Douglas DC-6 accident at Gallup, New Mexico, 11 Nov 1947 (NC-37510)

Accident narrative

On October 24, 1947, a United Air Lines Douglas DC-6 disintegrated in flight near Bryce Canyon, Utah. Less than three weeks later, on November 11, 1947, an American Airlines DC-6 made an emergency landing at Gallup, New Mexico, due to an in-flight fire. Because of the similarity of the initial fires, the Civil Aeronautics Board released its findings for both accidents in a combined report, following simultaneous technical investigations and testing at Santa Monica, California.

### The Flights

The United Air Lines DC-6, NC-37510, had accumulated 933 hours since its manufacture. It departed Los Angeles, California, for Chicago, Illinois. The crew consisted of Captain F. L. McMillan, who had approximately 15,000 flight hours including 138 in the DC-6; co-pilot G. G. Griesbach; and stewardesses H. F. Morrissey, Shirley C. Hickey, and Sabina R. Joswich.

The American Airlines DC-6, NC-90741, had accumulated 142 hours since manufacture. It departed San Francisco, California, for Tulsa, Oklahoma. It was crewed by Captain E. W. Chatfield, who had 9,747 hours with 455 in the DC-6; co-pilot V. B. Brown; and stewardesses Evelyn Apitz and Marilyn Humphreys.

### What the Investigation Found

The Bryce Canyon aircraft disintegrated extensively, leaving torn and burned pieces strewn along a 28-mile flight path. The wreckage was transported to Santa Monica and reconstructed from the leading edge of the wings to the rear pressure bulkhead. Investigators found that at least one barium nitrate flare had ignited in flight, but fire damage sequences indicated this happened several minutes after the fire had started. Smudge samples were taken to trace the metallic contents of the fire pattern, using barium to track the flares and lead to track aviation gasoline.

While the Bryce Canyon wreckage was being reconstructed, the American Airlines DC-6 made its emergency landing at Gallup. Preliminary examination of this aircraft revealed a well-defined aviation gasoline stain on the underside of the fuselage. The stain extended rearward in a fan shape from the No. 3 alternate fuel tank vent and centered directly upon the cabin heater combustion air intake scoop, located approximately 10 feet aft.

Flight tests were immediately conducted using a water and dye mixture. The tests indicated clearly that when the No. 3 tank was full and the cross-feed from another tank was opened, the booster pump could force fuel through the vent at approximately 12.5 gallons per minute. A considerable quantity of this overflow entered the cabin heater air scoop. Ground tests introducing a fuel and air mixture into the combustion air ducts resulted in severe backfiring and intense fire in the ducts. In one test, a violent explosion ruptured the heater combustion chamber. The heaters themselves were found to have been functioning properly prior to the fires.

Tests showed that under high boost, the No. 3 alternate tank would fill from the No. 4 alternate tank at 20 gallons per minute. Once full, the vent outflow was only 12.5 gallons per minute. The Board determined that overflow could continue for approximately 60 seconds without damaging the tank, but the difference between inflow and outflow eventually created pressures that would distort the structure. Inspection of the Gallup aircraft's No. 3 alternate tank revealed structural deformation from internal pressures exceeding seven PSI, though no leaks were observed.

The Board concluded that in both aircraft, the flight crews had transferred fuel from the No. 4 alternate tank to the No. 3 alternate tank and inadvertently failed to stop the process before the tank filled, causing gasoline to overflow the vent and enter the heater scoop.

The investigation noted two major differences in the fire patterns: * The Bryce Canyon aircraft had a higher concentration of heat in the right wing fillet, which the Board attributed to the ignition of a flare and the probable consumption of fuel lines. * The Gallup aircraft suffered more severe fire damage to its supercharger oil cooler and aftercooler because its aftercooler exhaust valve was open, drawing flames into the cooler.

The Gallup crew had discharged carbon dioxide into the air conditioning compartment, but its effectiveness was considerably minimized because a pressure control valve remained open, rapidly exhausting the extinguishing agent. The aircraft was operating with the valve open due to an inoperative cabin supercharging system and a placard instructing the crew not to close the door.

Other potential fire sources, including engine oil, electrical systems, alcohol, and baggage, were ruled out. The Board also conducted a study on photoflash bulbs and found the possibility of inadvertent ignition extremely remote, noting they could not dissipate enough heat to ignite surrounding materials.

However, inspections of DC-6 aircraft revealed considerable hydraulic fluid leakage accumulated in the belly cargo compartment linings. While the Board noted this fluid could not be ignited by the aircraft's systems, it would intensify any fire starting from another source. The Civil Aeronautics Administration issued a directive on November 5, 1947, to remove all lining with traces of hydraulic fluid saturation.

The Board found that the DC-6 fuel system was capable of permitting flow between tanks and overflow in flight, despite Civil Air Regulations stating this should not be possible. The CAA had approved the vent locations without conducting tests to determine if discharging fuel constituted a hazard, as it was not contemplated that gasoline would discharge from the vent. Although fuel transfer was not described or authorized in the CAA-approved or manufacturer operating manuals, the Board found it was a common practice. Manufacturer pilots had demonstrated the method to air carrier crews, and CAA inspectors had frequently witnessed it.

Following the investigations, a government-industry Modification Board agreed on changes before DC-6 aircraft returned to service. Modifications included check valves to prevent fuel transfer between tanks, relocated alternate tank vents, a separate intake scoop for cabin heater combustion air, stainless steel ducting, and shrouds around all flare containers.

### Probable Cause

The Board determined 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 Board found that contributing factors were "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."