Is There A Design Flaw In The Boeing 737 MAX 9?

The door plug on an Alaska Airlines Boeing 737 MAX 9 dramatically came off the fuselage inflight, causing rapid depressurization. Fortunately nobody was sitting next to the hole on that trip out of Portland, and pilots got the plane down swiftly and safely. Since shortly after that flight the new aircraft type has been grounded.

  • The 737 MAX 9 is a stretched version of the 737 MAX 8 (that was grounded after Lion Air and Ethiopian crashed).
  • Airlines that don’t cram as many passengers into a plane as an ultra low cost carrier have a ‘door plug’ instead of an extra emergency exit door, which would be needed to evacuate a larger number of passengers.
  • The specific door plug was manufactured by Spirit AeroSystems in Malaysia. Boeing has had quality control issues, but so has Spirit.

Numerous 737 MAX 9s have been inspected, and some others have had loose door plugs. There are four bolts that are supposed to hold in onto the frame. But we don’t know whether these were installed incorrectly or whether they actually were installed correctly and loosen over time as the plane flies.

  • If it’s just poor installation of the door plug, no big deal, just inspect and fix the issue and everyone may be good to go.

  • But if the door plug was installed correctly, and came loose there’s actually a much bigger problem.

According to the chair of the National Transportation Safety Board, “We’re looking at, was there any sort of structural flexing of the aircraft? It may not be bolts.”

If we don’t know why the door plug was loose, we cannot be sure the issue won’t re-occur. Enilria points out that it’s possible that “the much heavier engines of the MAX 9 had resulted in structural flexing of the fuselage…[which] could, over time, shear off or unscrew the bolts due to vibration if they were positioned at a stress point.”

[V]ibration in airplanes tends to travel more toward the rear of the plane with forward momentum inflight and the area where the Door Plug is located is one of the first areas of the fuselage that does not have the additional structural support of the attached wing which could lead to additional vibration and flexing in that area.

This hasn’t been identified as the cause of the door plug detaching from the airframe of a 737 MAX 9, or of loose bolts found on other aircraft, but the possibility needs to be investigated. The fact that it is not yet known what the cause is precisely is what generates concern.

If this continues to be a possibility then frequent inspections (which involve removing interior wall panels to verify bolt tightness) would seem necessary – not just a one-off.

And if it turns out that bolts loosen as the plane continues to operate, then there’s a design issue with the aircraft. Perhaps there needs to be greater reinforcement. But are there stress points elsewhere on the fuselage that would be affected? So a number of questions about this aircraft type remain.

About Gary Leff

Gary Leff is one of the foremost experts in the field of miles, points, and frequent business travel - a topic he has covered since 2002. Co-founder of frequent flyer community, emcee of the Freddie Awards, and named one of the "World's Top Travel Experts" by Conde' Nast Traveler (2010-Present) Gary has been a guest on most major news media, profiled in several top print publications, and published broadly on the topic of consumer loyalty. More About Gary »

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  1. What I don’t understand is if there is a door there instead of a plug, wouldn’t the door just be bolted onto the aircraft as well? That would seem to be a systemic issue.

  2. Does the relative newness of the Alaska Airlines plane weigh against the problem being due to bolts that loosen over time?

  3. The scans they’re doing should be able to reveal whether there were ever bolts and how they were tightened to certain tolerances

  4. Bolts such as these in an aircraft cannot “loosen” if they are correctly installed as they will have some form of locking, in this case, probably split pins. Something would therefore have to break for them to come out. If they simply loosened, then they were not correctly locked.

  5. Sigh. What “Enilria” states is complete uniformed rubbish. Door plugs have also been used on the 737-900 which has been flying for decades. And probably on other aircraft models.

    Good info on the door plugs is here:

    Lets wait for the NTSB report before making wild speculations.

    – Aircraft structures engineer

  6. Good comments. @Joseph, you’re mostly correct, an operable Emergency Exit Door in that location would be bolted into the opening, but through hinges (on one side) and a latch on the other. That dramatically affects the performance of the bolted joint. @William, I would expect them to use Mass Spec and/or ESCA, in order to check for residue of the plated bolt. It’s less clear to me that it could determine the initially applied torque. The fact that they did not use castellated nuts on this application is interesting, it tells me that the anticipated loosening torque was significant relative to the shear strength of the safety wire or pin.

  7. @SteveW – yes, I have written about the door plugs on the 737-900 but the point here is the change in engine (I’d note, location as well) may matter – Enilria doesn’t say that the use of door plugs is new.

