Douglas DC-6 accident at Near Eagle, Colorado, 22 Aug 1950

Accident narrative

On August 22, 1950, American Airlines Flight 14, a Douglas DC-6, suffered a propeller blade failure and explosive decompression at 21,000 feet near Eagle, Colorado. A portion of the blade pierced the cabin, and the resulting imbalance tore the number three engine from the aircraft. The crew made a safe emergency landing at Stapleton Airport in Denver 19 minutes later. Five passengers and one stewardess sustained minor injuries, and one male passenger died, presumably from heart failure. The aircraft was extensively damaged.

### The flight

Flight 14 was a scheduled non-stop service from Los Angeles, California, to Chicago, Illinois. The aircraft departed the ramp at 2302 on August 21 and took off at 2312. It carried 54 passengers, including two infants, and a crew of five. At departure, the aircraft weighed 88,647 pounds, which was 978 pounds under the allowed maximum, and its center of gravity was within allowable limits.

The flight crew consisted of Captain Robert K. Baker, who had approximately 10,000 total flying hours; First Officer Robert E. Reinicke, with about 7,500 hours; Flight Engineer Daniel J. Niemiec; and Stewardesses Joan Robinson and Margie Peterson.

### The emergency

The flight proceeded uneventfully in clear weather, initially cruising at 19,000 feet. At 0128, Air Route Traffic Control cleared the flight to cruise under instrument flight rules at 21,000 feet. At 0229, the flight reported over Eagle at 21,000 feet, estimating arrival over Denver at 0252.

At approximately 0237, the crew sensed engine roughness and checked their instruments. The check did not disclose the source, and the first officer reached to retard the throttles. As he did so, the number three propeller failed. A portion of a blade was thrown into the right side of the fuselage, passing upward and exiting through the top. The cabin depressurized almost instantly. Warning lights for the number three engine's fuel and oil pressure illuminated as the engine wrenched free and fell from the aircraft. The cabin lights went out. An estimated five seconds elapsed from the first sensing of roughness to the failure.

The crew reduced power, began a descent, and applied engine-out procedures. At approximately 18,0C0 feet, power was increased on the remaining three engines to continue toward Denver. The captain broadcast a "Mayday" call to the Denver Airways Communication Station at 0237, reporting an explosive decompression, a hole in the fuselage, and the apparent explosion of the number three engine. He requested fire equipment and ambulances to stand by. At 0240, the flight reported that everything was under control.

The aircraft held an altitude of about 12,000 feet until it was past the mountains, then descended for a straight-in landing on Runway 8 at Stapleton Airport at 0256. The landing was made without flaps due to a loss of hydraulic pressure, though the captain later testified he was dubious about the structural integrity of the damaged aircraft and would not have used them regardless. The aircraft was stopped on the runway using reverse pitch on the three remaining propellers and emergency compressed air brakes.

### What the investigation found

Inspection of the aircraft revealed a nearly vertical slit, approximately 35 inches long by 4 inches wide, on the right side of the fuselage. A large, irregular opening of about 250 square feet extended back along the top and upper sides of the fuselage above the forward three rows of passenger seats. The rapid decompression caused significant internal distortion, bulging bulkheads, ceilings, and floors, and blowing the left forward bunk out of the aircraft. The passenger seats and safety belts were undamaged.

The number three engine had separated at the mount ring. Following a search, the engine was located on September 1 in a mountainous area at an altitude of about 10,000 feet, approximately 84 miles west of Stapleton Airport. The outer portion of the failed blade was found four days later, about one and a half miles west of the engine.

The failed blade was a Curtiss Wright hollow steel model. The National Bureau of Standards analyzed the failure and found it was caused by a fatigue fracture originating at defects on the inside surface of the blade's flat side. These defects were gouges made inadvertently during the manufacturing process. When the two surfaces of the blade were welded together, an internal collapsible mandrel was used to control the distance between them. The gouges on the failed blade corresponded to the location and spacing of Allen head set screws used to lock the mandrel's cam adjustments. The maximum depth of the gouges was approximately one-sixth of the blade's wall thickness.

The Board found that original X-ray photographs taken by the manufacturer at the time of fabrication showed faint marks corresponding to these gouges. Testimony indicated that factory inspectors had seen the indications but considered them not serious enough to warrant rejecting the blade. The blade had accumulated 4,536 hours of service and had not been damaged in operation.

Teardown of the recovered engine revealed no internal failures, but bushing wear in both dynamic dampers exceeded allowable tolerances, rendering them inoperative. While inoperative dampers increase vibratory stresses on the propeller blades, the Board noted this engine model was prone to such wear, and the additional stress had been accounted for in the blade's design. The Board concluded the damper wear did not cause the blade failure, though it may have hastened its occurrence.

Following the accident, the propeller manufacturer immediately reexamined X-rays of all similar blades in service and conducted new X-rays where originals were missing. A small percentage of blades with significant defects were discarded, and inspection procedures were tightened.

The Board commended the flight crew for the highly efficient professional airmanship they displayed in safely landing the badly damaged aircraft under conditions of extreme emergency.

### Probable cause

The Board determined that the probable cause of this accident was "the internal gouging of a propeller blade during the manufacturing process which resulted in a fatigue fracture and subsequent failure during flight."