What happened
The aircraft arrived in Chicago at 0952 local time while en route from Los Angeles to New York City. After a routine station inspection and servicing, the plane departed for New York with a new crew at 1044. The flight carried 39 passengers, a crew of four, cargo, and fuel, all within certified weight limits. The aircraft climbed to its planned altitude of 17,000 feet and proceeded on course.
At 1227, the crew acknowledged a clearance to descend between 13,000 and 11,000 feet. Four minutes later, at 1231, an unidentified voice urgently called out over company radio, followed by a transmission stating, "This is an emergency descent." Another United crew flying behind heard screaming voices calling for New York. Ground witnesses observed the aircraft flying southeast toward Shamokin, Pennsylvania, at approximately 4,000 feet above ground level.
As the plane approached the terrain north of Shamokin, it descended to between 500 and 1,000 feet and executed a shallow left turn. The course was directed toward rising hills, with elevations reaching approximately 1,600 feet. Five miles east of Shamokin, the aircraft entered a right climbing turn at only 200 feet above the ground. As it passed north of Mount Carmel, the climbing attitude increased sharply before striking a hillside at an elevation of 1,649 feet. The airframe disintegrated on impact, resulting in 43 fatalities.
Findings
The investigation determined that the crew was incapacitated by high concentrations of carbon dioxide gas in the cockpit. This occurred after a fire warning prompted the discharge of CO2 extinguisher bottles in the forward cargo pit. Although six 15-pound bottles and valves were recovered, their damaged state prevented determination of how many had been released prior to impact.
A key contributing factor was that the emergency cabin pressure relief valves remained closed at the time of impact, preventing the venting of hazardous gas levels. The investigation noted that no mechanical failure or in-flight fire was found other than the apparent failure of a fire detection instrument. Additionally, the approved emergency procedures for the DC-6 fire system had not been tested in the specific descent configuration (160 mph with gear and flaps down) that likely applied during the incident, as the landing gear was found in the "up" position.
The board highlighted the physiological and toxic effects of high CO2 concentrations, which rendered the crew physically and mentally incapable of performing their duties. It emphasized that under extreme stress, pilots may not strictly adhere to complex emergency procedures. In response, Douglas Aircraft began testing a modified fire extinguishing system operable by a single control and an additional vent to reduce cockpit CO2 levels. The Civil Aeronautics Authority subsequently advised all flight crews to wear oxygen masks and utilize smoke clearance procedures whenever carbon dioxide is released from fuselage compartments.