  8. Now the actual engineering is being looked at instead of using speculation and/or bias. If the bolts were tight but not torqued to the correct specifications, the possibility of them loosening would be greater. Was thread locking compound, such as one of the versions of Loctite, used or not used when it was required? Simulations during the design process should have indicated any vibration related potential for loosening. Were the simulations done correctly? Fortunately the loose bolts and the tight bolts of the door plugs that didn’t fail will give a lot of clues to what is going on.

  9. I’m no expert but it seems to me that step one is to scrap door plug’s entirely. The Max 9 was designed to have actual emergency doors in that space and that’s what should be there going forward.

  10. Good read, I’m anxious to hear what the root cause(s) was.

    The nuances of the situation go far beyond just plane design and manufacturing. Boeing is a staple of the US economy and an American icon — politicians and Presidents on both side of the aisle have rightly or wrongly depending on your point of view revered and embraced the company. Hopefully this renewed scrutiny will lead to positive changes, whatever they may be (I’ll leave that to people much smarter than I am) that optimize safety over profits and we never have to deal with such a situation again.

  11. Loose bolts, Improperly rigged, improperly torqued, or improperly installed by a “novice” and inspected by an equally “novice” inspector are “severe quality disconnects.” The problem with Boeing and Spirit is they are unwilling to pay for top-end experienced techs and inspectors. In contrast, attracting technicians straight out of A&P school and/or from the local tire shop. Send them through 4 weeks of training and assign them to install and inspect aircraft critical systems and functions. You get what you pay for. IMPHO (in my professional humble opinion) at the bottom it was an Engineering Design Flaw. Do a redesign pay for top-end proven experienced personnel to regain the public’s confidence.

  12. Gary, have other planes used this door plug before?
    Any problems?
    I saw a report somewhere that Alaska had been ignoring pressure warnings.
    Have you heard anything on this?

  13. @DavidP

    Having an actual door in place instead of a door plug would require that row of seats to be spaced farther back to allow for an emergency exit. Just like the other emergency exit rows. This would require one full row of seats to be removed from the plane (6 seats).

    The airlines don’t want to lose the revenue so instead of just having an emergency exit here which would allow for a faster evacuation in an emergency the airlines would rather have the plug.

  14. The photo I saw of a “good” lower bolt assembly had not only a nut but also a cotter pin to keep the nut from coming off of the bolt. Unlikely to “shake loose” in my opinion.

  15. Why not a flange on the inside of the plug? Say about 2″ and would not have to be continuous, could have gaps where the bolts are. This would provide redundancy. When pressurized, the flange would hold the plug in place even tighter than when not under pressure. I’m asking as I’m certainly no expert of aircraft engineering, although I have been a pilot since 1982 and have built my own plane, which I currently fly.

  16. Boeing’s CEO was very public and very clear in his/Boeing’s assertion that the cause was a “quality escape” on the assembly floor of Spirit. The NTSB has now seen preliminary inspections data…for them to cast doubt on the “quality escape” explanation from Boeing management is nothing less than massive. Beyond implying continued design issues with the MAX platform, the NTSB’s Chair’s statement makes obvious that Boeing management and Boeing data remains not to be trusted.

    Unless this is quickly resolved back to a “quality escape” issue, there is now strong risk that the MAX 10 will never be certified. Has anyone run the financial numbers on that scenario?

  17. My reading suggests there are three variations in the MAX9. 1. Have an exit there (I think none like that are in service). 2. Have the opening and a door plug, like Alaska and United. 3. Don’t have an opening at all (some non-US carriers have these). I assume those having option 2 think it gives them (or a future buyer) the ability to add an exit easily.

  18. Steve W is right
    Speculating on a million different reasons for the failure hoping one will stick is indeed rubbish.
    If the weight of the engines creates structural stress, the chances that the door plug is the only place where problems appear is more than a stretch in logic.
    Let’s let the NTSB do their job. The problem for MAX 9 operators including AS and UA Is that the cancellations continue to build.
    And if the issue is really the length of the MAX 9, then the MAX 10 will be even more of a problem.

    Just fess up that someone behind Malaysia – in the door plug supply chain – Wichita or final assembly in Washington State – forgot to tighten everything.

    It is equally as unbelievable that the door plug were the only fasteners that were not fully tightened but that is more believable that structural stress shows up just on the door plug fasteners.

