As Engine Failed, F.A.A. Was Considering Rules That Might Have Made a Difference

The engine failure that knocked a hole in the side of a Southwest Airlines Boeing 737 and left a woman dead on Tuesday was an extraordinarily unusual event, seemingly the result of a rare engine defect that scattered debris and knocked out one of the plane’s windows at 32,000 feet.

Yet it was foreshadowed by a similar episode less than two years earlier.

“The fact that this happened is a really, really freakish condition,” said Mark Millam, a vice president at the Flight Safety Foundation. “If I was going to look at all the probabilities for a fatal accident, it would not have been this.”

The initial cause of the engine failure this week, which forced the flight to make an emergency landing in Philadelphia, is believed to have been metal fatigue — the same problem that investigators think caused an engine failure on another Southwest 737 in August 2016.

That episode caused no deaths, but it prompted federal transportation regulators, the engine manufacturer and airlines to agree that more thorough engine inspections were necessary. Nearly two years later, those inspections are not yet mandatory in the United States.

The process, like so much else in government, has many steps and takes lots of time.

A few weeks after the 2016 flight, from New Orleans to Orlando, the National Transportation Safety Board issued its initial findings: Metal fatigue had caused part of the engine to come apart. The stresses of normal flight had degraded a fan blade, causing it to crack and break loose. The blade destroyed part of the engine and opened a gash in the fuselage.

By January 2017, the engine’s manufacturer, CFM International, had updated its maintenance manual. CFM, a joint venture between General Electric and Safran that also made the engine involved in this week’s accident, recommended using electromagnetic testing to detect flaws in engine fan blades that were most at risk of the same fatigue failures. In June, the company issued a so-called alert service bulletin, recommending ultrasound inspections of the components involved in the 2016 engine failure.

Such bulletins, while not legally binding, are treated seriously by the airlines, Mr. Millam said. “You don’t generally sit and wait around as an airline to wait and see what happens with an alert service bulletin,” he said.

In August, the Federal Aviation Administration proposed a new regulation modeled on the manufacturer’s guidelines. Many major carriers seemed to support the proposal.

The 2016 episode was not deemed serious enough for the agency to issue an emergency directive. Instead, it followed its normal procedure: It researched the issue, released a proposal and accepted public comments for 60 days. Those comments are analyzed before a final rule is issued.

The public comments, which are posted online, show that CFM wanted airlines to complete inspections within 12 months of the directive. The Federal Aviation Administration recommended 18 months, which most airlines, including Southwest, seemed to prefer.

The agency was still analyzing the public comments when the engine failure happened on Tuesday. A day later, it announced that it planned to issue a regulation within two weeks.

European regulators had issued their own version of the rule in late March. It requires carriers registered in the European Union to complete their inspections within nine months.

American Airlines, United Airlines, Delta Air Lines and Southwest all said they were working pre-emptively to inspect their engines based on CFM’s recommendations. Delta said it had already completed its inspections.

Southwest announced that it would accelerate its inspection program and finish it in the next 30 days.