Airlines have privately complained to Boeing about poor quality control in recent deliveries of 787-10 Dreamliner airplanes.

The complaints center around Boeing's North Charleston, South Carolina, factory, The Post and Courier reported. Boeing also assembles the jetliner at its Everett, Washington, facility.

In internal surveys, the airlines described issues like production delays, dented panels, scratches on cockpit windows, loose nuts and bolts, and unsecured fuel-line clamps. Publicly, the airlines all stand by the plane, which also receives good feedback from passengers because of its high-tech, comfort-focused features.

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Airlines flying Boeing's 787-10 Dreamliner have complained to the plane maker about "unacceptable" production mistakes and inconsistent quality.

The problems center around Dreamliners built at Boeing's North Charleston, South Carolina, factory, according to a report from The Post and Courier.

Issues at the North Charleston plant were reported in April in a comprehensive New York Times investigation, which found evidence of shoddy production, poor oversight, and a culture that "made speed a priority over safety." The report came a month after Boeing's 737 Max jet was grounded worldwide after the second fatal crash in five months. The Department of Justice expanded an inquiry into the 737 Max to include issues at the North Charleston factory in June.

The new report surfaced complaints from a global cadre of airlines that fly the jet and have received orders from the South Carolina plant, one of two locations where the Dreamliner is assembled — other orders are built at Boeing's Everett, Washington, factory.

While the issues are not limited to either the South Carolina plant or the 787 — similar problems have been raised in Everett with both 787s and military tankers — the complaints surfaced by The Post and Courier focus on recent deliveries of Boeing's newest and largest variant of the Dreamliner, the 787-10. It was not immediately clear whether the airlines made similar complaints about other variants of the plane, including the 787-8 and 787-9.

Read more: Photos show how Boeing's grounded 737 Max planes are piling up at the company's Seattle plant

Although the 787 had a troubled start, with a grounding ordered because of battery fires in 2013 and some customers facing issues with Rolls-Royce Trent 1000 engines used on some of the planes — customers have the option of choosing between the Rolls-Royce engine or one made by General Electric Co. — the Dreamliner has generally been favored by passengers and airlines for its comfort-focused features and fuel efficiency.

The complaints were communicated as part of internal surveys Boeing asks airlines to complete after a plane is delivered. Survey results are inherently unscientific because not all airlines participate, and The Post and Courier said supplier issues outside Boeing's control could have skewed the results.

In a statement provided to Business Insider, Boeing said airlines "continue to express great confidence in the 787."

"We are committed to transparency with our customers and regulators throughout the assembly and delivery process to ensure world-class safety and quality for all of our airplanes," the company said. "One example of that transparency is the real-time feedback our customers provide through a variety of channels."

However, the specific issues described by numerous airlines were consistent with past whistleblower complains and news reports.

KLM Royal Dutch Airlines described the factory's quality control as "way below acceptable standards" when talking about a new 787-10 delivered in spring. Among several issues noted were loose seats, missing and incorrectly installed pins, nuts and bolts not fully tightened, and a fuel-line clamp left unsecured.

Notably, while the airline privately expressed concerns related to the plane — "who looks at quality in this facility?" the airline asked — it publicly described the plane, which was painted specially for the airline's 100th anniversary, with enthusiasm.

—Royal Dutch Airlines (@KLM) June 30, 2019

"We're very happy to have this beauty on board," KLM tweeted.

The airline also confirmed to Business Insider that it was happy with the Dreamliner — and downplayed that the survey answers reflected any loss of confidence in the Boeing.

"We appreciate our long-standing partnership with the Boeing team and we regularly exchange feedback with them," KLM said. "This practice continually improves the airplanes for our passengers and we see that in the performance of our 787 fleet, including the new 787-10 that is now in service."

United Airlines gave the plant above-average marks in its survey results but still noted 20 issues when it inspected a 787-10 that was delivered in April, including two dented panels.

"The safety of our customers and employees is our top priority," a representative for United told Business Insider. "Our teams conduct thorough inspections and reviews of every single new aircraft before it enters service, and we have full confidence in our 787 fleet."

American Airlines complained about a program that allowed Boeing's mechanics to inspect most of their own work, speeding up production by eliminating a second reviewer. The airline said that problems not being caught ended up slowing down the production process, delaying test flights.

A spokesman for American, Ross Feinstein, told Buiness Insider that the airline had full confidence in its fleet of Dreamliners. American has 42 of the planes, with 47 more on order.

"American monitors the manufacturing process of our aircraft on-site, and inspects each aircraft from start to finish," he said. "Prior to taking delivery of any aircraft, our team conducts additional inspections. Lastly, once we take delivery of an aircraft, it is flown to one of our maintenance bases for additional inspections, prior to any aircraft entering commercial service."

Read more: I flew first-class in Delta's 6-month-old A220, the plane Boeing tried to keep out of the US

Airlines inspect and run test flights on newly delivered planes before they begin flying passengers, meaning any production or safety issues should be caught before the plane ever enters service.

"Boeing and our customers demonstrate the highest standards of safety and quality, which is evident by the 787 Dreamliner's excellent record of safety and reliability in-service," Boeing told Business Insider.

However, the process of fixing these issues can take time, and with Boeing already struggling to meet an ambitious production schedule, that can cause logistical problems for both the plane maker and the airline.

When Singapore Airlines received a 787-10 in April, it noticed several missing pieces and a scratched cockpit window, saying that the delivery process was problematic compared with past deliveries of other variants of the jet.

"It doesn't help that resources were stretched, resulting in slow rectification of these issues," the airline said in the survey, according to The Post and Courier.

In a statement provided to Business Insider, Singapore Airlines said it was fully satisfied with the jet.

"We have not experienced significant quality issues with the 787-10 aircraft that we have taken delivery of from Boeing," the airline said.

KLM also said that the Boeing staff seemed to be stretched thin, potentially affecting quality-control standards.

"A lot of Boeing personnel, factory and management, works way too much overtime," the airline said. "In this customer's opinion this reflects in quality and the inability to make schedule."

Etihad, a United Emirates airline based in Abu Dhabi, said that aside from the quality, Boeing was not communicating the delays, which could cause issues when the airline is building its future schedules.

"Consequently, we had to find out the true status of the airline on our own and the hard way," Etihad said. It described the delivery as "very bad and not good for Boeing and Etihad."

Read more: Boeing says it could suspend 737 Max production if grounding continues, putting tens of thousands of jobs at risk

The problems come as Boeing faces intense scrutiny over the 737 Max planes, which are built in Boeing's Renton, Washington, factory. Two crashes involving the 737 Max plane in October and March killed a total of 346 people and forced the plane to be grounded worldwide.

Boeing has faced criticism that it rushed development of the Max in order to catch up with its rival Airbus, which had unveiled an updated version of its successful A320 family of planes.

As the Max remains grounded, and Boeing faces mounting costs from delivery delays, the plane maker is more dependent on revenue from the 787 and its upcoming 777X to offset losses.