A fan blade broke off and destroyed part of the structure that houses the engine, the NTSB found in issuing its probable cause for the deadly incident. A metal latching mechanism flew out and smashed against the plane, blowing out a window and leading to a violent decompression that prompted panic in the cabin, investigators said.
Passenger Jennifer Riordan, a philanthropy executive and mother of two from Albuquerque, was seated next to the blown-out window. She was sucked halfway out of the plane and died of blunt impact trauma, a medical examiner found.
The latching mechanism was part of the overall airplane engine-housing structure, known as a fan cowl, that played a key role in the incident. The NTSB said it is crucial that interactions between different engine and airplane components that played any part in the deadly incident “be well understood to preclude a failure of the fan cowl structure on Boeing 737NG-series airplanes.”
To that end, the NTSB issued several recommendations to the Federal Aviation Administration.
It said the FAA should require Boeing to “redesign the fan cowl structure on all Boeing 737 next-generation-series airplanes to ensure the structural integrity” of that structure in case of another broken fan blade.
The FAA also should require Boeing to install that redesign on new 737 Next Generation jets and require airlines to “retrofit their airplanes with the redesigned fan cowl structure,” the NTSB said.
NTSB Chairman Robert L. Sumwalt III said it is not the role of his organization to consider the costs of such a change but to make sure planes are as safe as they can be with new technologies that have become available in the more than 20 years since the 737 Next Generation was certified. Sumwalt said he did not fault Boeing for the design issue, saying the company used the best technologies at its disposal in the 1990s. But there is now an opportunity to make the planes safer, he said.
Sumwalt also said that airlines have increased the frequency of inspections and made them more robust, which will enable them to catch cracks in fan blades in the current fleet.
In a statement, Boeing said “all 737 NGs are safe to continue operating normally as the issue is completely mitigated by the fan blade inspections.”
The company said that “enhancements are being introduced” into the fan cowl design and to another component “to enhance their ability to withstand” a broken fan blade. The enhancements “would fully address the safety recommendation from the NTSB. Once approved by the FAA, that design change will be implemented in the existing NG fleet,” Boeing said.
Southwest said it will review the recommendations and work “with the manufacturers to prevent this type of event from ever happening again.”
The NTSB also recommended that the FAA “mandate that airplane and engine manufacturers work collaboratively” to ensure that their engine and airplane designs are completely integrated and “that the analysis findings are fully accounted for,” a reference to NTSB findings that the consequences of the broken fan blade on the planes’ structures were different from what was originally predicted in preparation for safety certification.
GE, which is part of a joint venture — CFM International — that made the engine, said that “our standard practice is to develop and certify engines in close collaboration with our airframe partners, working within the appropriate regulatory framework.”
The FAA said it would “carefully review and respond” to the recommendations, adding that it had ordered expanded inspection requirements and “is committed to a philosophy of continuous improvement.”
The April 2018 incident was preceded by a similar Southwest engine failure in August 2016, raising questions about how the airline and federal regulators approached the issue. The questions about the rigor of FAA oversight would grow more pointed still after two crashes of the Boeing 737 Max that killed 346 people .
The deadly Southwest case also added to the safety concerns for Boeing, which has come under intense scrutiny for its internal safety practices and response when safety problems arise.
Boeing said that after the two failures of engines built by its supplier, CFM International, “the FAA and [the European Union Aviation Safety Agency] mandated additional fan blade inspections recommended by the engine manufacturer. Boeing fully supports these inspections and recommends operators comply with all regulatory requirements.”
CFM developed new testing methods for airlines after the 2016 Southwest engine failure. Southwest did not use those new methods to check the fan blades on the plane that ended up having the deadly engine problem but says it now does so routinely.
Southwest pilot Tammie Jo Shults, a former Navy fighter pilot, guided the plane to a safe emergency landing on one engine.