Airlines say they found loose parts in door panels during inspections of Boeing Max 9 jets

Federal investigators are learning more about how a door panel flew off an Alaska Airlines jetliner last week

Federal investigators say a door panel slid up before flying off an Alaska Airlines jetliner last week, and they are looking at whether four bolts that were supposed to help hold the panel in place might have been missing when the plane took off.

The comments Monday from the National Transportation Safety Board came shortly after Alaska and United Airlines reported separately that they found loose parts in the panels — or door plugs — of some other Boeing 737 Max 9 jets.

“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,” Chicago-based United said.

Alaska said that as it began examining its Max 9s, “Initial reports from our technicians indicate some loose hardware was visible on some aircraft.”

The findings of investigators and the airlines are ratcheting up pressure on Boeing to address concerns that have grown since the terrifying fuselage blowout Friday night. A plug covering a spot left for an emergency door tore off the plane as it flew 16,000 feet (4,800 meters) above Oregon.

Boeing has called an online meeting for all employees Tuesday to discuss safety.

The door plugs are inserted where emergency exit doors would be located on Max 9s with more than about 200 seats. Alaska and United have fewer seats in their Max 9s, so they replace heavy doors with the plugs.

The panels can be opened for maintenance work. The bolts prevent the mechanism from moving upward on rollers when the plane is in flight.

During Alaska Airlines flight 1282 on Friday night, roller guides at the top of one of the plugs broke, allowing the entire panel to swing upward and lose contact with 12 “stop pads” that keep the panel attached to the door frame on the plane, NTSB officials said at a news briefing in Portland.

NTSB Chair Jennifer Homendy said the safety board was investigating whether four bolts that help prevent the panel from sliding up on rollers were missing when the plane took off from Portland or whether they blew off “during the violent, explosive decompression event.”

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Koenig reported from Dallas and Krisher reported from Detroit. Associated Press reporter Terry Spencer in Fort Lauderdale, Florida, contributed to this report.