Boeing is scrutinizing the flight-deck windows of some of its 787 Dreamliners as the planemaker expands its search for potential manufacturing flaws that have delayed deliveries of its marquee jetliner, according to people briefed on the matter.

Boeing has been testing the cockpit windows in a limited batch of aircraft after learning that a supplier modified its production process, two of the people said, asking not to be identified because the matter is sensitive. Boeing wants to ensure that the windows still meet its requirements after the change, but the testing isn’t expected to affect March deliveries, one of the people said.

The emergence of yet another potential glitch comes as Boeing’s mechanics and engineers work furiously to try and restart 787 Dreamliner deliveries by the end of this month, in line with what executives promised during a January earnings call.

The U.S. manufacturer hasn’t handed over any of the jets since October after discovering more of the tiny dimples in the inner lining where the carbon-fiber fuselage barrels are fused to form the jet’s frame. New issues with the plane could pose problems for shipments beyond this month, making it more difficult for Boeing to meet its delivery target for the year.

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When asked about the window checks, a Boeing representative referred back to the company’s March 9 statement that said it is “progressing through inspections and rework as necessary on undelivered airplanes.”

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“Based on our current plans, we continue to expect to resume delivering 787s by the end of March; however, we will continue to take the time necessary and will adjust any delivery plans as needed,” the statement said.

A supplier revising its manufacturing methods typically wouldn’t make headlines. But with Boeing under scrutiny from regulators and customers after two fatal crashes involving its 737 MAX and a slew of production glitches, officials need to ensure the modified processes still meet all requirements. The testing of flight-deck windows in other batches of Dreamliners is still ongoing, one of the people said.

The production disruptions have forced Boeing to store more than 80 Dreamliners around its factories and in the California desert, creating a new worry just as the crisis surrounding its 737 MAX subsides. For some customers, the delays threaten to hamstring aircraft for which financing is already lined up, while for others, the expanding inspections provide an excuse to avoid taking expensive new jets at a time when international travel has been laid low by COVID-19.

Boeing has been working with its suppliers since late last year to find the source of manufacturing flaws with its 787 Dreamliner aircraft. Spirit AeroSystems, which makes the Dreamliner’s nose and cockpit, said last month that it’s conducting an engineering analysis of “noncomformities” on its portion of the carbon-fiber frame.

A key 787 buyer, Air Lease, has cautioned that production issues “seem to have mushroomed” for the twin-aisle jet. “There’s just greater and greater levels of inspections going on,” John Plueger, CEO of the Los Angeles-based aircraft lessor, said.