History of Flight
On July 2, 2013, a Korean Air Lines (KAL) Boeing 777-300ER, registration HL8275, powered by two General Electric GE90-115B turbofan engines, experienced a No. 1 engine (left) in-flight shutdown (IFSD) while crossing the Bering Sea (60°4'16.32"N/178°40'33.60"W) en route to Seoul, South Korea. The pilots diverted the airplane to Ugolny Airport (DYR), a mixed-used military and civil airport in Siberia, Russia. An uneventful single engine landing was made and no injuries were reported. The incident flight was a 14 Code of Federal Regulation Part 129 regularly scheduled international flight from O'Hare International Airport (ORD), Chicago, Illinois, to Incheon International Airport (ICN), Seoul, South Korea. The Korean Aviation and Railway Accident Investigation Board (ARAIB) initially opened an investigation and provided the National Transportation Safety Board (NTSB) initial notification. The Korean ARAIB subsequently requested delegation of the investigation, and the NTSB accepted responsibility.
Transfer Gearbox Damage
Post-landing examination of the No. 1 engine revealed that the transfer gearbox (TGB) housing was fractured and the internal gears were damaged. The No. 1 engine was removed and shipped to the GE-Wales facility in Cardiff, Wales for removal of the TGB. Removal of the TGB revealed a separated radial bevel gearshaft (subsequently referred to as radial gearshaft) within the TGB. TGB hardware, including the separated radial bevel gearshaft, were sent to GE-Aviation facility in Evendale, Ohio for metallurgical examination.
Test and Research
Visual examination of the radial gearshaft found three X-shaped cracks in the short shaft side outer diameter at the shaft-to-web transition radius, with one X-shaped crack (associated with fractured and missing material) linked to a 0.049-inch axial crack. The axial crack was considered the primary crack and the fracture origin area. Scanning Electron Microscope (SEM) examination of the axial fracture region revealed striation features indicative of fatigue propagation but did not exhibit morphology typical of axial-axial induced fatigue fracture surfaces; instead, it was more consistent with initial fatigue initiation under a torsional and/or biaxial stress condition. Published literature for fatigue cracking under torsional and/or biaxial loading conditions was consistent with the X-shaped cracking. GE subjected several steel test bars to various biaxial fatigue loading conditions in an attempt to reproduce fracture features observed within the axial fracture. The results indicated biaxial loading conditions could reproduce those features.
Microhardness tests of the short shaft revealed a reduction in near surface hardness (lower than required) while core hardness met the part drawing requirement. Other than the near surface hardness loss, the material was consistent with properly processed material. No evidence of material inclusions or other discrepancies was observed within the initial axial fracture or on the adjacent outer diameter surface.
During manufacturing, gear teeth and splines are case hardened using carburization, which adds a carbon layer to the outer surface. Surfaces not requiring carburization are copper plated prior to the carburization cycle. The area of cracking is not case hardened and would have been copper plated. After carburization, copper plating is stripped, then the entire part is re-copper plated prior to the hardening cycle to prevent decarburization. GE concluded that the near surface low hardness values were attributable to decarburization due to marginal (thin or detached) copper plating on the gearshaft surface and a hardening atmosphere prone to causing decarburization. Through computer modeling and testing, GE concluded that the axial crack was likely the result of a combination of high residual tensile stresses produced by local decarburization coupled with the operating stresses experienced at the outer diameter surface.
Additional Information
Corrective Actions: GE issued four service bulletins (SBs) to remove suspected radial gearshafts from service, one SB to provide a specific crack detection inspection, and two SBs to repair and toughen the radial gearshafts. To expedite removal, the Federal Aviation Administration issued three Airworthiness Directives (AD), including two emergency ADs requiring compliance within 5 days. According to GE, all affected radial gearshafts (186 in total) addressed by the FAA ADs have been removed from service.