Fresh question marks have been raised over the troubled 737 Max after Boeing discovered debris left inside the wing fuel tanks of several undelivered aircraft during the assembly process.
Boeing has ordered inspections of all 737 Maxs, about 400 of which are stored at various locations awaiting delivery to airlines.
The manufacturer also recommended inspections for 385 of the aircraft that were delivered to airlines but have been grounded by almost a year following two crashes in Indonesia and Ethiopia which killed a total of 346 people.
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The head of Boeing’s 737 programme, Mark Jenks, told staff that the discovery was “absolutely unacceptable”.
The company said it discovered so-called ‘Foreign Object Debris’ left inside the wing fuel tanks of several undelivered 737 Maxs.
Foreign Object Debris is an industrial term for rags, tools, metal shavings and other materials left behind by workers during the assembly process.
A company spokesman told the BBC: “While conducting maintenance we discovered Foreign Object Debris (FOD) in undelivered 737 Max airplanes currently in storage.
“That finding led to a robust internal investigation and immediate corrective actions in our production system.”
The US Federal Aviation Administration (FAA), told the BBC that it was monitoring Boeing’s response to the new issue.
“The FAA is aware that Boeing is conducting a voluntary inspection of undelivered aircraft for Foreign Object Debris as part of the company’s ongoing efforts to ensure manufacturing quality,” the regulator said.
“The agency increased its surveillance based on initial inspection reports and will take further action based on the findings.”
Boeing said it did not expect the issue to cause any additional delays to the 737 Max’s return to service, which the company said could happen by mid-summer.
A Boeing spokesman said the company did not see the issue further delaying the Max return to service.