An emergency landing Friday of an Alaska Airlines Boeing 737 Max 9 plane in Portland, Oregon, led the Federal Aviation Administration to order some U.S. airlines to stop flying some Max 9 planes until they were inspected. The order affects about 171 planes owned by Alaska, United and other airlines. The episode also raised troubling new questions about the safety of a cargo plane design beset by years of problems and multiple fatal accidents.

No one was seriously injured in Friday’s incident. The plane returned to the Portland airport shortly after a piece of the plane’s body broke off in mid-air, leaving a door-sized hole in the side of the plane.

A few hours after the episode, Alaska Airlines saying would ground all 65 Boeing 737 Max 9 planes in its fleet until mechanics could carefully inspect each plane. Later on Saturday, the FAA ordered the temporary grounding of planes in other airlines’ fleets. United Airlines, with 79 Max 9s, had the largest number of affected aircraft.

On Saturday night, Jennifer Homendy, chairwoman of the National Transportation Safety Board, the body charged with investigating air crashes, said a plug in one of the plane’s unnecessary emergency doors broke 10 minutes from the airport. while the plane was at an altitude of about 16,000 feet.

Homendy said investigators would compare the plug from a second emergency door, at the opposite end of the hallway, to the one that had blown out, in hopes of determining what went wrong. She added that investigators would also examine things like the plane’s pressurization system and maintenance records.

And while the particular problem that caused Friday’s scare seemed unique, Boeing’s 737 Max planes have perhaps the most troubling history of any modern aircraft currently in service.

As of Sunday night, Boeing and the FAA were still working on drafting a message to airlines with instructions on how to inspect the planes, according to a person familiar with the process.

Alaska Airlines Flight 1282, carrying 171 passengers and six crew members bound for Ontario, California, made an emergency landing at Portland Airport on Friday night, 20 minutes after takeoff.

Passengers on the flight reported hearing a loud sound before realizing that a section of the fuselage had opened in mid-air.

In the minutes before the emergency landing, with oxygen masks hanging from the ceiling and the wind howling through the gaping hole in the wall, passengers could not hear the urgent announcements made over the public address system.

The plane involved in Friday’s incident was virtually new by commercial airline standards. It was first recorded in November and only recorded 145 flights.

Two crashes involving Boeing 737 Max 8 aircraft killed a total of 346 people in less than five months in 2018 and 2019. Both crashes were later associated with a system malfunction that overrode the pilot’s commands.

Those crashes led to the global grounding of Boeing 737 Max aircraft, parking hundreds of planes on runways around the world for nearly two years as engineers worked to identify and resolve the problem so regulators could recertify the planes.

The first accident took place in October 2018, when a passenger plane carrying 189 people from Jakarta, Indonesia, crashed into the Java Sea just minutes after takeoff. Four months later, another 737 Max, flown by Ethiopian Airlines, crashed just after takeoff on its way to Nairobi, Kenya, killing all 157 people on board, including the flight’s eight crew members.

Days later, President Donald J. Trump announced that U.S. regulators would temporarily suspend all flights of the Boeing 737 Max, as investigators — and Boeing — sought to determine how a software system that was supposed to make the plane safer played a role. role in disasters. .

U.S. regulators were among the last to quash the model, but they did so after pressure mounted and as 42 other countries took drastic measures to prevent more accidents.

Reports by The New York Times and others eventually revealed that competitive pressure, flawed design and problematic oversight had played a role in the troubling story of the plane, Boeing’s best-selling plane in history, and one with hundreds of thousands of millions of dollars in pre-orders from airlines. all over the world when he was punished.

Boeing agreed to pay $2.5 billion in a settlement with the Justice Department in 2021 to resolve a criminal charge of conspiracy to defraud the Federal Aviation Administration, which regulates the company and evaluates its planes.

In 2022, Boeing paid $200 million more in a settlement with U.S. securities regulators over allegations that the company had misled investors by suggesting human error was to blame for the two deadly crashes and ignoring public concerns. company on the plane.

When the planes were recertified, 20 months after the crashes in Indonesia and Ethiopia, Boeing estimated that the crisis had cost the company $20.7 billion.

As part of Boeing’s single-aisle 737 Max series, the Max 9 can carry up to 220 passengers, depending on its seating configuration. United Airlines has 79 Max 9s in service, the most of any airline, according to Cirium, an aviation analytics company. In total, there are 215 Max 9 aircraft in service worldwide, Cirium said. United and Alaska Airlines have about two-thirds of them.

Other companies flying the Max 9 include Panama’s Copa Airlines and Aeroméxico in America, Kazakhstan’s SCAT Airlines, Iceland Air, Turkish Airlines and FlyDubai.

A FlyDubai spokesperson said the three 737 Max 9 aircraft in its fleet completed its necessary safety checks over the past 24 months and that the company was awaiting guidance from Boeing before conducting further inspections.

Major aviation safety incidents, including those that do not cause injuries or loss of life, typically trigger immediate reviews by regulators in the United States, the European Union and China.

Investigators analyze everything: the design of the plane; its manufacturing, maintenance and inspection history; climate; air traffic control decisions; and flight crew actions. They look for causes of an incident and lessons for aviation safety.

In the case of the Alaska Airlines incident, the plane was manufactured in the United States and lost a section of the fuselage while flying in the United States. Therefore, the National Transportation Safety Board will be the primary agency responsible for investigating the incident.

Security investigations can take many months. They involve technical experts from the government, the airline that operated the plane, unions, and the plane manufacturer (in this case, Boeing).

The FAA does not need to wait for the safety board’s report before deciding whether to ground a model airplane or order immediate inspections. Airlines typically rush to check their planes as soon as they know what to look for.

Grounding one of the industry’s main workhorses could put pressure on travelers, as airlines sometimes have to cancel flights because they lack the plane to replace the grounded model.

On Sunday afternoon, United said it had canceled about 270 flights aboard Max 9 planes scheduled for Saturday and Sunday, although some of those cancellations may also have been due to bad winter weather in the Northeast.

Alaska Airlines said it had canceled 170 flights as of Sunday night, affecting about 25,000 customers. Foreign airlines such as Turkish Airlines and Copa Airlines in Panama also parked their Max 9 planes.

Alaska has issued a “system for everything flexible travel policy”allowing passengers to cancel or change their flights without incurring any charges. The airline encourages travelers to use Alaska’s website or app themselves, rather than calling the airline’s customer service line.

United flight status updates can be found online. If a flight is significantly delayed, United waive exchange fees or provide a travel credit or refund. The airline has not issued a specific waiver related to aircraft inspections that would further relax policies.

And if a flight is delayed or cancelled, the traveler may be entitled to compensation, depending on the circumstances.

Boeing CEO Dave Calhoun said in a message to employees that Boeing would hold a livestreamed companywide meeting Tuesday to discuss its response to the crash. He reiterated the company’s commitment to “safety, quality, integrity and transparency.”