Fresh problems for Boeing and its troubled 737 MAX aircraft.
An investigation has found debris that could pose safety risks in the fuel tanks of several planes waiting to be delivered to airlines, according to an internal memo seen by Reuters on Tuesday (February 18)
So-called foreign object debris - rags, tools, metal shavings or anything else left behind by workers during the production process - has been an issue for various Boeing aircraft.
Mark Jenks, general manager of the 737 program, told employees in the memo that such debris was "absolutely unacceptable", and that the company was taking steps to address the issue in its production system.
A Boeing spokesman confirmed the memo's authenticity, but said the firm does not see the debris adding to delays in the jet's return to service.
The objects were found during maintenance work on some of the hundreds of 737 MAX jetliners Boeing has built but not delivered.
They've been grounded since last March following two crashes that killed a combined 346 people.
The U.S. Federal Aviation Administration did not immediately respond to a request for comment.