The regulatory hurdle could push back the resumption of 737 MAX flights, a potentially serious issue for Boeing if it delays the scheduled first deliveries of the $110 million plane to airlines this month.

The delivery of a plane is a crucial step for Boeing since it receives the bulk of its payment when it turns over the aircraft keys and airlines fly their new planes home from the factory.

Boeing and engine maker CFM International must submit data to the U.S. Federal Aviation Administration, and the FAA has to “review it and determine that those engines can be cleared for flight,” said Jamie Jewell, a spokeswoman for CFM, a joint venture between General Electric (GE) and Safran SA (SAF) of France.

It was not immediately clear how long that process might take. The FAA is working with CFM and Boeing “to develop a proposed plan to clear the engines,” said spokeswoman Laura Brown.

The FAA certified the engine in March, but the emergence of the problem disclosed on Wednesday was an unusual occurrence that created uncertainty over the next steps.

Boeing said it still plans to deliver the first of the planes to customers this month. It said on Wednesday it had suspended 737 MAX test flights due to a quality issue with some of the low-pressure turbine discs in the engine, known as the LEAP-1B.

The nickel-based alloy discs had a flaw from forging that could make them prone to cracking, Jewell said, adding that discs from a second supplier were not affected.

However, the FAA review applies to all LEAP-1B engines, Jewell said, not just the 30 to 40 with suspected bad parts.

“In their mind, all LEAP-1B engines are equal,” she said.

KEY MONEYMAKER

Boeing has engines at the factory without the suspect parts, but CFM and Boeing “have to demonstrate to the FAA’s satisfaction that the issue is not present,” Jewell said.

The engines could be installed in short order once approval is granted, Jewell said.

The “suspect engines” will be shipped to CFM facilities in the United States and France for inspection and possible repair.

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Safran said earlier Thursday that it expected inspections to be completed within a few weeks. “We are doing everything to ensure that the first delivery can go ahead in May,” Safran Aircraft Engines’ Chief Executive Olivier Andries told reporters.

The 737 MAX replaces an older version of Boeing’s best-selling single-aisle aircraft, a moneymaker for the aerospace company. The 737 MAX 8, the first version of the plane to be built, seats 162 passengers in a typical two-class configuration.

Boeing had been set to deliver the first 737 MAX on Monday to Malindo Air of Malaysia.

Boeing has built up an inventory of planes outside its Renton, Washington, factory, and at a nearby Seattle airport known as Boeing Field. Reuters counted 23 Boeing 737 MAX airplanes at the facilities this week, excluding test aircraft.

The 737 MAX 8 carries a list price of $110 million but typically sells at a steep discount.