A harrowing flight over the weekend is again forcing Boeing to confront concerns over its planes, particularly the 737 Max, already one of the most scrutinized jets in history.

No one was seriously injured in the episode on an Alaska Airlines flight Friday night in which a portion of a 737 Max 9 fuselage blew out in midair, exposing passengers to howling wind. The plane landed safely, but the event, on a flight from Portland, Ore., to Ontario, Calif., has spooked travelers and prompted immediate safety inspections on similar planes.

Federal authorities focused attention on a mid-cabin door plug, which is used to fill the space where an emergency exit would be placed if the plane were configured with more seats.

The Federal Aviation Administration said on Saturday that it had ordered the inspection of 171 Max 9 planes operated by U.S. airlines or in U.S. territory, causing hundreds of flight cancellations over the weekend. It said the inspections should take four to eight hours per plane to complete, though at least one airline said it was still waiting for more detail on what those inspections should entail.

“We agree with and fully support the F.A.A.’s decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane,” Jessica Kowal, a Boeing spokeswoman, said Saturday.

Alaska Airlines’ woes continued into Sunday, with 163 cancellations, or 21 percent of its scheduled flights, by early afternoon, according to FlightAware. The airline has 65 Max 9 planes. Travelers took to social media to complain about continued long hold times on the phone for customer service, and inadequate compensation as they waited at the airport and faced lengthy delays and abrupt cancellations.

United Airlines said that it had canceled about 180 flights planned for the Max 9 on Sunday, while preserving another 85 flights by swapping in other planes.

United has 79 Max 9 planes, more than any other carrier. In a statement on Sunday, the airline said that it had parked all of those planes and had started removing door panels and performing preliminary inspections on those jets while it awaited further F.A.A. instructions on what inspections will need to be carried out to get the planes flying again.

“We’re continuing to work with the F.A.A. to clarify the inspection process and requirements for returning all Max 9 aircraft to service,” the airline said in a statement. “We are working with customers to reaccommodate them on other flights and in some cases have been able to avoid cancellations by switching to other aircraft types.”

It is not clear whether Boeing is to blame for what happened to the Alaska Airlines plane, but the episode raises new questions for the manufacturer and puts additional pressure on it. Another version of the Max, a 737 Max 8, was involved in two crashes that killed hundreds of people in 2018 and 2019 and led to a worldwide grounding of that plane.

“The issue is what’s going on at Boeing,” said John Goglia, a longtime aviation safety consultant and a retired member of the National Transportation Safety Board, which investigates airplane crashes.

Last month, the company urged airlines to inspect the more than 1,300 delivered Max planes for a possible loose bolt in the rudder-control system. Over the summer, Boeing said a key supplier had improperly drilled holes in a component that helps to maintain cabin pressure. Since then, Boeing has invested in and worked more closely with that supplier, Spirit AeroSystems, to address production problems.

“We are seeing increased stability and quality performance within our own factories, but we’re working to get the supply chain caught up to the same standards,” Boeing’s chief executive, Dave Calhoun, said on a call with investor analysts and reporters in October.

Spirit AeroSystems also worked on the fuselage for the 737 Max 9, including manufacturing and installing the door plug that failed on the Alaska Airlines flight.

Deliveries of another Boeing plane, the twin-aisle 787 Dreamliner, were virtually stalled for more than a year, until the summer of 2022, while the aircraft maker worked with the F.A.A. to address various quality concerns, including paper-thin gaps in the plane’s body.

Another flaw discovered last summer slowed deliveries of the plane again. And production of both the 737 and 787 has been slow to speed up amid those and other problems with quality and the supply chain.

The Max was grounded in early 2019 after two crashes killed a total of 346 people in Indonesia and Ethiopia. Over 20 months, Boeing worked with regulators around the world to fix problems with the plane’s flight control software and other components.

By the time passenger flights aboard the Max resumed in late 2020, the crisis had cost the company about $20 billion.

The two midsize variants of the plane, the Max 8 and Max 9, have been flying since then. But the smallest, the Max 7, and the largest, the Max 10, have yet to be approved by regulators.

The Max is the best-selling plane in Boeing’s history. The more than 4,500 outstanding orders for the plane account for more than 76 percent of Boeing’s order book. The plane is also popular among airlines: Of the nearly three million flights scheduled globally this month, about 5 percent are planned to be carried out using a Max, mostly the Max 8, according to Cirium, an aviation data provider.

Investigators with the National Transportation Safety Board have started looking into the case and are expected to examine a wide range of factors, including Boeing’s manufacturing process and the F.A.A.’s oversight of the company and any work Boeing or Alaska Airlines carried out on the plane. Investigators have also identified an area where the door probably landed and asked for help from the public in finding it.

“This is the kind of thing where, until you really get into the investigation — you identify all the facts, conditions and circumstances of this particular event — do you determine whether this is just a one-off or a systemic problem,” said Greg Feith, an aviation security expert and former N.T.S.B. investigator.

In the meantime, those who make, service, operate and regulate the planes will all be in the spotlight.

“Every American deserves a full explanation from Boeing and the F.A.A. on what’s gone wrong and on the steps that are being taken to ensure another incident does not occur in the future,” Senator J.D. Vance, Republican of Ohio, said in a post on Saturday on X.

Mark Walker, Christine Chung and Safak Timur contributed reporting.

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