Here’s The Difference Between ‘Plug Doors’ And ‘Door Plugs’ And How Both Keep You From Blowing Out Of A Plane

If you’ve been reading or watching the news lately, you’ve probably run into multiple stories about Alaska Airlines Flight 1282. During this flight, the Boeing 737-9 MAX aircraft involved lost one of its fuselage door plugs. Rapid depressurization followed and the skilled crew got the aircraft on the ground with haste. Thankfully, there were no serious injuries and, after repairs, the aircraft will fly another day. But what are door plugs? And what are plug doors? You, like me, have probably seen both terms used interchangeably in news media lately, but they aren’t the same thing. Let’s dig in.

On January 5, Alaska Airlines AS-1282 departed Portland, Oregon, bound for Ontario, California. The Boeing 737-9 MAX, registration N704AL, carried 171 passengers and 6 crew. The aircraft was on its downwind-leg climb from Portland International Airport’s runway 28L when a bizarre incident struck just ten minutes or so after departure. The door plug located at row 26 blew off of the aircraft, taking its interior panel and the fabrics of the window seats with it. The force of the aircraft’s depressurization was severe enough to tear the shirt off a boy seated in seat 26C, who was being held by his mother in the same seat. Reportedly, the depressurization also blew several phones out of the aircraft.

Oxygen masks dropped from the aircraft as the pilots arrested the aircraft’s ascent at 16,000 feet. After a descent and a quick loop, the pilots got the aircraft back on the ground only about 20 minutes after departure. As you may have read in the news, passengers were shaken, and some sent what they thought would be their final text messages to their loved ones. A rapid depressurization event is a dramatic situation, and as the passenger you almost certainly have no idea what’s going on. Thankfully, planes are more than capable of flying after incidents like these, and the skilled pilots in the cockpit will get you on the ground as soon as safely possible.

Alaska Airlines says door plug failures are rare. I searched a few databases of aviation incidents and came up empty-handed. That doesn’t mean a plug failure has never happened before, but it’s so rare you’d probably have better luck winning the lottery and witnessing a Sharknado on the same day.

If you’ve been following the news about this incident, you may have heard the terms “plug door” and “door plug.”  Some news outlets have seemingly confused the two, which only complicates matters because a plug door and a door plug aren’t exactly the same thing. They serve two different functions. So, what exactly blew off Alaska Airlines Flight 1282?

Plug Doors

The entry and exit doors you walk through when you board a modern pressurized airliner are known as plug doors, and a plug door literally plugs the opening it fits within.  

To better understand plug doors, let’s take a look at an aircraft door that isn’t of a plug design. As illustrated below, the door may rely on a system of locks, cams, and hooks to stay closed. You aren’t going to board a plane through a door like this, but you will see cargo doors like this. The FAA shows offers this as an example of a typical aft cargo door:

There are plug-type cargo doors, but as they open inward, they take up space that could be occupied by cargo. Instead, they open out and allow cargo to be swallowed in. During the early decades of jet airliners, cargo doors sometimes blew off, with varying levels of destruction caused afterward. Design changes to these doors made them safer.

While an outward-opening door braces itself with mechanical locks, plug doors use our good friend physics to seal the door to the fuselage. Plug doors come in different designs, but they are built so that the pressure differential between the aircraft’s interior and the outside pushes the door into the fuselage, creating a seal so strong that even Dwayne ‘The Rock’ Johnson won’t be able to open it in flight. Plug doors also prevent those nasty door failures mentioned in the above paragraph. Watch the process of closing the cabin door of a Boeing 757:

Plug door designs can vary between manufacturers and even aircraft types. A common plug door design is a door that’s larger than the opening it has to fit through. To open these types of doors, you would first open the door inward, turn it, and slide it through a gate that allows it to open outside of the aircraft. Other plug doors may slide into the ceiling rather than require the previous dancing act but still use the same concept of pressure to keep the door closed.

There are other doors that are still considered to be plug doors but do not rely on the door being larger than the opening. Instead, these doors lower into strong tabs, and then use cabin pressure to lock the door into the tabs. You’ll find these on Airbus aircraft as well as aircraft like the Boeing 777. Below, I’ve provided an example of what these tabs look like. This blurry screenshot comes from an Airbus A320 emergency door video, the tabs are the blue structures I circled in red:

You can watch the video for yourself. The door-opening action happens at 2:30 and 10:20, respectively:

No matter the design, these doors form a strong seal against the fuselage that ensures a safe, uneventful flight for the vast majority of flights these planes will ever fly. As I said before, mere mortals aren’t opening these doors when the aircraft is at altitude. Cabin doors are locked during the flight. Even if you beat these locks, you’d have to figure out how to pull the door inward to make any sort of difference.

Unless you have superhuman strength, that’s unlikely to happen. The exception to the rule is when the cabin equalizes with the outside pressure. Then the doors can open with mere human force. And that has happened before! Last year, a man reportedly opened a cabin door during an Asiana Airlines flight. However, at the time, the reported altitude of the aircraft was just 700 feet.