  19. Gary, I disagree with your assessment that a design flaw would be worse:
    If it IS a design flaw (of a known component that has worked without issues elsewhere for decades), they’ll come up with a better design (stronger bolt, better lock, whatever), replace the current solution and are done for good.
    If it is manufacturing issue (“quality escape”), it’s a lot harder to fix. Inexperienced staff, insufficient supervision, insufficient quality control at Spirit, Boeing and Alaska- any/all of that will take a lot of time & effort to fix.
    And manufacturing issues could show up in other parts of the plane – if they missed loose bolts or assembled then incorrectly, what else did they miss or assemble incorrectly? Fixing a manufacturing issue with the plug doesn’t necessarily mean other manufacturing issues are addressed as well.
    I fear, if this is a manufacturing issue, this is a much bigger problem for Boeing than the plug…

  20. There have been good diagrams published that show the details. There are two pins from the airframe extending from the sides of the door area. The “plug” (and possible the doors if installed_ have a sloped groove in a substantial metal section. The plug is placed at the top of the groove, and it slides down and tightens against the fuselage. THEN there are bolts (2 each side) thru the groove fitting that make it impossible to move upward in that groove. The photos published that show this detail show the nuts on those bolts are secured with metal tie wire through a hole in the end of the olt. Just like in many airplane engines of the past.

    The accident report says the plug moved upward and then off.

    The bolts were never installed!!

  21. Boeing has been trying to put lipstick on a pig for years. They keep stretching and massaging this plane and things are really get out of hand. One can point fingers everywhere but, when Boeing and McDonnell Douglas merged, that’s when things started going into the toilet. Move headquarters away from Seattle and settle on Chicago. WHA’??? Remember when Boeing was brought in to save the Apollo program that initially killed 3 astronauts? “Wha’ happen'” to that spirit of “can do!”? The 737, like the 777 has reached its limit. The 737-900 is about the size of a 757 but it’s a pig where the 757 can fly high, fly fast and has great short field capabilities (SNA MDW, DCA, EYW). But will Boeing make a 757NG…nope…you gotta buy our 737-900. Airbus, meanwhile, continues to trudge along with aircraft that, for the most part, only require a differences class to fly it from one model to the next. They fly high, fast, burn less gas and are very quiet. It used to be said, “If it ain’t Boeing, I ain’t going.” Now, “If it’s a MAX, I ain’t a pax.” Once I went to “the dark side” and became familiar with Airbus, I found that I love them!

  22. Boeing splits off Spirit but its cultural soul stays with McDonnell-Douglas. Accountants have plenty of coffee and long fingernails. Once they get their caffein induced strength into those fingernails and grasp engineering by the throat, their financial logic woos the upper level management team into neglectful but profitable compliance. A few hundred or a few thousand will die. But the cost/benefit model will hold and a few settlements later, we’ll be just fine. Accounting based companies are the bane of American corporate culture.

  23. There are 4 bolts total that keep the door plug from moving upward out of its locked position, where when locked it rests against 12 stop pads that prevent outward movement due to pressurization. The 2 bolts on bottom are part of a hinge assembly that allows the plug to be hinged open for maintenance when the interior panels are removed. The 2 on top are on rods that lock into grooves when the door is slightly lowered into its locked position. ALL 4 of these bolts have castle nuts and safety pins preventing them from loosening. It’s pretty easy to google search for pictures of the plug door. If it were correctly installed, as in the bolts and nuts and safety pins were installed in the first place, I seriously doubt there would be any problems. It’s pretty well engineered. This failure is either on Spirit on assembly, or on Boeing if they removed and reinstalled the plug door in Renton. Somebody, an assembly mechanic, left the bolts, nuts, and pins out. And then they missed it when checking their own work, and then at least one more, and perhaps two more people missed it on an inspection check off. Heads should roll over this.

  24. 20 years Ive watched this once great engineering company spiral thanks to the legacy of the Jack Welch disciples…Stonecipher, McNerney who authorized a reworked old 737 into the max instead of a new airplane, Mulinberg who we had hope would turn it around, only to be sacrificed because of McNerney’s decisions, and now we have a Jack Welch mini-me…And now the new COO, Stephanie Pope, another accountant, is the heir apparent. Where does it end?

  25. Now, we often see in comments , jokes like this one :
    “If it’s a Boeing, straight in heaven or hell you’re going”

  26. Modules outsourced to the lowest bidder, and then cobbled together with Boeing’s “don’t care” US workforce, by management who put profit over schedule…

    What could go wrong?

