The Federal Aviation Administration will have to give its approval before Boeing’s 737 MAX airplanes can fly again. Neither Boeing nor the FAA would elaborate on what inspection procedures may be required to allow that to happen.

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The Federal Aviation Administration (FAA) will have to give its blessing before Boeing’s 737 MAX airplanes can fly again.

Boeing grounded the jets on Wednesday after the discovery by engine supplier CFM International of a flaw in the manufacturing of some metal discs inside the jet’s new engines at one of its sub-tier suppliers.

“The FAA is working closely with CFM and Boeing to develop a proposed plan to clear the engines,” Laura Brown, a spokeswoman for the air-safety agency, said Thursday.

Neither Boeing nor the FAA would elaborate on what inspection procedures may be required before a return to flight can be approved.

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The problem is a potential material flaw in a metal part inside the engines, detected at one of several suppliers of that part. Not all the engines delivered to Boeing have this potential flaw.

No problems have been found with those parts during 16 months of flight tests, but the discovery of an incorrect manufacturing process raised quality-control concerns.

CFM said Wednesday that its quality inspections had found an anomaly in the process of forging the discs that are part of the low-pressure turbine at the rear of the engine.

Because that could lead to a flaw in the metal that would potentially leave the disc vulnerable to cracking, CFM said that “in an abundance of caution” Boeing decided to ground the aircraft until inspections can be completed.

The affected engines are being flown back to CFM facilities either in Lafayette, Indiana, or Villaroche, France, for inspection of the discs and replacement where necessary.

CFM is a 50-50 joint venture between U.S. engine-maker GE and French engine-maker Safran. Safran is responsible for the low pressure turbine system.

The engine for the MAX, the LEAP-1B, is the critical new feature of the aircraft, making it 13 percent more fuel efficient than the current model 737.

CFM said only the Boeing MAX version of the engine is affected, not the LEAP-1A, the engine on the Airbus A320neo, nor the LEAP-1C, the engine on the Chinese C919.

The first MAX delivery, to Malaysian carrier Malindo Air, a subsidiary of Asian low- cost carrier Lion Air, was supposed to happen next week, with a second delivery soon after to Norwegian Air.

Boeing said Wednesday it still hopes to be able to make its planned deliveries this month.

CFM spokeswoman Jamie Jewell told Reuters Thursday that the FAA will have to review all the LEAP-1B engines produced so far, not just those with the suspect discs made by the supplier where the forging process was faulty.

Boeing has so far rolled out about 25 MAXs, all of which are now parked either at Renton Municipal Airport or at Boeing Field. All flight tests of those aircraft are on hold.