ATL94IA097
ATL94IA097

On May 5, 1994, about 0855 central daylight time, a Douglas DC-9-32, N986US, had an uncontained engine failure during the takeoff roll at Memphis, Tennessee. Flight 661 was operated by Northwest Airlines under Title 14 Code of Federal Regulations (CFR) Part 121 as a scheduled, domestic, passenger flight. Visual meteorological conditions prevailed. An instrument flight rules flight plan was filed for the flight to Phoenix, Arizona. There were no injuries to the airline transport pilots, the three flight attendants, nor the 64 passengers; and minor damage to the airplane. The flight was originating at the time of the accident.

During the takeoff roll, approximately 125 knots, the flight crew heard a loud explosion and noted that all of the left engine instruments were reducing to zero indication. The takeoff was rejected, and the engine shut down. The engine fire extinguisher was discharged. As the airplane was taxied onto a taxiway, tower personnel informed the flight crew that smoke was coming from the engine. The flight crew obtained information from fire/rescue personnel and determined that an evacuation was not required.

A post incident examination of the engine, which was installed in left position of N986US, showed that an uncontained failure of the combustion case had occurred. The engine cowling was partially opened up and distorted. There was minor shrapnel damage to an unpressurized portion of the fuselage, adjacent to the engine. The engine, a Pratt and Whitney JT8D-15, serial number 657510, was removed to the Northwest Airlines engine overhaul facility in Atlanta, Georgia, for additional visual examination. The combustion chamber outer case (CCOC) had fractured through a bolt hole of the rear flange, at the engine's three o'clock position. All of the combustion cans were in place and there was no evidence of an external fire. The outer case opened along an axial plane with the upper and lower halves displacing up and down, respectively, as if opening a briefcase. The fracture surface on the lower portion of the case was discolored black from the rear flange bolt hole forward about four inches. Its coloration then changed to red for about 3.75 to 4.00 inches. The fracture surface then abruptly changed to a lighter color (bronze then steel) for about five inches. Subsequently, the engine was forwarded to Pratt and Whitney facilities for in-depth examination. A metallurgical examination was conducted by Pratt and Whitney which revealed that the fracture resulted from low cycle fatigue in the separated bolt hole, of the rear flange. Low cycle fatigue cracks were observed in numerous additional bolt holes. The case material conformed to composition and hardness requirements.

Rear flange fractures of the CCOC are the subject of Federal Aviation Administration Airworthiness Directive 87-10-01 R1, effective November 13, 1989, and Pratt and Whitney Alert Service Bulletin 5676 dated July 15, 1986. The incident engine had been inspected in accordance with both documents on November 28, 1990. The combustion case had 5,940 cycles since the inspection, and had 2,060 cycles remaining until the required inspection was again due at 8,000 cycles.

Two previous CCOC fractures have occurred, similar to the fracture in this incident. The first occurred during takeoff and the second occurred during routine reassembly following engine maintenance. An All Operator's Wire was issued by Pratt and Whitney, following the second fracture occurrence. The wire discussed a "low toughness condition" of older cases that, under certain conditions, could result in an intergranular crack.

Following the additional examinations of the engine, Pratt and Whitney, in conjunction with the Federal Aviation Administration, are pursuing the issuance of inspections to preclude similar, undetected CCOC cracks.

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