NTSB Public Hearing Held on Southwest Fan Blade Failure

The NTSB held a pubic hearing yesterday, Wednesday, November 14 as part of the agency’s ongoing investigation of the fatal, April 17, 2018, Boeing 737/CM International fan-blade-out and depressurization accident on Southwest Airlines flight 1380.

There were two panels; CFM56-7B Fan Blades: Design and Certification, Consequences of the Fan Blade Out, Inspection Intervals and Procedures, and B737/CFM56-7B Inlet and Fan Cowl: Design and Certification, Structural Capability following a Fan Blade Out Event.

Witnesses from Boeing, CFM, the Federal Aviation Administration, and United Technologies Aerospace Systems provided testimony on the following topics during the second panel:

Certification of the Inlet and Fan Cowl structure for an FBO event Design, analysis and testing of the Inlet and Fan Cowl structure for an FBO event. Structural Capability of the Inlet and Fan Cowl structures following FBO.

NTSB had previously issued an investigative update about the ongoing investigation, but the hearing focused on the engine design, various design changes to address cracking and an incident in 2016 where another Southwest Airlines aircraft engine experienced a failure.

The engine fan blade design of the CFM56-7B engine was a challenge since the engine’s design, according to statements made by the manufacturer during the hearing.CFM International told the hearing the CFM56-7B initially failed its certification test in the 90s but ultimately passed. Design changes to prevent fan blade failures were implemented during the certification stage as well as after the engine was put into service. “The coating system was not staying intact as well as we had anticipated with the design change,” said Mark Habedank, engineer for the CFM56 engine. A shim and additional lubrication were added to correct the issues.

More cracked blades were later found on other airlines’ planes, according to the engineer. Additional inspections were required after the 2016 failure. However, the inspections were done methodically but in no particular order. “We had no basis to prioritize these fan blades versus any other blades in our fleet,” said Mark Wibben, a Southwest engineering manager.