United Airlines on Monday said preliminary inspections of grounded Boeing 737 Max 9 planes have turned up loose bolts and other issues with the part of the aircraft that failed on an Alaska Airlines flight over Portland, Oregan, US, last week, raising concerns of a systemic problem with the jetliner.

In some cases, the bolts needed additional tightening, the carrier said. The inspections of more than 100 Alaska and United planes manufactured by Boeing were ordered by the Federal Aviation Administration (FAA) after a door plug blew out of the Alaska jet midflight Friday, causing injuries and chaos.

The finding could compound the woes at Boeing, which has struggled to rebuild its reputation since an earlier model of the Max was grounded after two crashes killed 346 people several years ago. Investigations revealed problems with the design of an automated system on that plane, which had not been fully disclosed to the FAA.

Friday’s incident did not cause any fatalities, even though a piece of the plane broke off midflight, leaving passengers exposed to the open air. The National Transportation Safety Board continues to investigate the cause, gathering evidence that scattered across the Portland area, including the door-like plug, which landed in a schoolteacher’s backyard.

Science teacher Bob Sauer found the missing Alaska Airlines door plug that was nestled in dense shrubbery in his backyard in Portland, Ore., on January 7.

NTSB officials said at a news conference Monday evening that guide tracks on the Alaska Airlines door plug were fractured, and that the agency is investigating the absence of four bolts designed to keep the plug from moving upward in a motion that would normally help disengage it for maintenance and repairs.

It is unclear whether the four bolts were absent at the start of the flight Friday, or if they went missing after the accident, NTSB chair Jennifer Homendy said. The door plug on the right side of the plane, which remained intact, had no discrepancies or missing parts, she added.

Homendy also said the cockpit door, which blew open during the flight, was designed to do so during rapid decompression — a detail that flight crew had not been made aware of. Boeing is expected to update its manual to make that clear, Homendy said.

Boeing said in a statement: “As operators conduct the required inspections, we are staying in close contact with them and will help address any and all findings … We are committed to ensuring every Boeing airplane meets design specifications and the highest safety and quality standards. We regret the impact this has had on our customers and their passengers.”

United’s discovery of the loose bolts occurred as part of preliminary inspections that have been ongoing since Saturday. The discovery was first reported by the Air Current, an aviation news site. On Monday, the FAA said airlines can begin inspections in earnest using guidance from Boeing and the agency.

One hundred seventy-one Boeing 737 Max 9 planes have been grounded amid the investigation into the rapid depressurisation accident Friday, which triggered an emergency landing and resulted in extensive damage to an Alaska Airlines plane.

Video from passengers showed part of the plane’s wall missing from the aircraft midair.

There were no serious injuries in the accident, but the dramatic midair blowout caused the FAA to order immediate inspections of the jetliners. Guidance issued Monday by Boeing and approved by the FAA allows the inspections to begin. The FAA previously said the inspections could take four to eight hours per plane, but on Monday said that was no longer accurate, without offering a revised estimate.

“The FAA’s priority is always keeping Americans safe,” the agency said in a statement, adding that all Max 9 aircraft would remain grounded until operators have completed the enhanced inspections.

Two airlines in the US — Alaska and United — have Boeing Max 9 aircraft in their fleets. Alaska Airlines did not respond to requests for comment Monday.

Operators must also complete corrective-action requirements based on findings from the inspections before bringing any aircraft back into service, the FAA said.

“The safety of our airplanes and everyone who steps onboard is a core Boeing value,” Stan Deal, commercial airplanes president and chief executive, and Mike Delaney, chief aerospace safety officer and senior vice president of global aerospace safety, wrote in a message to employees Monday. “We agree with and fully support the FAA’s decision to require immediate inspections of 737-9 MAX airplanes with the same configuration as the affected airplane.”

Jeff Guzzetti, a former accident investigator with the FAA and NTSB, said the door plug was a key piece of evidence that will allow investigators to confirm whatever failure occurred and how it occurred.

“I still believe that because it was such a new airplane and this is such a unique type of event that it’s manufacturer-related,” he said.

Anthony Brickhouse, an associate professor who teaches aviation safety and investigation courses at Embry-Riddle Aeronautical University, said the door plug would offer important clues about what might have caused the failure.

“When it comes to structures, metals tell the story,” he said. “Whenever a metal fails, it leaves a signature. It tells a story — are there any dings, dents or deformation? There will be telltale signs that the NTSB is already looking at.”

On Monday evening, Homendy of the NTSB said that there were “no indications whatsoever” that three previous activations of the auto-pressurisation fail light on the Alaska Airlines aircraft were linked to the door plug accident.

The light, designed to signal failures in the control of cabin pressure, had illuminated on three flights in the weeks leading up to the accident, prompting tests and a reset by maintenance. A later request from Alaska Airlines for a deeper look had gone unfulfilled before the accident, she said.

Flight 1282 was on its way to Ontario, Calif., from Portland when it had to return to the airport shortly after takeoff because the door plug — an exit that is panelled off, usually because it is deemed optional in safety regulations — had separated from the plane midair.

The blowout caused a loud banging sound and allowed frigid, whipping winds into the aircraft.

An United Airlines plane taxis at Los Angeles International Airport (LAX) in Los Angeles, California on January 4, 2024. Photo: AFP

The cockpit door immediately flew open, banging into a lavatory door and jolting the first officer forward, causing her to temporarily lose her headset, Homendy said, citing interviews with flight attendants. The captain and the first officer were able to put on oxygen masks and turn on a speaker, but “communication was a serious issue,” she said.

Flight attendants described trouble getting information from the flight deck, and passengers in the cabin struggled to hear announcements. “It was very violent,” Homendy said.

“This was a really significant event with zero information at the time,” she added at the news conference Monday. “There’s a lot of trauma that they are working through.”

Investigators who examined the grounded plane after the accident found that the headrests of two seats directly adjacent to the door plug were missing, as well as the back of one seat.

One flight attendant suffered minor injuries, according to the union that represents Alaska crews, The Washington Post reported. Several passengers required medical attention for injuries, the airline said. The flight was carrying 171 passengers and six crew members.

Pieces of trim, paneling and insulation were ripped from the interior of the plane, Homendy said, and damage was visible in at least 12 rows, including the interior side of some windows. However, these parts are “not critical to the structure of the aircraft,” she said, adding that the tubing of several oxygen masks had been “sheared off.”

“My impression, when I saw that, was it must have been a terrifying event to experience,” she said.

NTSB investigators were unable to uncover communications from the cockpit voice recorder, which overwrites itself every two hours and was not recovered before the recording had been automatically erased. Homendy called on the FAA to implement a rule that would require the automatic overwrite time to be increased to 25 hours, a standard she said the NTSB has called for and is “consistent with Europe and many other countries.”

“That information is key not just for our investigation but for improving aviation safety,” she said.

Members of the Portland community had launched drones to help search for the plug, and two cellphones were turned in after they were discovered in a yard and on the side of a road. A plastic window frame and headrest were also turned in.

The door plug and other components will be sent to an NTSB lab in Washington, DC, for further examination. NTSB investigations can take “anywhere from a year to 18 months,” NTSB aerospace engineer Clint Crookshanks said at the news conference Monday.

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