As Alaska Airlines Flight 1282 made its ascent on Jan. 5, few, if any, passengers knew that a panel called a “door plug” — hidden behind the interior surface of the cabin at both window seats in Row 26 — was all that stood between them and the cold evening sky. Nor did they know that when the jet reached an altitude of 14,830 feet, warning lights began flashing in the cockpit.
Federal investigators say those lights indicated a drop in the cabin’s air pressure — perhaps a clue that the panel was failing. At about 16,000 feet, pilots heard a loud boom, and the pressure dropped further: One of those door plugs had completely torn off.
A New York Times analysis of how the door plug is supposed to work, a review of photos and documents, and interviews with aviation experts suggest that manufacturing or installation problems allowed it to come loose and break away just two months after Boeing delivered the 737 Max 9 to Alaska Airlines.
Filling the space that would have been occupied by an emergency exit door if the plane had more seats, the plug relied primarily on two pairs of bolts at the top and bottom, as well as metal pins and pads on the sides to stay in place.
The New York Times
When investigators recovered the plug from a backyard in Portland, Ore., they found that the door plug itself was largely intact, with the stop pins in place. The bolts, though, have not been recovered.
Bolts at the bottom of the plug are supposed to prevent it from sliding up vertically, which could lead the stop pins to slip past their contact points, or stop pads, on the plane’s body.
The New York Times
Bolts at the top work together with the bottom bolts to prevent the plug from sliding out of the guide rollers and to keep the pins and pads in place. Misalignment of the pins and pads could allow the door plug to open and be blown out, aviation experts said.
The New York Times
The blowout did not seriously injure anyone, but it exposed passengers to powerful winds while 16,000 feet in the air. The incident could have been much worse if the plane had been at a higher altitude and passengers and crew members had been walking around the cabin.
The National Transportation Safety Board has not yet concluded what caused the blowout and is considering various possibilities. It could be that the pressure change contributed to the failure or that the plane had a major structural or design flaw.
But two experts in aviation mechanics said the visual evidence suggested some sort of bolt failure, while adding that there was not enough information to seal the case. Though the four bolts at the top and the bottom are seen as critical to keeping the door plug in place, there are other bolts on the structure that could also have failed.
Jeff Simon, a pilot and mechanic who is authorized by the Federal Aviation Administration to inspect aircraft, pointed to the intact pins, pads and door plug as evidence that bolts had failed gradually. One possible explanation, he said, is that vibrations could have loosened improperly tightened nuts on the bolts or severed the so-called cotter pins securing some of the nuts in place. If some bolts had been missing entirely, Mr. Simon added, excessive stress on the others could have led to their failure.
“In the world of aircraft maintenance, anytime we look at a failure we look as much at what’s intact as we do at what’s broken,” said Mr. Simon, who is also the creator of a website for aviation enthusiasts. “It appears that the plug left the aircraft following a similar pathway to how it’s designed to be removed for service,” he added, basing his analysis on publicly available information.
“And therefore the next logical conclusion is to look at what locks the plug in place in its normal operations,” he said. “Those are the bolts I would be focusing on first.”
Gary Peterson, an aircraft mechanic who is a vice president of the Transport Workers Union of America, said the intact plug and other evidence also led him to conclude that bolts were the most vulnerable elements of the door plug.
National Transportation Safety Board staff members inspected the door plug from the Alaska Airline 737 Max 9 jet. The plug was largely intact, along with its stop pins.
National Transportation Safety Board
Boeing’s chief executive, Dave Calhoun, has suggested that a manufacturing lapse was responsible for the door plug blowing out. In an interview with CNBC this month, he said factories operated by Boeing and one of its major suppliers, Spirit AeroSystems, had suffered a “quality escape.”
The door plug on the Alaska Airlines jet was manufactured by Spirit in Malaysia and then brought to the company’s factory in Wichita, Kan., where it builds the fuselage, or body, of the 737 Max 9. There, the plug was installed on the fuselage, which was then transported by rail to Boeing’s factory in Renton, Wash., where the plane maker assembles the jet.
When Max 9 fuselages arrive in Renton, Boeing employees inspect the door plugs and conduct a pressurization test, according to two people familiar with the process, who asked to speak on the condition of anonymity because they were not authorized to speak publicly while the National Transportation Safety Board conducts its investigation.
Boeing declined to comment. A spokesman for Spirit, Joe Buccino, said in a statement that a team from Spirit was “supporting the N.T.S.B.’s investigation directly, and commenting on the scope or conclusions of the investigation is up to the N.T.S.B.”
“As a company, we remain focused on the quality of each aircraft structure that leaves our facilities,” Mr. Buccino added.
The Alaska Airlines plane at Portland International Airport on Jan. 8 showing the hole in the fuselage left by the door plug that blew off midair.
Mathieu Lewis-Rolland/Getty Images
A warning light indicating a problem with the plane’s pressurization system had gone off on earlier flights, and the N.T.S.B. said it could not rule out those warnings as a clue.
“In the aftermath of Flight 1282 and in light of the N.T.S.B. investigation, it’s clear to us we received an airplane from the manufacturer with a faulty door plug,” Alaska Airlines said in a statement. “We won’t return these planes to service until we are confident they are completely safe.”
Door plugs have been used safely on passenger and cargo planes for decades. An older Boeing model, the 737-900ER, has the same design for its door plugs as the Max 9. On Sunday, the F.A.A. recommended that airlines conduct a visual inspection of the plugs on the 737-900ER.
The F.A.A. has grounded about 170 Max 9 planes until they can be inspected under a new Boeing process that the agency must still approve.
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