Weeks away from the first delivery of its new 737 MAX airliner, Boeing on Wednesday grounded its fleet of planes because of a quality problem with the new LEAP engine.

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Just days away from its first 737 MAX delivery after a remarkably problem-free development effort, Boeing’s new single-aisle jet program got a nasty jolt when a freshly discovered engine-manufacturing problem prompted it Wednesday to ground the entire MAX fleet.

Boeing learned that metal discs inside some of its new LEAP engines could potentially crack, although no problems had been detected during extensive flight testing of the MAX jets, which began in January 2016.

“Out of an abundance of caution, we decided to temporarily suspend MAX flights,” said Boeing spokesman Doug Alder.

About 25 MAX planes, both test jets and production planes intended for customers, are parked at Boeing Field or Renton Municipal Airport and have been making regular test flights.

Boeing said the affected LEAP engines on those airplanes are being sent for inspection to engine maker CFM International’s facilities either in Lafayette, Indiana, or Villaroche, France.

Because some LEAP engines already delivered to Boeing are unaffected by the issue, Boeing said it still intends to go ahead with initial deliveries of the MAX this month.

CFM informed Boeing late last week of “a potential manufacturing quality escape with low pressure turbine discs (LPT)” in the engines already delivered, Boeing said.

CFM spokeswoman Jamie Jewell said quality inspectors discovered an anomaly in the process of forging the discs during manufacturing.

“It could lead to cracking,” said Jewell.

She said most parts on the LEAP engine, including the LPT discs, are produced by multiple suppliers. The forging problem arose at just one of the disc suppliers, which Jewell declined to name.

No in-flight problems

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.

In the LEAP-1B engine used to power the MAX, the LPT system is a series of five rotating discs with blades around their circumferences located at the rear of the engine. Hot, high-energy air passing out of the engine’s combustion chamber spins these discs, turning a central shaft that drives the big fan at the front of the engine.

The problem identified with the LEAP was detected only during manufacturing.

“At no time have we experienced an issue associated with the LPT during our ongoing MAX testing program,” Alder said.

Jewell said that in addition to the more than 2,000 hours of flight tests Boeing has conducted on the LEAP engines without finding a problem, CFM did some 300 hours of its own flight testing and before that thousands of hours of ground testing, all without any issue in the LPT disc.

“We have found no problems in the field,” Jewell said.

CFM now has inspectors on site at the supplier of the affected discs, to make sure no more are produced with potentially defective forgings.

Jewell declined to specify exactly how many engines already delivered to Boeing are affected and need to be inspected, but said it’s “a very small number.”

However, because the engines must fly to either a GE facility in Indiana or a Safran facility in France for the inspections and disc replacement where necessary, “it’ll take some time” to get the all clear, she said.

The first MAX delivery, to Malaysian carrier Malindo Air, a subsidiary of top Boeing customer Lion Air, was supposed to happen next week. Boeing said that delivery will still go ahead this month.

Jewell said a second delivery to another airline will also go ahead in May. Norwegian Air is the second customer in line.

That implies Boeing has at least four unaffected engines ready to fly.

Alder declined to comment on whether subsequent deliveries to airlines may be delayed.

Clearly, Boeing hopes the engine issue will prove temporary. It plans to maintain its production rate in Renton in the expectation that CFM can catch up with a supply of good engines.

“We will work closely with CFM to understand the precise scope and root cause of the quality issue,” Alder said. “MAX production will continue, as will production and delivery of our (current model) 737 airplanes.”

A jolt to the stock price

The MAX is an upgrade of the 737 jet that comes in three sizes and has won more than 3,700 orders.

The new LEAP engines are its most critical feature, providing a 13 percent boost in fuel efficiency compared with the current 737 model.

With production of 777 widebody jets in Everett set to fall this summer, a quick ramp-up of MAX production in Renton is essential to Boeing’s cash-flow plans.

The rival Airbus A320neo jet has been plagued with problems on its Pratt & Whitney geared turbofan engine that have severely delayed some deliveries of that jet.

But until now, Boeing seemed to have escaped engine issues.

CFM also supplies a different, bigger version of the LEAP, the LEAP-1A, as an alternative engine for Airbus’ A320neo jet, and that model is now flying with airlines.

Jewell said the LEAP-1A doesn’t have the problem afflicting Boeing’s -1B engine.

“There is no impact to the A320neo fleet in service,” she said. Likewise, the LEAP-1C, which powers the Chinese-built C919 jet, is unaffected.

“The LEAP-1A/-1C has a different design,” said Jewell. “We use different suppliers.”

When news of the MAX engine problems broke shortly after 11 a.m. PST Wednesday, Boeing stock fell sharply, dropping by more than $5.

The price later recovered and shares closed at $183.10, down $2.39 or 1.3 percent for the day.