The plane, a Boeing 737-700 carrying 143 passengers and five crew, was on a flight from New York's La Guardia airport to Dallas Love Field when the explosion happened about 20 minutes out of NY. A woman sitting next to the window died of her injuries. The crew was forced to use its remaining two engines to steer the plane to an emergency crash landing at the airport in Sioux City, Iowa.
Sumwalt said there was evidence of metal fatigue where the blade attached to a hub.
The National Transportation Safety Board also blamed metal fatigue in preliminary findings after an engine broke apart on another Southwest Boeing 737 over Florida in 2016.
Southwest crews were inspecting similar engines the airline had in service, focusing on the 400 to 600 oldest of the CFM56 engines, made by a partnership of France's Safran and General Electric, according to a person with knowledge of the situation. It is something the FAA proposed making mandatory almost a year ago, but the draft directive was never approved. It will require inspections of some CFM56-7B engines, the type on the Southwest flight, and FAA officials acknowledged the total number of engines could be higher than the 220 initially estimated.
"The affected engine count for the fleet in costs of compliance ... appears to be vastly understated", it said.
Southwest said in its submission on the federal website it would have to inspect some 732 engines in one of two categories of engines under review - much higher than the FAA's total estimate of 220 engines needing to be inspected across the whole USA fleet.
Southwest, meanwhile, said Tuesday that it is "accelerating its existing engine inspection program relating to the CFM56 engine family". United Airlines executives said Wednesday that they had begun to inspect some of their planes.
The blades, which sweep air backwards to help provide thrust, can be changed and repaired independently of the rest of the engine, meaning airlines that don't keep tabs have to examine more engines than anticipated, which adds time and cost.
It's not clear whether the rule, were it in effect, would have applied to the failed engine in Tuesday's flight, because there are many CFM56-7B variants.
Tuesday's accident broke a string of eight straight years without a fatal accident involving a USA passenger airliner.
Southwest CEO Gary Kelly said that the plane involved in Tuesday's incident had undergone an inspection two days before the explosion, but he said he was not aware of the nature of that inspection or whether it included a specific inspection of that engine.
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Passengers described scenes of panic as a piece of shrapnel from the engine shattered a plane window, nearly sucking Riordan out.
Texas rancher Tim McGinty, of Hillsboro, said Tuesday that he and Needum struggled to pull 43-year-old Jennifer Riordan back into the plane.
The pilots began a harrowing descent and made an emergency landing in Philadelphia. Pieces of the plane were found in rural Pennsylvania by investigators who tracked them on radar.
The leading edge of the left wing was damaged by shrapnel from the engine explosion.
It is unknown whether the FAA's original directive would have forced Southwest to quickly inspect the engine that blew up.
The engine had 40,000 cycles on it, a quarter of those since it was overhauled, the airline said.
Before Wednesday's announcement, critics accused the FAA of inaction in the face of a threat to safety. Based on witness testimony, authorities are now working under the assumption that a piece from the broken plane engine flew into the window, causing it to shatter, and hitting the woman.
William Waldock, a safety expert at Embry-Riddle Aeronautical University, predicted the FAA's decision. Investigators found signs of metal fatigue.
No one was injured, Southwest said.
The two-year-old incident was revealed following Tuesday's deadly incident that killed a mother of two aboard a Southwest flight. The NTSB's Sumwalt said, however, that the kind of wear seen where the missing fan blade broke off would not have been visible just by looking at the engine. "So careful (visual) maintenance inspection from outside the fan blade would not have detected it, more than likely".
"The window had broken and the negative pressure had pulled her outside the plane partially", Peggy Phillips, a registered nurse who was on the plane, told WFAA-TV in Dallas. Engines are typically removed for work during a D check.