What happened
On July 29, 2004, a Northwest Airlines Boeing 747-200B, registration N645NW, was operating a scheduled cargo flight from Kansai International Airport to Inchon International Airport. During the engine start sequence, the crew noted that the No. 1 engine and No. 2 engine revolutions failed to stabilize, necessitating restarts for both engines. Following a successful takeoff, the aircraft climbed to approximately 300 feet when the crew experienced a loud noise, a sudden yawing motion, and a significant drop in thrust from the No. 1 engine.
Air traffic controllers at Kansai Tower observed flames emanating from the engine and heard a loud bang. The crew declared an emergency and requested to return to the airport. The aircraft landed safely at 11:44 JST with no injuries to the three crew members on board. Post-flight inspections revealed that fragments from the engine had pierced the No. 1 engine cowling and the high-pressure turbine (HPT) case.
The investigation
The investigation, conducted by the Aircraft and Railway Accidents Investigation Commission (ARAIC), included a detailed teardown inspection of the No. 1 engine at a Northwest Airlines facility in the United States. Investigators examined the high-pressure turbine section, specifically focusing on the second-stage turbine blades and the lenticular seal. The analysis also involved reviewing flight data from the Digital Flight Data Recorder (DFDR), although the investigators noted that the recording contained erroneous data and gaps during the period of the incident.
Findings
Technical analysis revealed that the high-pressure turbine section suffered extensive damage. Specifically, twenty-four second-stage turbine blades were found fractured. The investigation determined that the primary cause of the failure was the failure of the lenticular seal within the HPT section.
It was determined that the lenticular seal experienced high-temperature fatigue. This fatigue was driven by heat stresses resulting from insufficient cooling, which caused cracks to initiate and propagate through the seal. As the seal failed, portions of it separated and impacted the turbine blades. The resulting debris was powerful enough to rupture the turbine blades and penetrate the engine's outer case and cowling.