The smaller emergency exit doors located closer to the center of the fuselage are also plug doors. These doors will also often seal using tabs and stop fittings rather than being larger than their openings. Once again, these doors will be locked while the aircraft is in flight.

Door Plugs

That brings us to what failed on Alaska Airlines Flight 1282. Airliners must be able to be evacuated in 90 seconds during an emergency. To help facilitate this, the aircraft must have enough emergency exits. The Boeing 737-9 MAX can carry a maximum of 220 passengers. However, Alaska’s aircraft was certified for up to 189 passengers and the aircraft involved had just 178 seats. Thus, it did not require all of the emergency exits provided on 737-9 MAX aircraft.

Since emergency exit doors were not required in that location of the aircraft, door plugs were installed there instead. Door plugs are a neat solution to this problem. They fill the space of an emergency door, but they technically are not a door. You will not find any opening mechanism, nor will you find an emergency slide or a pressure vent. If you are a passenger sitting in that row, you may even get a full-size window. If you didn’t know any better, you would not know you are sitting in what would otherwise be an exit row. Check out the NTSB photo below:

Your clue would come when inspecting the outside of the aircraft. The door plug isn’t obvious, but you can spot it by looking at the outline of where an emergency door would otherwise be. As Chris Brady, an airman with over 20 years of experience with 737s, explains, 737 door plugs are held in place with 12 stop fittings and stop pads around the plug:

As I mentioned before, the stop fittings are what take the pressure loads and are what classify these door plugs as plugs. These plugs are neat because not only do they solve the problem of covering up unneeded emergency exits, but should the airline reconfigure the aircraft and need to use those exits, the structure is still there. The plug can be removed and an emergency door put in its place.

Additionally, the door plugs have lower hinges and straps. Yes, a door plug can open, but only for maintenance, and short straps hold the plug slightly open.

Otherwise, the door plug is sealed shut. Since the door plug is not supposed to be used as a door, there isn’t any mechanism to move the door out of the track that holds it closed. Instead, there are four bolts that lock the plug in place from moving vertically and outward. These bolts work by blocking the plug’s rollers from moving out of their closed position. A castle nut with a pin is used to prevent the bolt from backing out. Think something like an axle nut.

Alaska Airlines Flight 1282

So, with that in mind, how did the door plug fail on Alaska Airlines Flight 1282? The investigation is ongoing and as of right now, a cause is not known. However, images taken after the incident do provide some information.

Brady notes that the roller pins and stop pads, at least the ones visible in the aftermath photos, are intact. He believes that a part of the investigation may focus on those locking bolts. Some news publications have reported that the two-month-old aircraft had been giving pressurization warnings on three earlier flights.

If these reports are true, it’s possible there was a problem before the failure occurred. Reportedly, Alaska Airlines responded to the pressurization warnings by banning the aircraft from making flights to Hawai’i in case it needed to get to an airport for a failure. Again, it’s early on in the investigation, so it’s unclear whether those warnings were related to the incident.

The event also marks another bit of trouble for Boeing supplier Spirit AeroSystems, which is unrelated to Spirit Airlines. Spirit AeroSystems provides Boeing with the fuselages for its MAX aircraft, including doors. Last year, Spirit AeroSystems and Boeing discovered improperly drilled fastener holes in the aft pressure bulkheads of MAX fuselages. These bulkheads are dome-shaped structures at the end of the cabin. Reportedly, these defects did not cause safety issues but caused enough of an internal scandal that Spirit AeroSystems ousted its CEO.

At any rate, on January 6, Alaska Airlines grounded all 65 of its 737-9 MAX aircraft to conduct inspections. About a quarter of these inspections were reportedly completed “with no concerning findings.” Also on January 6, the FAA issued Emergency Airworthiness Directive 2024-02-51, which grounded all Boeing 737-9 MAX aircraft in the United States. These aircraft will be able to take to the skies again only after they’re inspected and after any necessary corrective action is taken.

Aside from that, we’ll just have to wait and see what caused this door plug to take flight. The investigation is ongoing and it’s unlikely the National Transportation Safety Board will have a clear answer until about a year from now. Until then, now you know how plane doors work. They’re a wonderful part of why commercial aviation is so safe and unless you’re looking for it, you probably won’t even notice.

Update: As ABC News reports, United Airlines has found loose door plug bolts in an unspecified number of Boeing 737-9 MAX aircraft:

United Airlines said Monday that it has found loose bolts during inspections of its 737 Max 9 fleet in the wake of a door plug getting blown out of an Alaska Airlines plane over the weekend.

United won’t say how many planes had loose bolts.

“Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug — for example, bolts that needed additional tightening,” United said in a statement. “These findings will be remedied by our Tech Ops team to safely return the aircraft to service.”

Update: If you’re curious, Lewin did some rough back-of-the-napkin math to find out how much force was being exerted on the plug. Also, how much force you’d need to produce to open a door at altitude. His conclusion is that the plug was holding back roughly 3,683 pounds of force. As far as opening a cabin door goes, you’d need to beat 10,371 pounds. Those are all rough numbers, of course, but underscores why a drunk isn’t opening a cabin door at 33,000 feet.