  27. The entire 737max series is an engineering patchwork, first and foremost suboptimal centre of gravity, kind of fixed with software…; astonishing airlines even buy such an aircraft; as things stand, a new cleansheet aircraft would have saved hundreds of lives, tens of billions in losses for Boeing it’s shareholders, ….
    Short term profit by Boeing Board turned into a graveyard

  28. If quality control is poor in this instance and it is the same level of quality control Boeing uses throughout then the question of safety needs to go beyond the bolts and this instance. Reacting as parts fail may be annoying For your washing machine, it can be deadly in an airplane. The mindet that all else is OK and lets just focus on this technical issue is not sufficient. The safety and quality control needs to go beyond this incident to all aspects of the max 9 and other Boeing planes as well.

  29. The Max is Boeing’s Dehavilland Comet. Public trust in this aeroplane is waning. Melt them all down and make the plane they should have made: an updated 757.

  30. Loctite? No… What’s actually holding the plug are 12 stop pads. There’s a castellaneted nut with a cotter pin on the 4 bolts preventing the plug from moving up and out and disengaging from the stop pads. Check out the video SteveW points to. There’s an updated one dated Jan 17.

  31. Using door plugs to eliminate the emergency exit door is an option for the airline. It is entirely possible to keep the operational emergency exit door with pull handles and cover this with the interior cladding. This would mean that there would be no window but equally if the airline weee to reconfigure the aircraft for more passengers then this door could still be used (as would be required) whereas the plug version with window could not easily be retrofitted back to a full emergency door.

  32. Throwing out design flaw. Ultimate click bait. Don’t recall you ever mentioning your aeronautical training.

  33. 1. Part quality
    2. Structural design integrity
    3. Installation accuracy
    4. Spirit QA buyopff
    5. Boeing QA buy off
    6.. Modal suppression due to vibration for heavier engine mounted further back that other 737 models

    UA and AS 7379Max fleet as been inspected so amount of defects are known either torquing or vibration suppression, part manufacturing changes.

    Takes time to analyze vibration issues as airline will volunteer an airplane to be instrumented and recording for takeoff/Landings when most vibration occurs then analyzed for energy levels and different frequency nodes. Time frame collecting data will be determined on total number of takeoff/landings per plane. So there is a method to prove/disprove vibration but it takes time collecting the data.

    Boeing has a history of hiding any worker involved is a manufacturing or engineering problem and only allowing the media to spit out junk of not a big problem.
    Mullenburg(former CEO , Hamilton(former chief engineer) two monkeys chatting in front of congress not knowing details.

    Now Boeing is run a Blackrock Hedge dodo-bird Calhoun, chief engineer McKenzie who replaced Hislop.

    SO many unqualified executives in Boeing who buy into the cheapness product and safety methodology they add zero value ask work is continuously re-done.

  34. Isn’t the whole point of a plug that it’s bigger than the opening it is plugging?

    You can’t open a door on a plane at altitude due to the pressure differential, how is the plug any different?

    Sewer covers never fall into sewers because they just don’t fit. How was the plug able to fit through the fuselage at all?

  35. With some help from the above linked videos to the 737 tech guy… Yes there is a blatant design flaw!

    Even if the bolts were never installed at all, the door plug design should not have allowed the door to open at altitude. That’s airplane design 101: Eliminate single points of failure.

    Turns out on the door “plugs”, the pins are on the plane and the guide tracks are on the door, the reverse of the actual doors, and if the door “plug” gets pushed up, it will simply open (and fly off, at altitude.)

    The kicker?

    Boeing put springs at the bottom of the door “plug” that constantly push the door up and into the open position (so it will stay open during maintenence.)

    To stop the door from opening AT ANY TIME, Boeing (maybe) installed bolts.

    That is just plain stupid. You don’t trade convenience in rare matenence for fail-safe operation in standard use.

    “plug” in quotes because the door “plugs” are not actual plugs at all, since they fit through the opening they are installed in.

  36. Imagine an aircraft with an inherent instability built in as a result of a design constraint that someone says must be used. Imagine the company decides to go ahead and manufacture this without a dedicated and focused workforce at a time of cut throat competition with competitors that are far superior in fuel efficiency and performance. You have all the perfect scenarios for a very damaging outcome. Coupled with arrogant top management who make bigoted remarks about African and Asian airlines which paid dearly for a most stupid transport product that no Aeronautics 101 student would consider OK, this is the outcome

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