Boeing’s largest factory is in “panic mode”, according to workers and union officials, with managers accused of hounding staff to keep quiet over quality concerns.

The US plane maker has been grappling with a safety crisis sparked by a cabin panel blowout during a flight in January, and intense scrutiny of its production line as regulators launched a string of investigations.

Its site at Everett, Washington – hailed as the world’s biggest manufacturing building – is at the heart of Boeing’s operation, responsible for building planes like the 747 and 767, and fixing the 787 Dreamliner.

One mechanic at the complex, who has worked for Boeing for more than three decades, has claimed it is “full of” faulty 787 jets that need fixing.

Many of these jets are flown from Boeing’s site in South Carolina, where the company shifted final assembly of the 787 in 2021 in what was characterized as a cost-cutting measure.

“There is no way in God’s green earth I would want to be a pilot in South Carolina flying those from South Carolina to here,” the mechanic, who requested anonymity for fear of retaliation, told the Guardian. “Because when they get in here, we’re stripping them apart.”

Managers at Everett “will hound mechanics” to keep quiet about quality-assurance concerns and potential repairs, the mechanic alleged, emphasizing speed and efficiency over safety. He added: “Boeing has to look in the mirror and say: ‘We’re wrong.’”

Boeing did not comment on claims that staff have been pressured not to raise concerns about quality. Work at Everett on 787 jets is taking place as part of an established verification program, it said.

The company met this week with US regulators to discuss how it plans to address quality-control issues. Executives have recently described how workers have been emboldened to speak up since January, with submissions to an internal portal for safety and quality concerns up 500%.

Earlier this year, a panel of experts, which was appointed by the US Federal Aviation Administration (FAA) after two fatal Boeing 737 Max crashes in 2018 and 2019 that killed 346 people, described a “disconnect” between Boeing’s leadership and workforce on safety, and made 53 recommendations to help resolve its concerns.

After January’s blowout, which took place on board a brand-new Max 9 plane, the FAA launched a sweeping investigation. After a six-week audit of Boeing’s production line found multiple failures to comply with manufacturing quality-control requirements, the agency gave Boeing 90 days to outline an action plan and address the panel’s findings.

Boeing submitted its proposed plan to the FAA on Thursday. In a statement, the company said it “welcomed” the expert panel’s 53 recommendations. Mike Whitaker, the FAA administrator, told reporters that Boeing had accepted them all.

Najmedin Meshkati, who served on the panel, said Boeing’s safety culture has “eroded” over the past two decades, in the wake of its merger with McDonnell Douglas in the late 90s, “under the direct watch of its leaders and board of directors, who have been complicit in and ultimately responsible for its present problems”.

“Boeing’s most valuable asset is its highly dedicated and skilled workforce, and, fortunately, most of its systems-related problems are fixable,” Meshkati, professor of civil and environmental engineering at the University of Southern California, said in an interview before the company presented its plan this week. “However, Boeing’s corporate governance and its perpetually cloned [board] needs to be overhauled and diversified,” he added, suggesting that it should move its headquarters back to Seattle and “systematically” implement all the panel’s recommendations.

The FAA launched another investigation this month, into production of the 787 Dreamliner, scrutinizing whether Boeing workers in South Carolina completed required inspections on the jet, and whether they “falsified aircraft records”, after Boeing informed the watchdog that it “may not have completed” all checks.

Sam Salehpour, an engineer at Boeing, testified in front of Congress in April that the 787 was riddled with quality defects and called for all 787s to be grounded for inspection. Boeing denied his allegations and said it was “fully confident in the safety and durability” of the plane.

The mechanic who spoke to the Guardian described how “giant failures” in Boeing’s 787 production line had put enormous pressure on the company as it scrambles to reassure regulators, airlines and passengers.

“Right now, we’re in panic mode in Everett,” they said, because Boeing managers “finally figured out that they got more people that have no idea what’s going on, than people that do”.

A spokesperson for Boeing said: “As we shared publicly on numerous occasions, we will put all undelivered 787s through our join verification program to ensure every airplane conforms to our exacting engineering specifications prior to delivery to our customers. Airplanes assembled since 2022 do not require this additional join verification work.

“In mid-2023, we announced that we would relocate all join verification efforts in Everett so that we can focus our South Carolina team on new airplane production positions. There has been no change to this plan and our team in Everett has continued to complete the join verification program.”

Some workers at Everett have characterized the shift of 787 production from Washington to South Carolina as an anti-union move: whereas Washington has a union for mechanics, Boeing’s plant in South Carolina is non-union, and was the site of a contentious union-organizing drive several years ago.

Another Boeing employee at Everett, who also requested anonymity, drew a contrast between productivity at the Washington state complex and the North Charleston factory in South Carolina, where final assembly of the 787 was moved but where no union mechanics were hired.

“We would build 10 to 12 planes a month, while South Carolina would build just over two, almost three a month,” they said. “I don’t see them ever getting to 10 a month.”

A “very robust union-containment strategy” at Boeing is partly responsible for the company’s broader safety problems, said Rich Plunkett, director of strategic development for the Society of Professional Engineering Employees in Aerospace (Speea), IFPTE Local 2001, which represents 17,000 employees at Boeing and Boeing supplier Spirit AeroSystems.

In addition to not hiring union mechanics in South Carolina, Boeing has significantly reduced the number of workers who write instructions for machinists in recent years, through outsourcing, voluntary resignations and layoffs.

“The existing employees are not the enemy,” said Plunkett. “They’re your solution. And you need to engage them collectively.

“Boeing has to implement meaningful non-retaliation policies, demonstrate by walking the talk, and quit threatening their workforce [that] if they can’t do everything they can cheaper that they’re going to send the work abroad. Because Boeing’s globalization is continuing, if not expanding, in the midst of Boeing telling the free world they’re changing the culture.”

Some veteran union employees at Boeing draw a link between its current issues and a move by the company more than two decades ago to introduce “team leader” managers, replacing a previous system whereby the most senior, experienced factory workers were in charge.

“The team leader isn’t picked by his skill on the airplane – he is picked by his relationship with another manager or another person,” said the mechanic. “Now we don’t have team leads who know what’s up.”

Today, the managers “doing the thinking have never done physical labor before”, they added. “There’s no way you can learn how to build an airplane in a school on the third floor. There are a lot of things you can learn in the classroom, but building an airplane is not one of them.”

Boeing said: “We value experience as well as other factors such as skill set, performance and leadership that go into making a strong team lead. Our team leads help develop employees while the overall training we provide builds the knowledge and skills of our teammates as they advance in their Boeing career.”

Recently, disagreements between Boeing and unions have stalled talks over the creation of a program for employees to raise safety concerns to regulators without fear of retaliation.

Speea claimed last month Boeing wanted to control the flow of information to regulators; Boeing said it had offered the union the same deal it had signed with another union, the International Association of Machinists and Aerospace Workers, and was willing to negotiate.

Boeing did not comment on the company’s position toward unions, criticism tied to the 1997 merger with McDonnell Douglas, or complaints about quality-assurance issues.