Watch: Plane loses pressure mid-air, portion of wall missing as oxygen masks deployed

The flight, which had been bound for Ontario, California, suffered the depressurization soon after departure and landed safely back at Portland

The National Transportation Safety Board on the US said late on Friday that it was investigating a depressurisation incident that forced an Alaska Airlines Boeing 737 MAX 9 to make an emergency landing shortly after taking off from Portland, Oregon.

Alaska Airlines Flight 1282, which had been bound for Ontario, California, suffered the depressurization soon after departure and landed safely back at Portland at 5.26pm. Pacific Time with 171 passengers and six crew, about 20 minutes after takeoff, according to the airline and Flightradar24 data.

The Federal Aviation Administration said the crew reported a pressurisation issue, and said it would investigate.

The new MAX 9 was delivered in late October to Alaska 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. Watch the video here:

Boeing and the Federal Aviation Administration did not immediately comment. "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."

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