PHILADELPHIA (AP) -- U.S. airline regulators have ordered inspections on engine fan blades like the one that snapped off a Southwest Airlines plane, leading to the death of a woman who was partially blown out a window.
The Federal Aviation Administration's announcement late Wednesday comes nearly a year after the engine's manufacturer recommended the additional inspections, and a month after European regulators ordered their airlines to do the work.
Pressure for the FAA to act grew after an engine on a Southwest plane blew apart on Tuesday, showering the aircraft with debris and shattering a window. A woman sitting next to the window was partially blown out and died of her injuries. The plane, which was headed from New York to Dallas, made an emergency landing in Philadelphia.
Investigators said a blade that broke off mid-flight and triggered the fatal accident was showing signs of metal fatigue — microscopic cracks that can splinter open under the kind of stress placed on jetliners and their engines.
The National Transportation Safety Board also blamed metal fatigue for an engine failure on a Southwest plane in Florida in 2016.
That led manufacturer CFM International, a joint venture of General Electric Co. and France's Safran SA, to recommend last June that airlines conduct the inspections of fan blades on many Boeing 737s.
The FAA proposed making the recommendation mandatory in August but never issued a final decision.
On Wednesday, the FAA said it would issue a directive in the next two weeks to require ultrasonic inspections of fan blades on some CFM56-7B engines after they reach a certain number of takeoffs and landings. Blades that fail inspection would need to be replaced.
It was not immediately clear how many planes would be affected. Last year, the FAA estimated that an order would cover 220 engines on U.S. airlines. That number could be higher now because more engines have hit the number of flights triggering an inspection.
Southwest announced its own program for similar inspections of its 700-plane fleet over the next month. United Airlines executives said Wednesday that they had begun inspecting some of their planes.
American Airlines has about 300 planes with that type of engine, and Delta Air Lines has about 185. It will not be clear until the FAA issues its rule how many will need inspections.
Tuesday's emergency broke a string of eight straight years without a fatal accident involving a U.S. airliner.
"Engine failures like this should not occur," Robert Sumwalt, chairman of the NTSB, said Wednesday.
Sumwalt expressed concern about such a destructive engine failure but said he would not yet draw broad conclusions about the safety of CFM56 engines or the entire fleet of Boeing 737s, the most popular airliner ever built.
Federal investigators were still trying to determine how a window came out of the plane. The woman sitting next to it, identified by family members as 43-year-old Jennifer Riordan, was wearing a seat belt. Philadelphia's medical examiner said the banking executive and mother of two from Albuquerque, New Mexico, died from blunt impact trauma to her head, neck and torso.
It is unknown whether the FAA's original directive would have forced Southwest to quickly inspect the engine that blew up. CEO Gary Kelly said it had logged only 10,000 cycles since being overhauled.
Before Wednesday's announcement, critics accused the FAA of inaction in the face of a threat to safety.
Robert Clifford, a lawyer who is suing American Airlines over another engine explosion that caused a fire that destroyed the plane, said the FAA should have required the inspections — even if it meant grounding Boeing 737s.
"There is something going on with these engines," he said, "and the statistical likelihood of additional failures exists."
William Waldock, a safety expert at Embry-Riddle Aeronautical University, predicted the FAA's decision. He said the scope of FAA action will depend on whether investigators find fatigue in other fan blades on the broken engine.
"The first thing they probably are going to do is pull every single one of those other blades off and X-ray them to see if they've got a similar type of failure waiting to happen," he said.
The Southwest CEO protested that it is too soon to say whether Tuesday's accident is related to any other engine failures.
Kelly said the plane was inspected on Sunday and nothing appeared out of order. A spokeswoman said it was a visual inspection and oil service of the engines. 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.