Alaska Airlines Flight 1282, which had been bound for Ontario, California, suffered the depressurisation soon after departing at 5.06pm local time and landed safely back at Portland at 5.26pm with 171 passengers and six crew, according to the airline and Flightradar24 data.
The Federal Aviation Administration said that the crew reported a pressurisation issue, and that it would investigate.
The new MAX 9 was delivered in late October to Alaska Airlines and certified in early November, according to FAA data.
“While this type of occurrence is rare, our flight crew was trained and prepared to safely manage the situation,” Alaska said in a statement. “We are investigating what happened and will share more information as it becomes available.”
Boeing said in a statement that it was looking into the emergency landing.
“We are aware of the incident involving Alaska Airlines Flight 1282,” the company said in a statement. “We are working to gather more information and are in contact with our airline customer. A Boeing technical team stands ready to support the investigation.”
Social media posts showed a window and portion of a side wall missing on the airplane and oxygen masks deployed.
Exterior photos of the aircraft appeared to show that the rear mid-cabin exit door separated from the aircraft during the flight, Flightradar24 said in a post on its website.
The MAX 9 features a rear cabin door behind the wings of the aircraft that can be “activated” in dense seating configurations to meet evacuation requirements, Flightradar24 said. However, they are permanently “plugged,” or deactivated, on Alaska Airlines jets.
Boeing and Alaska Airlines did not immediately respond to a request for comment about the door.
The aircraft reached a maximum altitude of 4,975m during the flight, data showed. “Whenever you have a rapid decompression such as this, it’s a major safety event,” said Anthony Brickhouse, an air safety expert at Embry-Riddle Aeronautical University. “I can’t imagine what these passengers experienced. It would have been loud. The wind would be rushing through that cabin. It was a probably pretty violent situation and definitely a scary situation.”
The incident shows the importance of passengers’ keeping their seatbelts buckled while seated in an airplane, even if the fasten seatbelt light is off, Brickhouse said, noting that the oxygen mask system appeared to have functioned properly.
“This is a very, very serious situation and it could have been a lot worse,” he said. “If someone had been sitting in that seat, and they weren’t buckled in, it would have been a different situation.”
Last week, Boeing said it was urging airlines to inspect all 737 MAX airplanes for a possible loose bolt in the rudder control system.
The FAA said it was closely monitoring Boeing 737 MAX inspections and would consider additional action if more loose or missing hardware was found.
The 737 MAX was grounded for 20 months worldwide after two fatal crashes in 2018 and 2019 killed 346 people in Ethiopia and Indonesia. Boeing is awaiting certification of its smaller 737 MAX 7 and larger MAX 10.
The FAA has carefully scrutinised the MAX for years. The FAA in 2021 said it was tracking all 737 MAX airplanes using satellite data.