Douglas DC-7C accident at Boston, Massachusetts, 29 Oct 1957

Accident narrative

On October 29, 1957, the No. 1 propeller of Scandinavian Airlines System Flight 912, a Douglas DC-7C, oversped and could not be feathered during a flight from New York to Copenhagen. The flight diverted to Boston, Massachusetts, where it landed safely after the propeller separated from the engine and fell into the sea. The aircraft received substantial damage, but the nine crew members and 50 passengers deplaned without injury.

### The flight

Flight 912 departed New York International Airport (Idlewild) at 1516 on an Instrument Flight Rules plan to Copenhagen, Denmark. The aircraft, Danish registry OY-KNB, had a takeoff gross weight of approximately 138,000 pounds, which was 5,000 pounds under the maximum allowable. The crew included Captain Kaare Herfjord, who had 8,324 flying hours with 521 in the DC-7C, and First Officer Erick Falkenberg-Nielsen, who had 2,119 hours with 294 in type.

The flight made routine reports while climbing to its cruising altitude of 21,000 feet, which it reached passing Nantucket at 1606.

### The emergency

At approximately 1715, about 200 miles off the Maine coast, the crew sensed engine roughness. Before they could identify the rough engine using the ignition analyzer, the No. 1 engine brake mean effective pressure dropped to zero, and its manifold pressure dropped to 20–25 inches. Oil temperature increased to 90–95 degrees centigrade, and oil pressure dropped.

The engine RPM increased to nearly 4,000. The crew found that closing the throttle had no effect on the high RPM. They attempted to feather the propeller using both the feathering button and the manual toggle switch, but without success, even though the loadmeter indicated the feathering pump was functioning normally. The mixture control was placed in idle cutoff; the tachometer dropped to zero, but the propeller continued to windmill at a very high rate. Sparks and flame appeared in the area of the No. 1 engine exhaust and cowling.

At approximately 1728, the crew transmitted a MAYDAY call. The autopilot was turned off, and the aircraft was flown manually. Air Route Traffic Control cleared the flight to descend and route back to Idlewild. The crew dumped approximately 2,800 gallons of fuel but could not maintain 8,000 feet. They requested and were granted a descent to 6,000 feet. After dumping an additional 700 gallons of fuel, they were able to maintain 6,000 feet at an airspeed of 140 knots, which noticeably reduced the propeller RPM and noise.

The captain changed the destination to Boston, which was 120 miles closer than New York. He stated he considered diverting to Halifax, Nova Scotia, but decided against it due to an overcast and short runways. A U.S. Coast Guard aircraft intercepted the flight at 1942 and escorted it.

Between 2026 and 2030, while descending from 6,000 to 4,000 feet during the Boston approach, the No. 1 propeller separated from the engine and dropped into the sea. Sparks and flame accompanied the separation, and the crew discharged one bank of fire extinguishing agent. The flight made a successful three-engine landing at Logan Airport at 2044. Airport fire equipment covered the No. 1 engine and left wing with foam as a precaution. The passengers deplaned safely via a standard loading ramp.

### What the investigation found

Examination of the aircraft showed that, aside from a small puncture in the left wing leading edge, all damage was forward of the No. 1 engine firewall. A major portion of the propeller shaft had broken away from the engine.

When the propeller governor was disassembled, it was found to contain small metal particles, and its pilot valve was seized in the "on speed" condition. The engine's nose case showed heat discoloration, but there was no fire damage around the cylinders, ignition wiring, or push rods.

Complete disassembly of the engine revealed heavy accessory drive gear damage and overheating throughout. The impeller drive system was completely destroyed. Laboratory analysis revealed that the impeller intermediate drive gear failed due to fatigue fractures.

Inside the power section, all connecting rods and master rods were broken. The crankshaft counterweights were heavily peened and scored, with the rear counterweight jammed against the rear crankcase. Two pistons were completely broken up, and the front and rear main bearings were nearly destroyed.

The Board believed the initial failure was the fatigue fracture of the impeller intermediate drive gear. This failure generated fine metal particles that contaminated the oil supply, causing the governor pilot valve to seize and preventing the propeller from being feathered. The Board noted that the gear train driving the fuel pump and tachometer was disrupted early in the sequence. This allowed the tachometer to register zero while engine rotation continued for approximately three hours, causing the extensive internal damage.

The Board believed that the rear counterweight partially separated during this continued rotation, and its sudden jamming against the crankcase diaphragm caused the already heated propeller shaft to fail, permitting the propeller to separate.

The Board found that the emergency was well handled by the flight crew, noting that dumping fuel enabled the aircraft to maintain altitude and that the decision to delay evacuation after landing was reasonable.

### Probable cause

The Board finds that "the probable cause of this accident was fatigue fractures in No. 1 engine impeller drive gear, resulting in an overspeeding propeller, loss of the propeller, and the subsequent emergency landing."