History of Flight
On June 3, 2017, at 0820 eastern daylight time, Southwest Airlines (SWA) flight 4635, a Boeing 737-7H4 (registration N765SW) with two CFMI CFM56-7B24 turbofan engines, experienced a right-hand (No. 2) engine failure while en route from Tampa, Florida, to Rochester, New York. The flight diverted to Washington Dulles International Airport (IAD), where an uneventful single-engine landing was performed. No injuries were reported among the 62 passengers and 5 crew members. The flight was conducted under instrument flight rules (IFR) as a scheduled 14 CFR Part 121 operation.
Engine Damage Examination
On-scene damage documentation and engine removal were conducted by SWA with NTSB approval; NTSB personnel were not present. The fan cowls and thrust reversers were latched and secured. The right-hand engine inboard fan cowl exhibited an 8.75-inch circumferential by 3.75-inch axial impact rip/tear/slice near the 9:00 o'clock position. Opening the fan cowls revealed loose debris including a piece of gear teeth rim with a part number corresponding to the accessory gearbox 47-tooth gearshaft assembly (also known as the control alternator gearshaft, L3 gearshaft, or L3 gear train). Parts consistent with the accessory gearbox housing and one oil supply nozzle were also found. The accessory gearbox housing had a 3-inch by 2-inch exit hole in line with the control alternator gearshaft.
The engine was shipped to the SWA maintenance facility at Love Field, Dallas, Texas. Before removing the accessory gearbox, an alignment check and examination of accessory mounting hardware were performed. Disengaging the transfer gearbox from the accessory gearbox revealed that the horizontal drive shaft locking nut was loose (finger tight). This nut was last torqued in January 2017 at the GE Celma facility per approved procedures. The locking nut threads and mating threads on the L3 gearshaft were in good condition with slight fretting; the lock nut retaining ring was properly installed and in good condition. The alignment check showed the accessory gearbox met installation requirements. No anomalies were noted on mounting hardware; dampers were installed and in good condition, and mount links appeared straight and were easy to remove. The transfer gearbox, horizontal drive shaft, and accessory gearbox were removed and retained. No anomalies related to the incident were found with the transfer gearbox.
Test and Research
The accessory gearbox was shipped to CFMI in France. Disassembly and visual examination of the housing and internal gearshafts found: (1) the L3 gearshaft bore was heavily damaged/scored with a 3-inch by 2-inch exit hole; (2) all gear teeth of the L3 gearshaft had separated from the gear web, and the remaining web was distorted; (3) gear teeth of the handcranking gearshaft (L4 gearshaft, which meshes with L3) were damaged but all present. Binocular examination of the L3 gearshaft web revealed the primary crack initiation site at the interface radius of the web-to-centerline shaft on the roller bearing side. Fine secondary circumferential fatigue cracks were observed on both sides of the web near the web-to-shaft interface, about 90° from the primary crack. Cracks were also visible at the bottom land of several gear teeth.
High-resolution images of the L3 gearshaft fracture surface showed features consistent with high cycle fatigue (HCF) with multiple arrest lines along the initial propagation path. The primary crack initiation site was more precisely located between microstructure grain boundaries where chemical etching and black oxide surface treatment were performed during manufacturing, consistent with intergranular corrosion; no embrittlement was noted around the fracture initiation site. The secondary web cracks also initiated on grain boundaries and showed HCF features. Chemical analysis and hardness checks confirmed the part was made of the specified material and to required hardnesses.
Additional Information
Review of manufacturing history showed the failed L3 gearshaft was one of 32 initially produced from the same batch. Due to corrosion during manufacturing, only 8 of the 32 (including the event shaft) were put into service after additional operations: corrosion removal, etching, deoxidation, and additional machining. At that time, shot-peening was not required on any accessory gearbox gears and gearshafts. Subsequently, CFMI changed specifications requiring shot-peening for all new gears and gearshafts. About 600 non-shot-peened L3 gearshafts were produced and entered service.
Maintenance records showed the failed L3 gearshaft was installed on the same accessory gearbox for its entire service life and was visually, non-destructively, and dimensionally inspected in May 2007 and January 2017 with no anomalies. At failure, the L3 gearshaft (and accessory gearbox) had accumulated 63,711 hours since new, 37,433 cycles since new, 668 hours since last shop visit, and 415 cycles since last shop visit.
After multiple attempts to replicate the failure condition, CFMI concluded that chemical etching alone could not account for the intergranular attack observed. Instead, it was thought that the additional machining process combined with chemical etching to remove corrosion caused an increase in quantity and density of microcracks along with grain boundary consumption, ultimately leading to primary crack initiation.
Corrective Action
Of the eight L3 gearshafts from the same production batch as the failed SWA gearshaft, two were no longer in service; the remaining six were positively identified as installed in accessory gearboxes on in-service engines. As a preventive action, CFMI released service bulletin CFM56-7B 72-1032-R00 on March 12, 2018, recommending removal of the seven remaining L3 gearshafts (including one out-of-service) from that batch at the next accessory gearbox overhaul.
Safran Transmission Systems now limits the number of additional manufacturing operations to address non-conformances during production (e.g., limiting acid etchings for steel gears/gearshafts). This process has been in use for several years, but in light of this event, the process is being reviewed for improvements in documentation.