After investigating an engine blowout that killed a Southwest Airlines passenger last year, federal safety officials on Tuesday said Boeing should be required to redesign the engine casing on its 737 NG airplanes, and airlines should retrofit more than 6,800 planes currently in service worldwide.

In a terrifying incident in April 2018, the left engine of the 737 exploded and flying metal broke a passenger window. Jennifer Riordan, 43, the mother of two children, was partially sucked out of the airplane.

The cause was a fan blade in the left engine that broke off and sent metal shrapnel ripping through the engine casing, piercing the fuselage and causing the cabin to decompress. Part of the engine cowl struck and broke the window by Riordan’s seat.

She died as a result of her injuries, the sole fatality on board a U.S. airline in the past decade.

The tragedy raised questions about safety oversight because of a similar engine explosion on a different Southwest flight 19 months earlier. After that earlier incident the Federal Aviation Administration (FAA) mandated inspections of engine fan blades — but the inspections failed to detect the cracked fan blade that caused the fatal accident.

At a public hearing Tuesday, the National Transportation Safety Board (NTSB) laid out its investigation into Southwest flight 1380, which departed from New York City bound for Dallas, but diverted to Philadelphia after the engine explosion.

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The NTSB issued a series of recommendations, including that the FAA require Boeing to redesign the casing around the engine to prevent penetration by hot metal shrapnel in the event of an engine explosion.

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On Flight 1380, it was the disintegration of the fan cowl that did most damage. That’s the middle part of the pod that wraps around the engine; a part that opens up on hinges to allow access for maintenance.

The design requirements for the engine casing — known as the nacelle — are developed by Boeing. The 737 NG nacelles are then designed and built by a division of United Technologies.

The 737 NG, with nearly 7,000 flying worldwide, is the model prior to the 737 MAX. The MAX has a different engine and casing and is not affected.

Boeing said it “is working on a design enhancement that would fully address the safety recommendation from the NTSB.”

“Once approved by the FAA, that design change will be implemented in the existing 737 NG fleet,” Boeing added.

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The NTSB said the accident investigation revealed that when the fan blade broke off, it hit the casing at a particularly vulnerable spot. It recommended that the FAA assess whether other airplane/engine combinations have similar critical locations where the casing may need to be strengthened.

A similar blowout in 2016

The NTSB found no fault with Southwest.

Captain Tammie Jo Shults, a former Navy fighter pilot, guided the plane carrying 144 passengers and five crew to a safe landing 20 minutes after the explosion.

However, the tragedy followed a similar engine blowout on a Southwest flight from New Orleans to Orlando in August 2016 that diverted to Pensacola, Florida. The 737 cabin decompressed, but no one was injured.

At the NTSB hearing Tuesday, FAA safety engineer Christopher Spinney said the Pensacola incident was the first broken fan blade on a CFM56-7B engine in 300 million flight hours.

Because it was such a rare event and resulted in only a slow depressurization, the FAA initially assessed “that we would require some corrective action, in that it was an unsafe condition, but we also determined we had some time,” Spinney said.

The FAA found the cause of the Philadelphia engine failure to be metal fatigue cracks at the root of a fan blade, and it ordered inspections of specific fan blades on the CFM56-7B engines powering that model of 737. The blade that broke on flight 1380 was not a part included in that directive.

In November 2017, a crack was found in a different fan blade not covered by the initial mandate and the FAA determined that the failure could potentially occur in any fan blade on a CFM56-7B engine.

The FAA said Tuesday that it then worked with engine maker CFM International — a joint venture between GE and Safran of France — to develop and approve a comprehensive inspection program for all the engine fan blades.

Before the fatal accident, it was preparing but had not completed an airworthiness directive to mandate the increased inspections.

The FAA added that CFM had already recommended to airlines the same inspection regime, and that Southwest Airlines and other airlines were proactively performing the inspections — though they were not fully completed.

Three days after the accident and the death of Ms. Riordan, the FAA issued an emergency airworthiness directive, which was expanded the following month to require airlines to perform detailed inspections on each fan blade before the fan blade accumulates 20,000 cycles of take-off and landing, and to repeat the inspection no later than each 3,000 cycles.

The FAA’s Spinney told the NTSB Tuesday that the blowout over Pensacola had been so unusual that “we thought it was an anomaly.”

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“When the Philadelphia event happened, we realized in coordination with our engine experts that indeed the Pensacola event was not an anomaly. That’s when we took immediate action with the engine airworthiness directive.”

The FAA said Tuesday that “the CFM engines at issue meet the FAA’s safety standards.”

Boeing likewise said that “all 737 NGs are safe to continue operating normally as the issue is completely mitigated by the fan blade inspections” in the FAA directives that followed the fatal accident.

Southwest in a statement said it will review the NTSB recommendations and will continue “ongoing work with the manufacturers to prevent a similar event from ever happening again.”

The NTSB recommendation adds to the mounting woes of the 737 jet family.

The MAX is still grounded eight months after the second of two fatal crashes. And on the 737 NG, cracks were recently discovered in a structural part called the “pickle fork,” a heavy-duty metal attachment between the wing and the fuselage.

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Last month, the FAA issued an airworthiness directive requiring inspection within the next 1,000 cycles of the pickle forks on all 737 NGs that have flown more than 22,600 cycles and immediate inspection for those with more than 30,000 cycles.

Rob Morris, head of consultancy at Cirium, an aviation data and analytics company, estimates that almost 1,800 aircraft will require this inspection. As of early this month, he said 1,100 aircraft had been inspected, of which 50 were found to need repairs that will take two to three weeks.

Airlines affected by pickle fork cracks include Southwest and Ryanair, each of which has two 737s under repair.