On April 17, 2018, about 1103 eastern daylight time, Southwest Airlines (SWA) flight 1380, a Boeing 737-7H4, N772SW, experienced a left engine failure while climbing through flight level 320 en route to the flight’s assigned cruise altitude. The flight had departed from LaGuardia Airport, Queens, New York, about 30 minutes earlier. As a result of the engine failure, the flight crew conducted an emergency descent and diverted to Philadelphia International Airport (PHL), Philadelphia, Pennsylvania. Portions of the left engine inlet and fan cowl separated from the airplane, and fragments from the inlet and fan cowl struck the left wing, the left-side fuselage, and the left horizontal stabilizer. One fan cowl fragment impacted the left-side fuselage near a cabin window, and the window departed the airplane, which resulted in a rapid depressurization. The airplane landed safely at PHL about 17 minutes after the engine failure occurred. Of the 144 passengers and 5 crewmembers aboard the airplane, 1 passenger received fatal injuries, and 8 passengers received minor injuries. The airplane was substantially damaged. The regularly scheduled domestic passenger flight was operating under the provisions of Title 14 Code of Federal Regulations (CFR) Part 121 with a destination of Dallas Love Field, Dallas, Texas.
The airplane was equipped with two CFM International CFM56-7B24 turbofan engines. The CFM56-7B engine has 24 fan blades installed in the fan disk. The left engine failure occurred when one of the fan blades fractured at its 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 fan blade impacted the engine fan case and fractured into multiple fragments. Some of the fan blade fragments traveled forward of the engine and into the inlet.2 In addition, the fan blade’s impact with the fan case caused the fan case to deform locally over a short period of time. This deformation traveled both around and forward/aft of the fan case. After reaching the airplane structure (the inlet attach ring, which was secured to the engine fan case A1 flange), the deformation generated large loads that resulted in local damage to the inlet. The forward-traveling fan blade fragments and the deformation compromised the structural integrity of the inlet, causing portions of the inlet to depart the airplane.
The impact of the separated fan blade with the fan case also imparted significant loads into the fan cowl (also part of the nacelle) through the radial restraint fitting, which was located at the bottom of the inboard fan cowl. These loads caused cracks to form in the fan cowl skin and frames near the radial restraint fitting. This damage then propagated forward and aft, severing the three latch assemblies that joined the inboard and outboard halves of the fan cowl, which caused large portions of both fan cowl halves to separate and depart the airplane. One fan cowl part that was recovered after the accident was the inboard fan cowl aft latch keeper. The left side of the fuselage near the location of the missing cabin window (row 14) had impact damage and witness marks that were consistent with the size and shape of the inboard fan cowl aft latch keeper and surrounding structure.
During the accident sequence, the fan blade fragments traveling forward of the fan case had a trajectory angle that was greater than that observed during the CFM56-7B engine FBO containment certification tests. Also, the inlet damage caused by the forward-traveling fan blade fragments was greater than that observed during the engine FBO containment certification tests and accounted for in Boeing’s 737-700 certification analyses (which used the state-of-the-art analytical modeling tools that were available at the time). In addition, FBO-generated loads were transmitted to the fan cowl through the radial restraint fitting, which was not accounted for in the fan cowl’s design, and the stresses in the fan cowl were greater than those calculated in the certification analyses. Since the time that the CFM56-7B engine and the Boeing 737-700 airplane were certificated (in December 1996 and December 1997, respectively), new technologies and analytical methods have been developed that will better predict the interaction of the engine and airframe during an FBO event and the response of the inlet, fan cowl, and associated airplane structures.
Metallurgical examinations of the fractured fan blade found that the crack had likely initiated before the fan blade set’s last overhaul in October 2012. At that time, the overhaul process included a fluorescent penetrant inspection (FPI) to detect cracks; however, the crack was not detected for unknown reasons. After an August 2016 FBO event involving another SWA 737-700 airplane equipped with CFM56-7B engines, which landed safely at Pensacola International Airport, Pensacola, Florida, CFM developed an eddy current inspection (ECI) procedure to be performed at overhaul (in addition to the FPI that was already required). An ECI has a higher sensitivity than an FPI and can detect cracks at or near the surface (unlike an FPI, which can only detect surface cracks).
The crack on the fan blade involved in the PHL accident was also not detected during the on-wing fan blade visual inspections (subsequent to the overhaul) that were conducted as part of fan blade relubrications, which CFM recommended to maintain the fan blade loads within the predicted range and prevent wear on the fan disk and the fan blade dovetail coating. After the August 2016 FBO event, CFM developed an on-wing ultrasonic inspection technique that could be performed at the time of fan blade relubrication. ECIs at the time of overhaul or ultrasonic inspections at the time of fan blade relubrication identified 15 blade cracks on separate engines (as of August 2019).
We determined 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 case at a location that was critical to the structural integrity and performance of the fan cowl structure. This impact led to the in-flight separation of fan cowl components, including the inboard fan cowl aft latch keeper, which struck the fuselage near a cabin window and caused the window to depart from the airplane, the cabin to rapidly depressurize, and the passenger fatality.
We made recommendations to the Federal Aviation Administration, Southwest Airlines and the European Aviation Safety Agency.