PORTLAND, Ore. — The depressurization incident on Alaska Airlines flight 1282 over Portland put an disconcerting spotlight on door plugs, a little-known component on certain aircraft including the Boeing 737 Max 9 from Friday's flight. Door plugs seal up unused emergency exits and are only visible outside of the plane. From the inside, the door plug on flight 1282 looked like any other part of the cabin wall — until it suddenly blew out at 16,000 feet.
The component is obscure, but its name might sound familiar to people who have seen previous news stories about depressurization accidents or scary moments where unruly passengers tried to open plane doors while in the air. Many of those stories repeat a simple and reassuring fact: it is physically impossible to open an airplane door midflight.
Plane doors function like plugs, these stories explain — hence the familiar term — meaning they're specifically built to stay closed while the plane is pressurized. And indeed, while there have been a handful of depressurization accidents over the years, you'd be hard-pressed to find one caused by a cabin plug door failure.
Except — wasn't it a plug door that just broke off in midair over Portland?
Technically no, although there's some understandable confusion. The term "plug door" has cropped up in several news stories about the incident, and even the Federal Aviation Administration used it in a tweet about its decision to ground all 737 Max 9s for inspection this week.
But despite the confusingly similar names, plug doors and door plugs are two separate things. When news stories declare that it's impossible to open a door midflight, they're talking about plug doors. But the thing that broke off Friday's flight is a door plug.
So what's the difference? And why is one apparently capable of popping out of the plane while the other is not? Let's break it down:
What is a plug door?
The phase "plug door" refers to a type of door that is designed to take advantage of the difference in pressure between the inside and outside of the aircraft. Modern airplanes generally fly at up to 38,000 feet high — far above the point where there isn't enough oxygen for humans to breathe, so they're pressurized to maintain a people-friendly atmosphere inside the cabin.
As Alaska Airlines Flight 1282 demonstrates, it's bad news if a large hole opens in the fuselage while a plane is pressurized — and that's where plug doors come in. Many cabin doors and emergency exits are plug doors, which means the door and its frame are wedge-shaped, making the outward-facing side narrower than the inward-facing side.
As a result, the door can only open inward, and if the air pressure inside the plane is higher than the outside, the force of that pressure pushes the door outward and into the frame with far more force than a person could overcome if they tried to pull the door open. WIRED published an explainer in 2019 that broke down just how much pressure is pushing on the door.
Even some doors that swing outward are still plug doors — their hinges allow them to pivot out through the frame, but only after they've already been pulled back from the frame on the inside first. This video of a main boarding door being opened and closed on a Boeing 737 shows how it works:
There have been reported cases of unruly passengers successfully opening plane doors, but in each of those cases the plane was either on the ground or at a very low altitude, so there was no pressure differential to stop them — and also no risk of the kind of explosive decompression that happened on the Alaska flight.
What is a door plug?
It's physically impossible for a plug door to cause a decompression accident, but not all doors on airplanes are plug doors. As this Federal Aviation Administration article shows, some plane cargo doors are just regular hinged doors, held shut by bolts and locks while the plane is pressurized — and they have caused depressurization accidents in the past.
Door plugs are another example. They're panels installed on certain planes to permanently seal off optional exit door openings in the fuselage. But despite the similar name, their design is closer to a bolt-and-lock door than a plug door.
The Associated Press reporter David Koenig published an article on 737 Max 9 door plugs earlier this week, explaining that they're installed by a separate company called Spirit AeroSystems and their presence comes down to customization and federal requirements governing the number of available emergency exits on planes.
The 737 Max 9 can be outfitted with anywhere from 162 to 220 seats, according to Boeing, depending on how a customer wants to configure it. Most Max 9s are built with an extra pair of emergency exits halfway between the wings and the rear doors. The extra set would be mandatory if a carrier goes for the maximum number of seats, Koenig explained, but Alaska and United don't put in enough seats to require the extra exits, so they instead opt to permanently seal them off.
Unlike plug doors, door plugs aren't shaped to be held in their frames by air pressure. As the National Transportation Safety Board explained this week, door plugs are slotted into place and anchored by hinges at the bottom, but critically, the design relies on a set of four bolts to restrict the door's vertical movement.
NTSB aerospace engineer Clint Crookshanks told reporters on Tuesday that the agency has not recovered the four bolts from flight 1282 and doesn't know yet if they were in place on the aircraft. Alaska and United have both reported finding loose bolts and other installation issues on other 737 Max 9 door plugs after beginning fleet inspections this week.
One final note, in fairness to door plugs: while they may not have the pressure-harnessing design of a plug door, they also do not have a history of popping off of planes prior to last week. Koenig's article included an interview with aeronautical university professor who said the Alaska flight was the first time he'd heard of a door plug separating in midair.
