If nothing else, this investigation should continue to shed light on a little-understood part of airliner design.
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Since Saturday, an Alaska Airlines Boeing 737 MAX 9 has riveted the nation after a small part of its fuselage blew out at an altitude of 16,000 feet. Throughout the course of its short lifespan, the MAX has been plagued by problems, yet Boeing still has thousands of orders from airlines that are waiting for the new planes.
But for now, the U.S. Federal Aviation Administration (FAA) has ordered inspections of all the affected planes that have similar door plugs, and Alaska Airlines has grounded all of its flights through at least January 13, the airline announced on Wednesday. That got us wondering: what happens when an airplane is grounded with a fatal safety flaw, and what exactly is a door plug, anyway?
Airplane groundings can be superficially similar to recalls that affect automobiles. If you’ve ever had a recall notice—like the wave of airbag recalls in the 2010s or the Toyota stuck accelerator recalls of 2009–2011—you know these are cues for you to take your vehicle to a dealership or other service provider in order to get an affected part or system replaced. The recall is never to return your entire car for a refund, like it is for consumer goods like toys or food. And like automobiles, airplanes can be affected by recalls, like one last year when an entire engine was recalled. It might take months or even years for manufacturers to repair the problematic part and for the FAA to re-certify the plane.
In this case, the specific airliner was a Boeing 737 MAX 9, part of a series that includes the MAX 7, 8, 9, and 10 at different sizes and passenger capacities.
What happened to Alaska Airlines Flight 1282 this week involves a door plug. This is different from a plug door (more on that shortly), and is simply the term for a door that comes standard on a particular airliner, but is not needed with the specific seating arrangement that the airline has chosen. The number of emergency exits required by air travel safety regulations varies by how many passengers are on the plane, and if an airline chooses to use fewer seats overall, they can proverbially “paper over” these doors.
Door plugs should be permanent, like most of a working airplane’s exterior, and firmly sealed by “a series of bolts, cables, hinges, and stop pads,” according to ABC News. If it sounds strange that an airline would decide some emergency exits aren’t needed and can therefore be sealed, remember that a door that opens and closes represents a different kind of dynamic risk than a panel that is bolted shut. It may introduce more chaos (and liability) in regular maintenance and inspection than it would provide aid in the case of an emergency.
The National Transportation Safety Board (NTSB) examined the Boeing 737 MAX 9 after its emergency landing, when it and many other 737 MAX airliners were grounded for inspections. They found that all 12 of the plug stops on the door plug had blown out, as reported by Airways magazine. These are the main way that the door plug is held in place, and they’re designed to be tolerant of some amount of distress.
Airways explains that 12 stop pins (or fittings) on the door slide down and inward, like a battery door on a remote control, to meet 12 matching stop pads. Then, four “stop bolts” are used to secure the door plug in that position. It also has rollers that guide the door plug into place. The bolts failed on the door plug on the Alaska Airlines flight, but investigators don’t know if those bolts stayed in place or whether they were blown off during decompression.
In a press briefing on Monday, NTSB chair Jennifer Homendy said that the 12 stop assemblies are like “high fives,” where one hand is “pressing inside the cabin, outward.” And if the stop bolts are what hold the door plug in place, once the 12 stop assemblies are in place, then, “it should not move up, if the door is bolted in,” as Homendy explained. But she clarified that the bolts and rollers are for installation and maintenance, not for securing the door plug. Bolts, and other threaded fasteners like screws, “naturally tend to loosen under vibration, impact, or alternating thermal load,” scientists explained in the Chinese Journal of Aeronautics in 2022.
The 12 stop assemblies have no locking mechanism. In this case, preliminary examination shows that the exact same door plug, but on the right side of the plane, was completely fine and had no signs of fault or loosening. Any changes to how airliners are made, including things as small as different hardware, can have consequences for aerodynamics, maintenance, and even the airplane’s “fighting weight” for flight.
In the 2017 documentary Secret Life of the Long Haul Flight, Qantas, the largest airline in Australia, explained that even the dirt that accumulates on the outside of the airliner can affect its fuel economy and performance. And just the paint on planes can weigh as much as up to eight passengers, the New Zealand Herald explained in 2023. Contrary to popular myth, white paint is 53 percent heavier than the equivalent amount of black paint. These small considerations can make a big difference in the finely balanced world of air travel.
This works fine for the overwhelming majority of flights, and air travel remains one of the safest forms of transportation. The NTSB estimates that the investigation into this flight and its ramifications for other 737 MAX airliners will take up to 18 months. There are just 215 737 MAX 9 airliners in operation around the world. Alaska Airlines and United Airlines have grounded their MAX 9 fleets, which account for about two-thirds of the total number of MAX 9 airliners, the Associated Press reports.
Confusingly, the two types of door-like things you see in an airliner have names that are reversals of one another. A door plug is used when an emergency door is not needed and is therefore plugged up. A plug door is, as the Autopian explains, the door that swings inward for passengers to board the airplane. These plug doors are physically larger than the opening that they fit into. Regular emergency doors, as opposed to the placeholder door plugs like the one on this Alaska Airlines flight, may be plug doors. (We know. It’s confusing.)
In 2023, a passenger was detained and charged with violating the Aviation Security Act after he opened an inward-opening emergency plug door as his Asiana Airlines flight prepared to land. But this was considered extraordinarily unlikely, as emergency doors are locked during flight. Experts speculated that the reason he was able to open the door was simply that the plane was so close to landing.
It’s not clear why a door plug is so differently designed, when a plug door seems simple and effective. Why can’t a plug door just become a plug that no longer opens at all, rather than a door plug that can still open outward by design? And NTSB chair Homendy said in the briefing that door plugs are opened outward to 15 degrees as part of maintenance, even though their use case is to never be opened or used at all during flight.
If nothing else, this investigation should continue to shed light on a little-understood part of airliner design. As with the Asiana Airlines passenger, what happened to this Alaska Airlines flight likely constitutes an edge case—a term used for examples that are considered at the extreme edges of what could or will happen to an engineered or coded product. But if an edge case can decompress the airliner’s cabin, something that could produce mass casualties at cruising altitude when seatbelt signs are off, then it should be accounted for going forward.
Today, or before the Alaska Airlines flight at least, the 737 MAX 8 and 9 are both in active service around the world. These airliners first flew in 2017, and quickly made news in the worst way. As scientists damningly summarized in a 2020 paper on the ethics displayed by Boeing and the FAA:
The two crashes killed a total of 346 people. In 2021, Boeing agreed to pay $2.5 billion in response to a Department of Justice charge that they conspired to defraud the FAA. Additionally, Boeing settled a claim from the U.S. Securities and Exchange Commission regarding its misleading sales and investor pitches for the 737 MAX in 2022 for $200 million.
In 2023, the Office of Inspector General of the U.S. Department of Transportation published an audit of the FAA’s process of returning the 737 MAX to regular service, which was completed by November of 2020. They concluded that the FAA had seven areas to improve in similar future processes, including that engineers’ judgment was considered “flexible” rather than standardized; and that they weren’t trained uniformly in how to deal with adverse events.
Caroline Delbert is a writer, avid reader, and contributing editor at Pop Mech. She's also an enthusiast of just about everything. Her favorite topics include nuclear energy, cosmology, math of everyday things, and the philosophy of it all.
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