The flight, AS 1282, bound for Ontario, California, safely returned to Portland with 171 passengers and 6 crew members, the airline said.
The flight climbed to as high as 16,300 feet, according to data from FlightRadar24, seven minutes after taking off from Portland, before quickly descending to below 10,000 feet and looping around to return to the airport.
Photos circulating on social media and local news appeared to show a hole the size and shape of an emergency exit "plug" on the left-hand side of the aircraft aft of the wing. The 737 MAX 9 features a spot for an extra emergency exit, which is required on models of the aircraft with certain higher density seating configurations. Airlines that put fewer seats on the aircraft can choose to place a "plug" in that place instead.
A source familiar with the matter described the moment of decompression to TPG as "explosive," while a passenger on the flight told local media that the force of the incident ripped a child's shirt off. Photos appeared to show emergency oxygen masks deployed throughout the passenger cabin.
Photos also appeared to show damage to the seat next to the hole in the bulkhead. Several unconfirmed reports suggested that the seat was not occupied when the incident occurred.
The plane, which has the registration number N704AL, was delivered to Alaska from Boeing on Oct. 31, 2023, according to data from Airfleets, and only began revenue service with the airline last month.
In a statement, Alaska CEO Ben Minicucci apologized to passengers who were on the flight and said that the airline would ground its 65 737 MAX 9 aircraft pending inspections.
"Each aircraft will be returned to service only after completion of full maintenance and safety inspections. We anticipate all inspections will be completed in the next few days," Minicucci said.
By noon ET on Saturday, inspections on more than a quarter of Alaska's 737 MAX 9 fleet had been completed, the airline said, and some of the planes had already returned to service. Several aircraft were operating revenue flights Saturday morning.
Alaska later clarified that it had reviewed maintenance records, and found that 18 of its 65 aircraft of the subtype had undergone the relevant inspection as part of routine heavy maintenance recently enough to be deemed safe.
The FAA on Saturday said it would temporarily ground some currently in-service aircraft pending inspections that would take four to eight hours per plane. The emergency Airworthiness Directive will apply to MAX 9 jets that are outside of certain routine inspection windows and will include about 171 aircraft, the agency said. There are approximately 215 of the aircraft subtype in service globally, according to aviation data firm Cirium.
United Airlines is the only other U.S. carrier that operates the MAX 9, the longest version of the jet that is currently operating for airlines. A larger version, the 737 MAX 10, is in the process of being certified by the FAA. It was not immediately clear whether the "plug" was an option on the MAX 10.
The inspection may be able to be performed at outstations, rather than at maintenance hubs, which would significantly speed up the process, according to a source familiar with the matter.
The National Transportation Safety Board said it was sending a team of investigators to Portland.
Boeing said in a statement that it supported the FAA's Emergency Airworthiness Directive.
"We agree with and fully support the FAA’s decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane," the Boeing statement said. "In addition, a Boeing technical team is supporting the NTSB’s investigation into last night’s event. We will remain in close contact with our regulator and customers."
The incident harkened back to the nearly two-year global grounding of the 737 MAX type, which was implemented in April 2019 following the second of two fatal crashes involving the relatively new aircraft type.
Investigators attributed the crashes to a flight control system that was designed to pitch the aircraft down in some situations to compensate for the fact that the MAX has larger engines than the previous model of 737, the Next Generation, or NG.
The system, Maneuvering Characteristics Augmentation System (MCAS), was found to rely on a single "angle-of-attack" sensor, which feeds information about the aircraft's pitch to the pilots and flight control computer. Without a backup sensor or other monitoring systems in place, if that sensor became damaged, investigators found that the plane could erroneously pitch down and cause the pilots to lose control.
Since the episodes, Boeing has found itself under a spotlight for its safety practices and records. Other potential manufacturing defects in various plane types — including the MAX — have been found since the grounding. While most of those were either minor issues or relatively commonplace advisories that plane makers routinely issue out of an abundance of caution, the U.S.-based manufacturer has struggled to shake the reputational stain, and even the most routine incidents involving a MAX aircraft have tended to draw disproportionate attention from the public.
The episode occurred just days after Alaska took delivery of its first 737 MAX 8, a smaller version of the jet.
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