Airlines began inspecting some Boeing 737 engines on Wednesday as an investigation gathered pace into an explosion which killed a passenger on a Southwest Airlines flight in the first fatal U.S. airline accident in almost a decade.
This month European regulators ordered checks following lengthy analysis of a non-fatal incident at Southwest two years ago but investigators warn it is too early to say whether the two problems are linked.
Southwest Flight 1380 made an emergency landing in Philadelphia on Tuesday after an engine ripped apart mid-air, shattering a window on the 737 and nearly sucking out a passenger. One of 144 passengers died.
French accident investigators said they would send a team on Wednesday to assist the investigation led by the Washington-based National Transportation Safety Board because the engine was developed by a French-U.S. joint venture, CFM International.
France's Safran, which co-produces the engines along with General Electric, will also provide technical support, a spokesman for the Paris-based BEA air accident agency said.
Safran shares slipped in Paris, lagging a higher market.
All recent Boeing 737s are powered by engines from CFM, a workhorse of the global airlines that has logged more than 350 million hours of safe travel but some of which were also being examined after the 2016 accident.
CFM says there are more than 8,000 of its CFM56-7B engines in operation on Boeing 737 passenger jets.
Although no cause has been ruled out, the first fatal U.S. airline accident since 2009 is expected to focus attention on the role of metal fatigue in engine accidents, which are rare.
An early review of Tuesday's failed Southwest engine found preliminary evidence of metal fatigue where a fan blade had broken off, Robert Sumwalt, chairman of the U.S. National Transportation Safety Board (NTSB), told reporters on Tuesday.
In August 2016, a Southwest flight made a safe emergency landing in Pensacola, Florida, after a fan blade separated from the same type of engine, and debris ripped a foot-long hole above the left wing. There too, investigators cited signs of metal fatigue.
"We are very concerned," the NTSB's Sumwalt said, referring to the general problem of detecting slow-developing metal fatigue.
"There needs to be proper inspection mechanisms in place to check for this before there's a catastrophic event."
Completing the latest investigation will take about 12-15 months, he said.
Southwest said it was speeding up inspections of all related engines, which it expected to complete within 30 days.
Investigators said they would be examining maintenance records of the airline, which operates one of the world's largest 737 fleets and has a strong safety record.
The 2016 incident prompted the U.S. Federal Aviation Administration (FAA) to propose ultrasonic inspections of similar fan blades and their replacement if needed.
Sumwalt said the NTSB would review whether the engines involved in Tuesday's incident might have been subject to that directive, which is not yet finalised.
European regulators, meanwhile, this month began requiring an inspection by early next year following a study of the 2016 incident.
A person familiar with the matter said U.S. regulators were close to finalising a similar rule.
Korean Air Lines said on Wednesday it planned to carry out voluntary inspections of engines used on its entire 737 fleet by November.
About 20-30 percent of its 35 Boeing 737 jets use the same type of fan blade as the one on the Southwest jet, a Korean Air official said.
Japan Airlines said two 737 jets in its fleet had engines with affected fan blades and inspections were due to be completed on Wednesday.
Dubai-based budget carrier flydubai said it had implemented the European directive ahead of the deadline.
The U.S. FAA has estimated that checks, originally prompted by a safety bulletin from CFM itself, would require two hours per inspection.
Not all airlines operating 737s are affected.