departure from Chicago. Investigations revealed that the failure of the left engine was caused by a fracture of the cylinder number 5.