When a Southwest Airlines plane was inspected on Sunday, no problems with the aircraft or its engines were uncovered.
Two days later, an engine on the plane broke apart shortly after takeoff from La Guardia Airport, killing a woman sitting in a window seat near the blast.
The fatal accident is focusing renewed scrutiny on the engine inspection process.
Investigators say a routine visual inspection might not have been sufficient to uncover problems in the engine. Two years ago, the manufacturer of the engine said the flaws suspected in the Southwest accident could be detected only with more thorough, ultrasound inspections.
The plane, a Boeing 737-700, and its engine are mainstays in the sky. Southwest’s fleet, more than 700 planes, is made up entirely of 737 models. They have a stellar safety record, as do the engines, which are made by CFM International, a joint venture between General Electric and France’s Safran. This is the first passenger fatality in the 51-year history of the airline, which flew about 3,500 flights a day last year.
After the fatal accident, several airlines, including Southwest, Korean Air and WestJet Airlines, say they plan to inspect the fan blades in their engines. Whether regulators will push for more action will depend on the early findings of their investigation.
The chairman of the National Transportation Safety Board, Robert L. Sumwalt, said a blade in the engine had broken in two places — where the blade attaches to the main hub and across its middle. He said that a crack “was on the interior of the fan blade,” and that it was “more than likely not detectable from looking from the outside.”
But Mr. Sumwalt said it was too soon to reach conclusions on the cause of the engine failure. The board, he said, had not yet examined maintenance records, and was still examining the plane and interviewing its crew. He did say parts of the exterior of the plane’s engine had been recovered on the ground in a rural area outside Philadelphia.
The authorities are still finishing up their investigation of a similar incident on a Southwest flight in 2016, when a fan blade separated and debris ripped a 16-inch-long hole in the fuselage. No one was injured.
That was when CFM International recommended that airlines conduct ultrasound inspections of the blades. In the United States, carriers aren’t required to follow manufacturers’ guidelines.
The Federal Aviation Administration said a directive that it proposed last year to compel airlines to perform such inspections would take effect within the next two weeks. The European Aviation Safety Agency established such regulations for European carriers in late March.
Finding flaws isn’t always easy.
So-called metal fatigue, which investigators suspect as a cause of this week’s engine failure, can be a visible or an invisible weakness that is the result of bending, vibration or other stress. While it is often associated with older airplanes and engines, it can sometimes be the result of manufacturing flaws that cannot be seen.
“The forces in the engine are extraordinary,” said John Gadzinski, a 737 pilot and the founder of Four Winds Aerospace Safety, an aviation consultant. “Those small cracks that you may only see with an electron microscope will go from small to catastrophic failure in an instant.”
What occurred midair on Tuesday — a failure that dislodges pieces of the engine — is exceedingly rare, aircraft analysts said. The fact that it happened twice with the same airline in such a short time span makes it even more worrisome.
“It’s unusual that a fan blade would fail twice on the same engine model, with the same carrier, over two years,” said Kevin Michaels, the president of AeroDynamic Advisory in Ann Arbor, Mich., who has worked as a gas turbine engineer.
In the 2016 incident, a fan blade and another component separated from the engine. The debris did not enter the cabin. It did produce a gash in the aircraft’s side, causing depressurization, according to a preliminary report from the transportation safety board.
The Boeing 737 involved was on its way to Orlando, Fla., from New Orleans and made an emergency landing in Pensacola, Fla.
If it turns out that this week’s engine failure had the same root cause, said Robert W. Mann, an airline analyst based in Port Washington, N.Y., the need to inspect 737 engines could be much more urgent.
“It could drive a significant event industrywide, but it’s hard to tell at this point,” he said. “It could be a defect of design, which would mean it’s subject to fatigue failures.”
In February, a United Airlines Boeing 777, which uses a different kind of engine, also experienced a blade separation and loss of other engine parts. That flight landed in Honolulu virtually on schedule, with no injuries reported and only minor damage, according to the safety board.
Aircraft experts say there’s no reason to worry about Southwest in particular, or about what this might say about the safety of flying in general. Southwest got into the aircraft business to “compete with the family car, which means they have saved tens of thousands of lives,” said Richard Aboulafia, an aviation analyst at the Teal Group in Fairfax, Va., alluding to the statistical fact that flying is safer than driving.
“The extreme worst-case scenario from these two incidents is that it might be prudent to inspect slightly more often,” he said.
Gary C. Kelly, Southwest’s chief executive, said in a news conference on Tuesday that the accident had not caused him to doubt the 737.
“The airplane, in my opinion, is proven,” Mr. Kelly said. “It’s very reliable. It has the greatest success of any other aircraft type.”