(Bloomberg) -- Boeing Co. is inspecting more than 400 stored 737 Max jets after discovering debris such as tools or rags in the fuel tanks of several newly built but undelivered aircraft, said a person familiar with the matter.

The checks aren’t likely to delay Boeing’s earlier estimate that the Max will be cleared to fly by midyear, said the person, who asked not to be identified as the matter is confidential. The company has been working to re-certify the 737 Max, which was grounded worldwide in March after two fatal accidents killed 346 people.

The debris problem is a new embarrassment for a company trying to reassure airlines, crew members and passengers of its commitment to manufacturing quality and safety. Boeing has struggled with similar quality lapses in other aircraft, including KC-46 aerial refuelers supplied to the U.S. Air Force.

Foreign-object debris “is absolutely unacceptable. One escape is one too many,” Mark Jenks, Boeing’s general manager of the 737 program, said in a message to employees that was viewed by Bloomberg.

The Chicago-based planemaker has held meetings with employees to share a new process for stopping foreign debris from winding up in Max frames, Jenks said.

The Federal Aviation Administration didn’t immediately respond to a request for comment.

An inspection of each stored plane takes about three days to be completed, according to Leeham News, which first reported that so-called foreign object debris had been discovered in some parked Max jets. The aircraft fuel tanks must be drained and fumes dissipated before work can begin.

To contact the reporter on this story: Julie Johnsson in Chicago at jjohnsson@bloomberg.net

To contact the editors responsible for this story: Brendan Case at bcase4@bloomberg.net, Susan Warren

©2020 Bloomberg L.P.