The move affects about 171 planes worldwide, according to a statement by the FAA. Alaska, the world’s second biggest operator of the type, already grounded its Max 9 fleet in the wake of Friday’s incident after takeoff from Portland, Oregon. United Airlines Holdings Inc., the model’s top operator, also took some of the jets out of service for inspections.

“Safety will continue to drive our decision-making as we assist the NTSB’s investigation into Alaska Airlines Flight 1282,” FAA Administrator Mike Whitaker said in a post on X.

The forced grounding marks the most severe response to an incident since the manufacturer’s entire fleet of Max aircraft was temporarily taken out of service in 2019 following two deadly crashes. The 737 Max is by far Boeing’s most popular aircraft and its biggest source of revenue, with single-aisle aircraft like the Max and the corresponding Airbus SE A320neo family used the most widely flown shorter routes.

Only two US airlines operate the 737 Max 9 variant: United, with 78, and Alaska with 65, based on data from FlightRadar24. Alaska Airlines said in an update that it had completed inspection on “more than a quarter” of its 737-9 fleet, without making any concerning findings. The carrier will begin returning the jets to service once the “inspections are completed with our full confidence,” it added.

Flight 1282 was carrying 171 passengers and six crew from Portland, Oregon, to Ontario, California on Jan. 5 when about 20 minutes into the journey, the crew reported a pressurization issue. What followed was a rear left part of the fuselage blowing out, leaving the hole resembling the opening for a door, all at a flight altitude of about 16,000 feet (4,800 meters).

Read More: After Boeing 737 Max planes crashed and killed hundreds of people about five years ago, one just lost a chunk of its fuselage in midair

Inside the aircraft, which was delivered to Alaska Airlines only in October, part of the cabin wall had also torn off, exposing insulation material, images on social media showed. Video footage showed the aircraft landing in Portland again in darkness, with passengers seated close to the gaping hole. Nobody was seriously injured.

“All of a sudden I heard a big bang,” Elizabeth Le, identified as a passenger on the flight, told KCAL News in an interview. “Then I look to my left and there’s this huge chunk, part of the airplane just missing and the wind is just extremely loud. There’s wind blowing everywhere, but everyone was in their seats.”

China’s aviation regulator is conducting an emergency meeting to consider a response to the incident, including a possible grounding of the Boeing Max fleet in the country, according to two people familiar with the situation, who asked not to be identified discussing private deliberations. The aircraft variant involved in the Alaska Air incident isn’t flown by Chinese carriers.

China was the first country to ground the 737 Max after the two crashes several years ago. Relations have only gradually improved, with China taking the first delivery of a larger 787 model in several years in December. It has yet to resume 737 deliveries.

The National Transportation Safety Board is also investigating the matter. Boeing said it’s gathering more information and is in contact with the airline, and a technical team is ready to support the probe. The European Union Aviation Safety Agency said it’s checking if it will need to mandate anything.

The 737 Max has modular fuselage layouts, allowing for emergency doors to be installed more variably depending on the number of seats. This gives operators greater flexibility with the cabin configuration.

On the 737-9 Max, Boeing includes a cabin exit door aft of the wings, but before the rear exit door. This is activated in dense seating configurations to meet evacuation requirements. The doors are not activated on Alaska Airlines aircraft and are permanently “plugged.”

Alaska Airlines had scheduled more than 5,000 flights with the Boeing 737 Max 9 model in January, according to aviation data provider Cirium. There are 215 Max 9 aircraft in service globally, with 76 on order, including 25 by Alaska Airlines, Cirium said.

The grounding, while voluntary, is a major setback for Boeing, which has grappled with manufacturing defects and costly repairs in recent years. Boeing has been forced to fix misaligned drilling holes in the rear section of the 737, and most recently the FAA said it’s monitoring targeted inspections of Boeing 737 MAX airplanes to look for a possible loose bolt in the rudder control system.

The Alaska Airlines aircraft experienced pressurization issues twice on Jan. 4, the Air Current reported, citing two people familiar with the matter. A warning light had prompted Alaska Air to remove the jet from extended-range operations, or ETOPs, the outlet said.

The temporary grounding, which will impact tens of thousands of customers with canceled flights, involves almost 30% of the Alaska Air’s 227 Boeing 737 family aircraft. Alaska Air is the second-biggest operator of the 737 Max 9 variant, behind United Airlines Holdings Inc.

Other airlines that operate the variant include Copa Airlines SA, with 29 units, and Aeromexico with 19. FlyDubai, which has three Boeing Max 9 aircraft, said it’s aware of the reports and said its planes have a different cabin configuration than the Alaska model.

Inspections are expected to be completed in the next few days, Minicucci said.

The jet didn’t appear to have suffered the type of powerful decompression that occurred on a Southwest Airlines Co. plane in 2018 when part of an exploding engine shattered a window of the Boeing 737-700, partly sucking a woman seated next to it from the plane and killing her. Video from the Alaska Air craft showed passengers seated near the gaping hole.

“While this type of occurrence is rare, our flight crew was trained and prepared to safely manage the situation,” the carrier said. Alaska Air operates an all-Boeing fleet.

— With assistance from Isabel Reynolds, Yi Wei Wong, Leonard Kehnscherper, Leen Al-Rashdan, and Danny Lee