Southwest B737 enroute on Jun 3rd 2017, engine shut down in flight, inlet cowl sliced
Last Update: June 28, 2018 / 14:16:28 GMT/Zulu time
Incident Facts
Date of incident
Jun 3, 2017
Classification
Incident
Airline
Southwest Airlines
Flight number
WN-4635
Departure
Tampa, United States
Destination
Rochester, United States
Aircraft Registration
N765SW
Aircraft Type
Boeing 737-700
ICAO Type Designator
B737
The NTSB reported inspection after landing revealed, the #2 engine suffered an accessory gear box failure resulting in a about 3 inches by 2 inches hole near the line 3 gear assembly. In addition, the left hand side of the engine's fan inlet cowl was sliced. The occurrence was rated a serious incident and is being investigated by the NTSB.
On Jun 20th 2018 the NTSB released their factual report. A post flight inspection revealed a drive shaft locking nut on the accessory gear box had become loose although the nut had been properly torqued and secured. The accessory gear box was removed and sent for inspection. The NTSB wrote:
The accessory gearbox was shipped to CFMI in France for examination; disassembly and visual examination of the housing and the internal gearshafts found that: 1) the accessory gearbox housing L3 gearshaft bore was heavily damaged/scored and exhibited a 3-inch x 2-inch exit hole, 2) all the gear teeth of the L3 gearshaft had separated from the gear web and what remained of the gear web was distorted, and 3) gear teeth of the handcranking gearshaft (L4 gearshaft) – this gearshaft meshes with the L3 gearshaft – were damaged but all present. Binocular examination of the web of the L3 gearshaft revealed that the primary crack initiation site was located at the interface radius of the gear web-to-centerline shaft on the roller bearing side of the gear web. Fine secondary circumferential fatigue cracks were observed on both sides of the web near the interface of the web-to-centerline shaft; secondary cracks were located about 90° from the primary crack initiation. Cracks were also visible at the bottom land of several gear teeth.
High resolution images taken of the L3 gearshaft fracture surface revealed 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 of the part was performed during manufacturing and was consistent with intergranular corrosion; no embrittlement was noted around the fracture initiation site. The secondary web cracks also initiated on the microstructure grain boundaries and the crack had features consistent with HCF. Chemical analysis and hardness checks confirmed the part was manufactured of the specified material and to the required hardnesses.
ADDITIONAL INFORMATION
Review of the manufacturing history of the failed L3 gearshaft revealed that it was 1 of 32 initially produced from the same production batch. Due to corrosion issues during the manufacturing process, only 8 of the 32, including the event gearshaft, were ever put into service. The 8 gearshafts that were put into service had additional manufacturing operations performed on them such as corrosion removal, etching, deoxidation, and additional machining to address the corrosion issue before they were deemed serviceable. At that time, shot-peening was not required on any of the AGB gears and gearshafts. Soon after, CFMI changed the manufacturing specifications requiring all new manufactured gears and gearshafts to be shotpeened. A total of approximately 600 non-shot peened L3 gearshafts were produced and went into service.
The NTSB reported with respect to corrective measures:
Of the eight L3 gearshafts from the same production batch as the failed SWA gearshaft, two are no longer in service, and the remaining six have all been positively identified as installed in an AGB on an in-service engine. CFMI, as a preventative action, releases service bulletin CFM56-7B 72-1032-R00 on March 12, 2018 that recommends removal of the seven remaining L3 gearshafts (including one that is currently out-of-service) from the same production batch as the failed SWA L3 gearshaft at the next AGB overhaul.
On Jun 28th 2018 the NTSB released their final report concluding the probable causes of the incident were:
The failure the control alternator gearshaft, which disconnected the accessory gearbox from the rest of the engine and resulted in an uncommanded in-flight shutdown of the left engine and subsequent flight diversion. The control alternator gearshaft failed because of high cycle fatigue intergranular corrosion cracking. Contributing to the failure of the control alternator gearshaft were additional manufacturing processing steps to address part non-conformances that reduced its high cycle fatigue capability and potential change in vibratory environment since the last shop visit.
Aircraft Registration Data
Incident Facts
Date of incident
Jun 3, 2017
Classification
Incident
Airline
Southwest Airlines
Flight number
WN-4635
Departure
Tampa, United States
Destination
Rochester, United States
Aircraft Registration
N765SW
Aircraft Type
Boeing 737-700
ICAO Type Designator
B737
This article is published under license from Avherald.com. © of text by Avherald.com.
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