Following an accident on Southwest Airlines (SWA) Flight 1380 in April 2018 that resulted in the death of passenger Jennifer Riordan, the National Transportation Safety Board announced the probable cause during a public board meeting held last week. A fractured fan blade from a CFM International CFM-56-7B engine, powering a Southwest Airlines Boeing 737-700, led to the engine inlet and fan cowl separating and subsequently damaging the fuselage, resulting in a rapid cabin depressurization.
As a result of the investigation, the NTSB issued seven new safety recommendations, with five issued to the Federal Aviation Administration, one to the European Aviation Safety Agency, and one to Southwest Airlines. These recommendations address the need to ensure the structural integrity of the fan cowl on Boeing 737 next-generation airplanes and assess whether other airframe and engine combinations have critical fan blade impact locations.
The NTSB determined that probable cause of the 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 to suffer fatal injuries.
The accident happened after SWA 1380 had departed New York’s LaGuardia Airport, bound for Love Field, Dallas, Texas. The flight crew, led by Captain Tammie Jo Shults, conducted an emergency descent and diverted to Philadelphia International Airport. There were 144 passengers and five crewmembers aboard. Along with the one passenger fatality, eight other passengers suffered minor injuries.
“This accident demonstrates that a fan blade can fail and release differently than that observed during engine certification testing and accounted for in airframe structural analyses,” said NTSB Chairman Robert Sumwalt. “It is important to go beyond routine examination of fan blades; the structural integrity of the engine nacelle components for various airframe and engine combinations needs to be ensured.”
The NTSB recommendations to the FAA included the requirement that Boeing determine the critical fan blade impact location(s) on the CFM56-7B engine fan case and redesign the fan cowl structure on all Boeing 737 next-generation-series airplanes to ensure the structural integrity of the fan cowl after a fan-blade-out event. Once that action is completed, the NTSB’s findings require Boeing to “install the redesigned fan cowl structure on new-production 737 next-generation-series airplanes, and require operators of Boeing 737 next-generation-series airplanes to retrofit their airplanes with the redesigned fan cowl structure.”
NTSB also recommended the expansion of Title 14 Code of Federal Regulations Part 25 and 33 certification requirements to “mandate that airplane and engine manufacturers work collaboratively to analyze all critical fan blade impact locations for all engine operating conditions, the resulting fan blade fragmentation, and the effects of the fan-blade-out-generated loads on the nacelle structure and develop a method to ensure that the analysis findings are fully accounted for in the design of the nacelle structure and its components.”
A statement from Boeing said “All 737 NGs are safe to continue operating normally as the issue is completely mitigated by the fan blade inspections. In addition, Boeing is working on the design enhancements to fully address the safety recommendation from the NTSB. Once approved by the FAA, that design change will be implemented in the existing NG fleet over the longer term. This issue is limited to the 737 NG and does not affect the 737 MAX.”
An abstract of the final report, which includes the findings, probable cause, and all safety recommendations, is available from NTSB, with links to the accident docket and other publicly released information about this investigation available on the NTSB website. The final report for the investigation of the accident is expected to post to the NTSB website in the next few weeks.
