Federal investigators said late on Monday that the bolts that were supposed to keep a fuselage panel in place might have been never installed before the panel blew off an Alaska Airlines Boeing 737 Max 9 in a near-disastrous accident on Friday night.

That is one of the theories that the National Transportation Safety Board is pursuing as it investigates the blowout, the board’s chair, Jennifer Homendy, said at a news conference in Portland, Oregon. Her remarks came hours after United Airlines said it had found loose bolts on similar panels on some of its Max 9 jets while preparing them for inspection after the midair emergency, and Alaska Airlines said it had also found “loose hardware” on Max 9s.

The panel that came off the plane, called a door plug, is placed where an emergency exit door would be if a jet had more seats. Homendy said on Monday that four bolts, known as stop bolts, should have prevented the door plug from moving upward and coming off the plane.

But the bolts were not on the door plug when investigators recovered it, and they are trying to determine whether they were there to begin with. “We don’t know if they were there or if, again, they came out during the violent explosive decompression event,” Homendy said.

The door plug came off the plane, Alaska Airlines Flight 1282, about 10 minutes after it took off from Portland International Airport, subjecting passengers to howling wind and forcing the pilots to quickly return to the airport. No serious injuries were reported. The door plug, phones, toys and other personal items all streamed out of the hole in the side of the plane and fell across Portland.

Airlines have cancelled hundreds of flights as they prepare to inspect nearly 200 Max 9 aircraft, which will be grounded until regulators and company officials decide they are safe. Some passengers’ travel plans could be disrupted for days. Alaska Airlines used 65 of the planes, about 20 per cent of its fleet, and United used 79, more than any other airline and about 8 per cent of its fleet, according to Cirium, an aviation data provider.

United Airlines said on Monday that it had found loose bolts in door plugs in some of its Max 9 planes as it took out seats and sidewall liners for inspections this past weekend.

New York Times News Service