Alaska Airlines and United Airlines canceled hundreds of flights after one of Alaska Airline's Boeing 737 Max 9 aircraft suffered a mid-air incident when an unused door exit blew out, requiring an emergency landing. 

As of Monday morning, Alaska Airlines had canceled 139 flights, or 20% of its scheduled departures, while United Airlines had canceled 204 flights, or 7% of its departures, according to FlightAware, which tracks commercial plane flights. 

Friday's incident prompted the FAA to ground all of the types of Boeing 737 Max 9s involved in the incident until the agency is "satisfied that they are safe," an FAA spokesperson said in a statement Sunday. 

Alaska and United are the only two U.S. passenger airlines that use Max 9 aircraft. The companies operate nearly two-thirds of the 215 Max 9 aircraft in service around the world, according to aviation analytics firm Cirium. The incident also prompted both Alaska and United to ground their entire fleets of 65 Max 9s. 

Alaska Airlines said passengers whose flights are canceled will be moved the next available flight, or they can request a change or a refund without incurring fees under a flexible travel policy. United said in a post on X (formerly known as Twitter) that it is working with customers to find other travel options. 

Meanwhile, the National Transportation Safety Board said the plug that was covering the unused door on the Alaska Airlines flight has been found. The agency's head said the discovery could prove vital in the investigation of the cause of the blowout, which forced the Boeing 737 Max 9 to return to Portland, Oregon, minutes after takeoff.

In a news conference Sunday night, NTSB Chair Jennifer Homendy said the plug was found near Portland in the backyard of a schoolteacher she identified only as Bob. 

How safe is the Boeing 737 Max 9?

The incident has also renewed questions about the safety of Boeing's Max aircraft, the newest version of the company's storied 737. There are two versions of the aircraft in service: the Max 8 and the Max 9, which is the larger of the two.

Aside from United and Alaska Airlines, six other airlines use the Max 9: Panama's Copa Airlines, Aeromexico, Turkish Airlines, Icelandair, Flydubai, and SCAT Airlines in Kazakhstan, according to Cirium.

Anthony Brickhouse, a professor of aerospace safety at Embry-Riddle Aeronautical University, said it's too soon to say whether the blowout involved an issue with Max 9s or that specific flight. Passengers should feel confident that regulators and airlines will make sure the grounded Max 9s are safe before returning them to service, he added.

Brickhouse also said it it was lucky that the emergency occurred shortly after takeoff when passengers were all seated with their seatbelts on. But he said that doesn't mean passengers should feel scared to leave their seats once the pilot turns off the "fasten seatbelt" sign because it's so unlikely for holes to open in the fuselages of airliners.

In 1988, a flight attendant for Aloha Airlines was blown out of the cabin of a Boeing 737 over the Pacific Ocean after an 18-foot-long chunk of the roof peeled away. Metal fatigue was blamed in that case, which led to tougher rules for airlines to inspect and repair microscopic fuselage cracks.

"When passengers board a flight they should feel confident that the aircraft they are flying on is safe," Brickhouse said. 

—With reporting from the Associated Press.

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