At least 60 airlines around the world are racing to check their older engines for signs of metal fatigue over the next two weeks in the wake of last week’s engine blowout on a Southwest Airlines 737, which killed one passenger and injured seven others.

To comply with an FAA order rushed out after the Southwest accident, airlines will have to take planes out of service for up to four hours to perform ultrasonic testing. Any CFM56-7B engines that have exceeded 30,000 landing and takeoff cycles—a milestone typically reached after 20 years of service—must be tested.

The inspections disrupted about 10 percent of the carrier’s 4,000 daily flights

Airlines will also need to inspect engines that have logged 20,000 cycles, but will have a few more months to comply. The engines in question are typically found on the 737-700 and -800 models. Airlines that rack up the largest number of cycles, which tend to be low-cost lines like Southwest, probably have more engines that need the emergency checks.

Southwest said it was “working around the clock” to meet the FAA mandate, and that means canceling a small percentage of flights and delaying many more. On Monday, the inspections disrupted about 10 percent of the carrier’s 4,000 daily flights, according to FlightAware.

Other airlines under the gun include United, which said it has 698 CFM56-7B engines in its fleet, and Ryanair, which said 70 of its 440 Boeing 737 jets have the older engines requiring immediate checks. A significant chunk of China’s commercial fleet—including planes operated by Air China, China Eastern, China Southern, and Hainan—will be impacted. American Airlines said it had already begun inspecting all engines of that specific model following an August 2016 incident aboard a Southwest flight that, in hindsight, appears to have been a precursor to last week’s deadly accident.

Indeed, that event a year and a half ago is sparking criticism of the FAA’s and the airline industry’s apparently desultory response to the problem of metal fatigue. The Southwest plane made an emergency landing in Pensacola, FL, after a fan blade separated from the same model of engine. Debris tore a foot-long gash above the left wing. No one was killed or hurt, but an inspection turned up evidence of cracking in the fan blade assembly.

In response, the engine maker CFM, a joint venture of GE and France’s Safran SA, called for airlines to inspect the engines within 12 months. But the industry pushed back, complaining that the FAA had “vastly underestimated” the high costs and inconvenience. In August of last year, the FAA agreed to give the airlines 18 months to complete the testing and replacement of any damaged components.

One factor that got the FAA's attention was the airlines’ resistance to checking all 24 fan blades in an engine rather than singling out only a few ones in particular for the checks. That was apparently a longtime industry practice. But in its order, the FAA specifically demanded airlines with the older engines to perform a “one-time check of all 24 fan blade” sides to detect cracks, and to remove any cracked blades before putting the plane back in the air. “Fan blade failure due to cracking, if not addressed, could result in an engine in-flight shutdown,” the FAA said, which could cause “an uncontained release of debris, damage to the engine and to the airplane and possible airplane decompression.” In other words, exactly what happened with Southwest Flight 1380 last week.

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Aviation safety experts say the previous engine explosion should have prompted a stronger response. “The FAA is developing a credibility issue,” says Greg Feith, a former official with the National Transportation Safety Board who is now an aviation consultant. The NTSB, too, has been criticized because it has yet to complete its probe of the 2016 incident, and there are signs Congress may investigate.

Sen. Chuck Schumer, for one, accused the FAA of lax oversight and said that a recent examination of agency records showed that enforcement of aircraft maintenance compliance and fines have slipped by 70 percent in recent years.

“Since at least 2014, it would appear the FAA has been in a nose-dive on safety and that simply cannot fly,” he said. And, as last week’s accident shows, the consequences of inaction could be even more serious the next time.

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