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Printed page viii. PDF page 11.
root (referred to as a fan-blade-out [FBO] event). The fan blade fractured due to a low-cycle fatigue crack that initiated in the dovetail (part of the blade root), which remained within a slot of the fan disk. The separated fanOpen official document at PDF page 11Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018
Printed page ix. PDF page 12.
Transportation Safety Board (NTSB) determines that the probable cause of this accident was a low-cycle fatigue crack in the dovetail of fan blade No. 13, which resulted in the fan blade separating in flight and impacting the engine fan caseOpen official document at PDF page 12Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018
Printed page label not indexed. PDF page 14.
engine failure occurred; and (4) Southwest Airlines' maintenance of the airplane. The low-cycle fatigue crack in the fan blade dovetail initiated because of higher-than-expected dovetail stresses under normal operating loads, and this crack was most likely not detectableOpen official document at PDF page 14Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018
Printed page 47. PDF page 63.
to the fracture origin area. At higher SEM magnifications, striations consistent with low-cycle fatigue crack growth were observed on the fracture surface. 79 The striation spacing generally became larger as the depth of the crack increased and as the crackOpen official document at PDF page 63Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018
Printed page 69. PDF page 85.
revealed no material anomalies. All of the cracks were located in the highest (maximum) low-cycle fatigue predicted stress areas-the leading edge on the convex side of the fan blade dovetail and the trailing edge of the concave side of theOpen official document at PDF page 85Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018
Printed page 71. PDF page 87.
Summary 2.2.1 Fan Blade Release Fan blade No. 13 in the left engine separated due to a low-cycle fatigue crack that initiated in the blade root dovetail outboard of the blade coating. Metallurgical examination of the fan blade found thatOpen official document at PDF page 87Left Engine Failure and Subsequent Depressurization: Southwest Airlines Flight 1380, Boeing 737-7H4, N772SW, Philadelphia, Pennsylvania, April 17, 2018