Here’s What You Need To Know About The Boeing 737-9 MAX Door Plug Situation

On January 5, Alaska Airlines AS-1282 departed Portland International Airport in Oregon, bound for Ontario International Airport in California. About ten minutes into the flight, a door plug, a panel covering a portal where an emergency exit door normally would be, blew off of the aircraft. Since then, the plug has been found and issues have been found on numerous other aircraft. Now, questions are mounting as an investigation tried to determine why this incident happened and why the plugs on other aircraft have been found to be not up to spec. Here’s what you need to know about Boeing’s door plug situation right now.

On that day, Flight AS-1282 was not a full flight as it hauled 171 passengers and six crew. When the door plug at row 26 was ejected from the aircraft, the force of the rapid depressurization ripped out the interior panel with it, as well as the padding from the window seats, cell phones, and other objects. The force was so strong that a shirt was torn off of a boy who was seated in 26C. Images from the NTSB seem to suggest that a seat frame may have bent during the event, too. The aircraft reached an altitude of 16,000 feet before descending down and returning to the airport. The whole flight lasted just 20 minutes.

Thankfully, nobody was seated in 26A or in 26B and the aircraft was just ten minutes after departure and climbing. Had there been people in those seats and had this incident occurred at cruising altitude, this would be a very different article.

Before I continue, I want to remind readers that our intent is not to scare you from flying to your next destination. The Federal Aviation Administration says it handles about 45,000 commercial flights every day. In a single month, U.S. airlines can shuttle over 80 million people around America and beyond. Airlines in America carry well over 800 million passengers each year. That’s literally thousands of planes in the air at any given moment, with tens of thousands of people safely reaching their destinations.

Sadly, incidents do happen sometimes. Thankfully, the vast majority of them don’t result in fiery death. A lot of it is thanks to the fact that when something happens, teams of experts identify causes and work to ensure the incident or accident doesn’t happen again. That’s what we’re witnessing right now with these Boeing 737-9 MAX door plugs.

So, what do you need to know about this developing situation?

Why These Planes Have Door Plugs

In my previous entry on this, I explained what door plugs are and how they’re different than plug doors. I touched on why planes have door plugs, but I’ll explain further here.

Safety is of extremely high importance in aviation. In an emergency situation, you have to be able to exit an aircraft within 90 seconds. That’s a minute and a half and it applies to everything from a puddle jumper with a dozen people to an Airbus A380 hauling the entire population of a rural American small town. To help facilitate this quick unloading, aircraft are equipped with safety features from emergency exits and slides to life vests and rafts for water landings.

The Boeing 737-9 MAX is designed to carry a maximum of 220 passengers. In order to enable the quick and safe exit of that many people, the 737-9 MAX is equipped with 10 emergency exits. The doors fore and aft are used for evacuation, as are the four common overwing emergency exits. Between the overwing exits and the rear doors are a pair of additional emergency exits. The MAX 9 is not the only aircraft with a configuration like this. To give another example, the Boeing 737-900 Next-Generation also has ten exits.

However, since cramming 220 people into a 737 should probably be a crime, most airlines configure their examples with fewer than the maximum allowable seats. The Boeing 737-9 MAX that operated Flight AS-1282, registration N704AL, was certified for up to 189 seats. Alaska Airlines was operating the aircraft with 178 seats. Thus, the two extra rear emergency exits weren’t needed. If you’re a passenger aboard an Alaska MAX 9 flight, the interior looks normal, so you’ll have no idea there’s a plug on the other side of your window seat:

Here’s a graphic showing Alaska’s seat map with the row featuring two door plugs boxed in red:

Boeing’s subcontractor, Spirit AeroSystems, assembles Boeing 737 fuselages, engine nacelles, pylons, and thrust reversers at its facility in Wichita, Kansas. Completed fuselages are loaded onto railcars, where they are transferred to Boeing in Renton, Washington for final assembly. Almost a decade ago, a train carrying 737 fuselages from Spirit AeroSystems derailed in Montana, producing a famous photo of river rafters floating by “crashed” 737s in the woods. We can’t publish that image here, so just click the link.

Anyway, the door plugs are initially mounted at Spirit AeroSystems, and this is one area of concern for the ongoing investigation. I’m going to return to the door plugs themselves for a moment. There is still a bit of confusion over how, exactly, these plugs work. Let’s look at Boeing’s illustration:

As I explained in the previous entry, door plugs work like plug doors. Many plug doors work by simply being larger than their openings. That way, the aircraft’s pressurization forces the door to seal against the fuselage. As our Lewin Day roughly calculated, at altitude, there are potentially thousands of pounds of force being enacted on the door’s fittings, so it is not going anywhere and you aren’t opening it. The other common way to have a plug door is to have the door forced into stop fittings. The door is not larger than its opening, but pressurization is still forcing it shut.

The door plugs used in the 737 are of that latter type of design. As you can see in the illustration, there are 12 stop pads and 12 stop fittings. These pads and fittings are taking the pressurization forces. The door plug also has a set of lower hinges, upper guides, and upper guide rollers. The plug is not officially a door, but it can be opened for maintenance or to aid in filling out the interior of the aircraft. In case you’re wondering, the plug weighs around 60 pounds.

