'Fan blade failure due to cracking, if not addressed, could result in an engine in-flight shutdown (IFSD), uncontained release of debris, damage to the engine, damage to the airplane and possible airplane decompression, ' the USA regulatory authority said.
Such inspections, noted The New York Times, can "detect flaws or cracks not visible to the unaided human eye".
The engines that must be inspected are manufactured by CFM International.
The agency said its order affects 352 engines in the USA and another 681 worldwide on "new generation" Boeing 737 jets. The plane, en route to Dallas from NY, made an emergency landing in Philadelphia.
In August 2016 a Southwest flight made a safe emergency landing in Florida after a fan blade separated from the same type of engine and debris ripped a hole above the left wing. Riordan died at a hospital after the plane made an emergency landing in Philadelphia. She was pulled back in by other passengers, but later died of her injuries.
The National Transportation Safety Board believes one of the blades snapped on the Southwest flight Tuesday, hurling debris that broke a window and led to the death of a passenger who was sucked partway out of the plane.
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Airlines must now make ultrasonic inspections of fan blades that have been used in more than 30,000 cycles, or have been in service for about 20 years, within the next 20 days.
"The unsafe condition is likely to exist or develop in other products of the same design", the FAA said in the order.
"The public should be anxious (because) a manufacturer sent out a warning, and Southwest and others didn't do it", said Mary Schiavo, a former inspector general of the Transportation Department, FAA's parent agency.
CFM International had issued a similar recommendation in 2016 after the failure of a CFM56-7B engine on another Southwest aircraft. CFM also urged the Federal Aviation Administration to issue an Airworthiness Directive to ensure prompt compliance with the recommended inspections. When asked why the airline resisted the proposal a year ago, spokeswoman Brandy King said Southwest needed more time to find individual suspect blades within certain engines. The company also recommended that fan blades with 20,000 cycles be inspected by the end of August.
NTSB chairman Robert Sumwalt told reporters that a fan blade had broken off due to metal fatigue and that a second fracture had been recorded about halfway along its length.
"There are very specific regulation guidelines to ensure that if an engine does fail, that it doesn't come apart and turn into little bombs". Uncontained engine failures are rare - about three or four a year, according to Sumwalt.