NTSB Identification: ENG12IA003
Scheduled 14 CFR Part 121: Air Carrier operation of DELTA AIR LINES INC
Incident occurred Sunday, October 23, 2011 in Detroit, MI
Probable Cause Approval Date: 12/07/2012
Aircraft: BOEING 747-451, registration: N661US
Injuries: 394 Uninjured.

NTSB investigators traveled in support of this investigation and used data obtained from various sources to prepare this aircraft incident report.

On October 23, 2011, 1600 eastern daylight time, a Boeing 747-451, registration number N661US, powered by four Pratt & Whitney PW4056 turbofan engines, experienced a left inboard engine (No. 2) low pressure turbine failure after takeoff from Detroit Metro Wayne Country International Airport (DTW), Detroit, Michigan. According to the flight crew statements, a large compressor stall and a muffled explosion were felt, followed by the No. 2 engine rollback and No. 2 engine fire warning indication. The flight crew performed an air turnback and made a successful three-engine landing at DTW. No injuries were reported.

Examination of the airplane revealed minor impact damage to the left wing, flaps, ailerons, and horizontal stabilizer. Initial examination of the engine revealed evidence of an undercowl fire, three burn-through holes on the No. 2 engine outboard core cowl, numerous exit hole penetrations in the rear turbine case, and a punctured No. 4 bearing compartment oil pressure supply line. Disassembly of the No. 2 engine revealed no damage upstream of the stage 3 low pressure turbine (LPT); one stage 3 LPT outer transition duct segment and two stage 3 LPT vane clusters were missing from their normally installed position, and extensive damage downstream of the stage 3 LPT vane clusters was found. The stage 3 LPT outer transition duct segments were approved but were an older configuration utilizing the riveted rear seal plate design that was prone to failure and had been superceded by a preferred design that incorporated an integral rear seal.

Comparing the airplane and engine damage to the requirements for engine debris containment and safety analysis at the time the engine was certificated revealed that the engine did not comply with the containment requirements set forth in Parts 33.19 and 33.75.

The undercowl fire was due to a failure of the rear riveted seal plates of the missing stage 3 outer transition duct segment that allowed hot gas path air under the duct segment, causing it to thermally distort and disengage from the rear turbine case. Once disengaged from the rear turbine case, the stage 3 outer transition duct segment released into the gaspath where it fractured all of the stage 3 LPT blades, which propelled blade fragments through the rear turbine case that punctured the No. 4 bearing oil pressure supply tube. The atomizing oil mist from the No. 4 bearing oil pressure supply tube ignited when it contacted the hot engine case, which created the undercowl fire.

The National Transportation Safety Board determines the probable cause(s) of this incident to be:

The penetration of turbine blade fragments through the rear turbine case, which punctured the No. 4 bearing oil pressure supply tube, allowing misted oil to contact the hot engine case and ignite the undercowl fire.

Contributing to the incident was the installation of approved (but not preferred) stage 3 low pressure turbine outer transition duct segments with the riveted rear seal configuration and the failure of the engine design to comply with the engine debris containment requirements of 14 Code of Federal Regulations 33.13 and 33.75.

Full narrative available

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