WASHINGTON (Reuters) - The U.S. National Transportation Safety Board on Tuesday called on Boeing Co BA.N to redesign the fan cowl structure on 737 NG planes after a passenger was killed on a Southwest Airlines LUV.N plane in April 2018 after an engine failure.

FILE PHOTO: Southwest commercial airliners taxied at McCarran International Airport in Las Vegas, November 19, 2014. REUTERS/Mike Blake/

The NTSB said the engine failure was caused by a broken fan blade, and the board said the U.S. Federal Aviation Administration should require Boeing to determine the fan blade impact location or locations on the engine fan case and redesign the structure to minimize the potential of a catastrophic failure. The board did not fault Boeing’s analysis in the mid-1990s when it developed the plane.

The NTSB had been investigating a 2016 engine failure on another Southwest 737-700 at the time of the fatal incident. The incidents in both flights were what is known as a “fan blade out” (FBO) event. The 737-700 is a model in the 737 NG family.

Boeing said all 737 NGs are safe to continue operating as the issue is “completely mitigated by the fan blade inspections.”

Boeing 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 NG fleet.”

The FAA noted that it had issued numerous directives requiring more fan blade inspections and said it would “carefully review and respond to the NTSB recommendations.”

The NTSB said Boeing’s postaccident analyses found that the fan cowl structure is “more sensitive and more susceptible to failure” when a fan blade hit the fan case than previously indicated.

NTSB Chairman Robert Sumwalt acknowledged the retrofit could be expensive.

“This accident underlines the vulnerability of the fan case to become separated when the fan blade detaches at a location that was not anticipated,” Sumwalt said after the hearing.

The NTSB did not call for the planes to be grounded and noted that airlines are now inspecting the fan blades on a more regular basis, essentially every nine to 12 months.

Southwest spokesman Chris Mainz said the airline would review the NTSB’s recommendations and work “with the manufacturers to prevent this type of event from ever happening again.”

Jennifer Riordan of New Mexico, a 43-year-old Wells Fargo vice president and mother of two, was killed after the engine exploded and shattered a plane window on Flight 1380. She was the first person killed in a U.S. passenger airline accident since 2009.

The accident occurred 20 minutes into the flight when a fan blade fractured as a result of a fatigue crack on a Boeing 737-700 jet powered by two CFM International CFM56-7B engines after taking off from New York’s LaGuardia Airport. The plane, bound for Dallas, diverted to Philadelphia International Airport. Eight of the 144 passengers suffered minor injuries.

The board noted that there are 14,600 CFM56-7B engines in service with 356,000 fan blades on the Boeing planes, with 400 million flights over more than two decades and two reported engine failures.

CFM said it “will continue to strictly comply with regulatory requirements, including any changes that might be adopted as a result of NTSB’s recommendations.”

Tammie Jo Shults, the flight’s captain, recounted in her book “Nerves of Steel” published last month that the engine explosion felt “like we’ve been T-boned by a Mack truck.” She said that the 737-700 rolled to the left and pulled into a dive, but that she and the co-pilot were able to level off the plane.

The engine on the plane’s left side spewed bits of metal when it blew apart, shattering a window and causing rapid cabin depressurization, the NTSB said. In 2018, the NTSB said two passengers eventually pulled Riordan, who was buckled into her seat, back inside the plane.

CFM International, the engine manufacturer, is a transatlantic joint venture between General Electric Co GE.N and France's Safran SA SAF.PA.

The issue does not impact the 737 MAX, the version of the plane that followed the 737 NG.