New York CNN Business  — 

Boeing has found a problem with the manufacturing of the 787 Dreamliner, the widebody jet that is one of the most successful products of the troubled aircraft maker.

Eight of the planes will be grounded for inspection and repair of how parts of the fuselage were joined together, Boeing said in a statement.

“Boeing has identified two distinct manufacturing issues in the join of certain 787 aftbody fuselage sections, which, in combination, result in a condition that does not meet our design standards,” said the company’s statement. It said it is conducting a thorough review into the root cause of the problem.

There are nearly 1,000 787s that have been delivered to 69 different airlines worldwide since it started operations in 2012.

Boeing would not identify the three airlines that own the planes that were grounded for inspections. The aviation news site The Air Current, which broke the news, reports that United, Air Canada (ACDVF) and Singapore Airlines own the eight planes.

Spokespeople for United and Singapore confirmed that each airline had one of the eight affected planes in its fleet. Both airlines said the affected planes were not in service when the problem was discovered. Most of the widebody jets worldwide have been grounded due to the Covid-19 pandemic. Air Canada did not immediately respond to a request for comment. It has 37 Dreamliners overall in its fleet.

Boeing is already struggling with problems with its best-selling plane, the single-aisle 737 Max, which has been grounded since March 2019 following two fatal crashes that killed 346 people. It is still awaiting approval for that plane to fly again.

The Federal Aviation Administration says it is aware of the problem with the 787 and “continues to engage with Boeing,” but aviation regulators did not say whether they plan to issue a safety bulletin to operators about the issue. Boeing says the FAA has been “fully briefed, and we will continue to work closely with them going forward.”

The 787 Dreamliner is a leading aircraft in the widebody portion of the market, in which Boeing dominates rival Airbus (EADSF). Made of composite material that weighs less than aluminum, the jet is extremely fuel-efficient. While it was briefly grounded in 2013 following some fires in its lithium battery, the plane has had few known problems since then.

But the plane is primarily used on longer international routes, and international air travel has been severely curtailed by the Covid-19 pandemic.

Boeing has announced it would slow production of the Dreamliner as a result of the pandemic, and said it is considering closing one of the two plants that build the plane to consolidate operations and cut costs. One plant, in Washington state, has union-represented workers, while the workers at the plant in South Carolina are not represented by a union.

A Boeing spokesman could not immediately say whether the manufacturing problems related to both plants or only one.

Shares of Boeing (BA) were little changed in morning trading Friday.