When the plug is locked into place, two bolts are inserted into the upper guide tracks. Two more bolts are inserted into an area above the assist springs in the lower hinges. These bolts feature castle nuts with locking pins in them to prevent them from backing out. These bolts prevent the door from opening. Aviation journalist Edward Russell uploaded an image to Twitter showing loose door plug bolts in a 737-9 MAX:

Following @jonostrower's scoop on United finding 737-9s with loose door plug screws, a source sent this image of one of the door plug's lower hinge bracket with a note that 2 screws are not screwed all the way in. https://t.co/sz1d997PdL pic.twitter.com/d6hgHAEb2u

— Edward Russell (@ByERussell) January 8, 2024

What Investigators Have Found Thus Far

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.

On January 7, the National Transportation Safety Board said the Alaska plane’s door plug had been located. The plug landed in the backyard of a Portland school teacher named Bob. The plug appeared to be in good physical shape and helped investigators learn more about what happened that day.

As reported by ABC 7 News, the NTSB has thus far found that the door plug appeared to be missing all four bolts that should have stopped it from moving. At this time, the NTSB is reportedly unsure if the bolts were even there to begin with. From NTSB engineer Clint Crookshanks via KGW8 News:

“We found that both guide tracks on the plug were fractured. We have not yet recovered the four bolts that restrain it from its vertical movement, and we have not yet determined if they existed there. That will be determined when we take that plug to our lab in Washington, D.C.”

This damage is believed to have allowed the plug to release from the stop fittings and thus blow off of the aircraft. The NTSB continues to investigate why all of this happened.

Since then, Alaska Airlines and United Airlines, the two operators of 737-9 MAX aircraft in the United States, have found loose door plug hardware on a number of aircraft. As Reuters reports, United has found around 10 of its 79 MAX 9 jets to have loose door plug parts. It’s unclear how many of Alaska’s 65 MAX 9 aircraft were found to have loose hardware but a statement from the airline indicated “some aircraft.“

The incident aircraft’s data recorder and cockpit voice recorder were sent to the NTSB for examination. As CNN reports, the cockpit voice recorder was found to be overwritten. CVRs run on a 2-hour loop. At the end of each loop, the data is overwritten. This is what happened with the incident aircraft as well as aircraft involved in 10 other NTSB investigations over the past five years. Understandably, the NTSB would like the standard changed from CVRs that record in 2-hour loops to CVRs that record 25-hour loops.

As the investigation continues, eyes are on both Spirit AeroSystems and Boeing. Spirit AeroSystems assembles the fuselages and Boeing finishes the aircraft. Investigators will be looking for where the process has gone wrong apparently multiple times. As of writing, the FAA alleges that Boeing may have “failed to ensure its completed products conformed to its approved design and were in a condition for safe operation in accordance with quality system inspection and test procedures.” Spirit AeroSystems is also working to help investigators understand the manufacturing process and again, to potentially figure out where things went wrong.

There is also some debate about whether Alaska should have grounded the two-month-old aircraft after auto-pressurization warnings were found on December 7, January 3 and January 4 flights. In response to these warnings, Alaska stopped the aircraft from making any flights to Hawai’i, which could have put it into a tricky situation if it lost pressure over the ocean. However, as PBS notes, the NTSB has not found evidence linking the pressurization warnings to the door plug blowout.

What This Means For You

As of right now, the greatest impact you will see is a canceled flight. United runs 79 examples of the Boeing 737-9 MAX while Alaska Airlines flies 65 examples of the jet. Hundreds of flights between Alaska and United are getting canceled. NBC News reports that 20 percent of Alaska’s routes are impacted by the grounding of the 737-9 MAX aircraft. Indonesia’s Lion Air also grounded three of its MAX 9 aircraft, even though they are of a different configuration than their American counterparts.

When will these planes take to the skies again? Well, the FAA says to hold tight, because safety is far more important than speed:

This incident should have never happened and it cannot happen again. FAA formally notified Boeing that it is conducting an investigation to determine if Boeing failed to ensure completed products conformed to its approved design and were in a condition for safe operation in compliance with FAA regulations. This investigation is a result of an incident on a Boeing Model 737-9 MAX where it lost a “plug” type passenger door and additional discrepancies. Boeing’s manufacturing practices need to comply with the high safety standards they’re legally accountable to meet. The letter is attached.

Every Boeing 737-9 Max with a plug door will remain grounded until the FAA finds each can safely return to operation. To begin this process, Boeing must provide instructions to operators for inspections and maintenance. Boeing offered an initial version of instructions yesterday which they are now revising because of feedback received in response. Upon receiving the revised version of instructions from Boeing the FAA will conduct a thorough review.

The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service.

The good news is that when you take flight in a MAX 9 again, these issues will be resolved. If anything, now you know a new fact about commercial airliners. If you see a weird door-shaped blank panel at the rear of your next flight, that might be where an emergency door could have been located.

We will continue to follow this investigation and give you major updates as they come. Incident investigations often take a year, sometimes longer, so don’t expect the cause of these loose parts to be determined quickly.