The Federal Aviation Administration on Saturday ordered U.S. airlines to stop using some Boeing 737 Max 9 planes until they were inspected, after one of those planes lost a piece of its body in midair, terrifying passengers until the plane landed safely.
Alaska and United Airlines on Saturday began canceling dozens of flights after grounding their Max 9 fleets so the planes could undergo the federally mandated inspections.
The Max 9 involved in Friday’s incident had taken off from Portland, Ore., as Alaska Airlines Flight 1282, bound for Ontario, Calif. The plane returned to Portland about 20 minutes after takeoff, and no one aboard was seriously injured. Those on board described wind blowing through a gaping hole that showed the night sky and the city lights below.
Though the F.A.A. has yet to publicly discuss what caused the episode, it ordered airlines to inspect what it called a “mid-cabin door plug.”
Some of the Boeing 737 Max 9s are configured with fewer than the maximum possible number of seats and therefore do not need all the exits originally designed for the plane. Those unneeded doors are filled with a plug. The Flight 1282 plane had two such plugged doors, located between the rear of the plane and the emergency exits on the wing.
Jennifer Homendy, chairwoman of the National Safety Transportation Board, the body in charge of investigating airplane crashes, said one door plug on the plane tore off 10 minutes out from the airport while the plane was at an altitude of about 16,000 feet.
The door plug was located nears seats A and B of row 26 — which were empty, Ms. Homendy said Saturday at a news conference in Portland. She also said that the outcome could have been much worse had it had happened at cruising altitudes, with seatbelt signs possibly off and passengers and flight crew moving around the plane.
Ms. Homendy said the investigators would be comparing the second door plug, on the opposite end of the aisle, with the one that had blown out in hopes of determining what went wrong. She added investigators would also be looking at things like the pressurization system and the plane’s maintenance records.
The Boeing 737 Max 9 in question is a relatively new plane for Alaska Airlines, having been delivered to the airline on Oct. 31. It was certified in November, according to the F.A.A. registry of aircraft. It entered commercial service that month and has since logged 145 flights, according to Flightradar24, another flight tracking site.
Forrest Gossett, a spokesman for Spirit AeroSystems, said on Saturday that his company installed door plugs on the Max 9s, and that Spirit had installed the plug on the Alaska Air flight.
The F.A.A.’s order affects about 171 planes. The agency said that the required inspections should take four to eight hours per plane to complete.
Dave Spero, the president of the Professional Aviation Safety Specialists, a union that represents more than 11,000 Federal Aviation Administration workers, including safety inspectors, said on Saturday that aviation safety experts from his union would be on the ground with the N.T.S.B. helping them determine how the plug was blown out of the plane.
“From our perspective, there is no acceptable type of situation where this kind of thing should happen; this sort of risk shouldn’t be introduced,” Mr. Spero said.
As the transportation safety board continued its investigation, it asked the public for help in finding the plane’s door, which they say most likely fell into the Cedar Hills neighborhood of Portland, according to radar.
Boeing’s Max aircraft have a troubled history. After two crashes of Max 8 jets killed hundreds of people within several months in 2018 and 2019, the Max was grounded around the world.
In 2018, Lion Air Flight 610, a 737 Max 8, crashed into the ocean off the coast of Indonesia, killing all 189 passengers and crew members. Less than five months later in 2019, Ethiopian Airlines Flight 302 crashed shortly after leaving Ethiopia’s capital, Addis Ababa, killing all 157 people on board.
The Max planes were grounded after the second crash. Boeing made changes to the plane, including to the flight control system behind the crashes, and the F.A.A. cleared it to fly again in late 2020. In 2021, the company agreed to a $2.5 billion settlement with the Justice Department, resolving a criminal charge that Boeing conspired to defraud the agency.
In December, Boeing urged airlines to inspect all 737 Max airplanes for a possible loose bolt in the rudder-control system after an international airline discovered a bolt with a missing nut during routine maintenance. Alaska Airlines said at the time that it expected to complete inspections for its fleet in the first half of January.
The planes are in wide use. Of the nearly 2.9 million flights scheduled globally in January, 4.3 percent are planned to be carried out using Max 8 planes, while 0.7 percent are slated to use the Max 9.
John Yoon, Victoria Kim, Orlando Mayorquin, Rebecca Carballo and Christine Chung contributed reporting.