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Exploding jet engine had 1-inch hub crack

 
Published July 9, 1996|Updated Sept. 16, 2005

Investigators have found a one-inch crack in the engine hub that blew apart Saturday and killed two passengers when it slashed the fuselage of a Delta Air Lines plane taking off from Pensacola.

The broken parts of the heavy titanium hub that holds the large engine fan blades will be taken to the National Transportation Safety Board laboratory in Washington to determine the cause of the crack.

One possibility under consideration Monday night was the kind of microscopic metal flaw that caused a similar part to fly apart on a United Airlines plane, sending it into a crash-landing in Sioux City, Iowa, in 1989.

The flaws, which can become cracks over a period of years, so rarely grow into undetected cracks that the Pratt & Whitney JT8D-200 series engine has never had a fan hub disintegrate before. About 40-million hours of flight time have been logged on 2,621 of the engines over 16 years of service on almost every type of small and medium-size airliner.

"The flight engineers I've been talking to tell me they've never seen anything like it," said Pratt & Whitney spokesman Mark Sullivan.

Safety board member George Black said that the board's metallurgist, Michael Marx, found the crack in less than two minutes of visual inspection of one part of the heavy hub. A Delta Air Lines metallurgist independently confirmed Marx's finding, Black said.

Delta Flight 1288 was rolling for takeoff Saturday afternoon, bound for Atlanta, when the left engine of the twin-jet McDonnell Douglas MD-88, mounted on the rear of the fuselage, blasted apart. The failure took place as the engine reached full takeoff power _ about 8,000 revolutions per minute.

Shrapnel flew into the passenger cabin, killing a mother and 12-year-old son, Anita and Nolan Saxton of Ludington, Mich.

Black said the board also is looking into whether an earlier oil seal leak might have played a role. The fan hub that flew apart had been installed on the engine in January because the oil leak had burned out a bearing on the original hub.

In any case, investigators will look carefully at Delta's maintenance practices, including when and how the fan hub was last tested for possible cracks.

The fan hub is one of the heaviest rotating parts on an engine. It holds the long fan blades at the front of the engine, and spins with tremendous force.

An engine outer cover, or cowling, is designed to contain almost any part that disintegrates or comes loose in an engine. But it could not hold back such a heavy part that breaks free at nearly its maximum rate of rotation. "Uncontained" engine failures, while rare, do occur, but seldom with such violence.

Under federal rules, the hub must be made so that it cannot fail under normal circumstances. The type of hub on the Delta plane must be replaced after 20,000 flights, and the one that failed had made more than 14,100 flights, the safety board said.

Black indicated that passengers at the rear of the plane were luckier than they might have thought. Almost all of the hub and blades missed the fuselage, although it blasted through the engine's protective outer cover and sent blades and other shrapnel into the plane.

Two-thirds of the hub was found 650 feet to the left of the plane; the other third flew over the passenger cabin and landed 2,400 feet away in a recreational area after flying over a road.

Black said only one piece of the hub, perhaps as big as a hand, shot through the fuselage. All the shrapnel hit with such force that most of it blasted through the fuselage and went out the other side. Black said investigators had counted five or possibly six entry holes and five exit holes. All of the holes were at row 37, the next-to-last row on the plane.

The last time that a heavy titanium assembly disintegrated on an airplane appears to have been July 19, 1989, when the tail-mounted center engine on a McDonnell Douglas DC-10 flown by United Airlines exploded and spread shrapnel throughout the tail section. The crew managed to limp to the airport at Sioux City, Iowa, but 112 of the 296 people aboard died in the crash landing.

In the DC-10 engine, made by General Electric Co., the titanium ring encircled the outside of the fan blades, meaning that it rotated with much greater force than the internal hub in the smaller Pratt & Whitney engine.