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. Metal fatigueInvestigators 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 engine 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 inspection of fan blades on some CFM56-7B engines after they reach a certain number of takeoffs and landings. Blades that fail will have to be replaced, the agency said. Planes affectedIt is not clear how many planes will 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 to inspect some of their planes.American Airlines has about 300 planes with that type of engine, and Delta Air Lines has about 185. It won’t be clear until the FAA issues its rule how many will need inspections.Tuesday’s accident broke a string of eight straight years without a fatal accident involving a U.S. passenger airliner.It is unclear 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.