The US National Transportation Safety Board (NTSB) has recommended that Boeing redesigns the fan cowl structure on all B737NG aircraft to prevent structural disintegration of related components in the event of an in-flight fan blade separation.

In its report based on the investigation of Southwest Airlines flight WN1380 from April 17, 2018, the NTSB said that the current design of the engine nacelle was not resilient enough to withstand a fractured fan blade hitting it at the bottom of the engine. The certification testing only covered a scenario in which an impact occurs at the more likely position, at the top of the engine.

"Boeing's post-accident analyses found that the fan cowl structure is more sensitive and more susceptible to failure when a separated fan blade impacts the fan case near the six o’clock position because of the proximity of this fan blade impact location to the radial restraint fitting (at the bottom of the inboard fan cowl)," the NTSB said.

As a result, the NTSB recommended that Boeing redesigns the engine nacelles for all B737NGs. The report further recommended that the manufacturer, after certifying the redesign, installs it on all new B737NGs and then retrofits it on all B737NGs in operation worldwide.

"All B737NGs 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 B737NG and does not affect the B737 MAX," the manufacturer said in a statement.

The NTSB underlined that the accident did not invalidate the certification process. The fan blade cracks "initiated because of higher-than-expected dovetail stresses under normal operating loads, and this crack was most likely not detectable during the fluorescent penetrant inspection".

The investigation concerned an accident in which a fractured fan blade from a CFM International CFM-56-7B engine, powering a Southwest Airlines B737-700, led to the engine inlet and fan cowl separating and subsequently damaging the fuselage, resulting in a rapid cabin depressurisation and the death of one passenger. Eight other persons were injured.