Last month, we blogged in this space about the Alaska Airlines flight that lost a door plug and decompressed at 16,000 feet on January 5. The aircraft involved was a Boeing 737 Max 9, and the door plug was recovered in the back yard of a Portland, Oregon resident. Fortunately, no one was killed, although several minor injuries resulted, and the plane landed safely.
On last Tuesday, the U. S. National Transportation Safety Board (NTSB) announced that its investigators determined that the four bolts which retain the door plug in place were missing before it blew out. Documents obtained from Boeing and its supplier Spirit AeroSystems show a sequence of events that points to a serious manufacturing problem, if preliminary indications are borne out by subsequent investigations. At this point, here is what we know, based on news reports and a preliminary report by the NTSB.
The 737 fuselages are manufactured at a Spirit facility in Wichita, Kansas, which used to be owned by Boeing. In 2005, Boeing spun it off to an investment firm, but it still makes fuselages and ships them via extra-long railcars to one of the main Boeing assembly plants in Renton, Washington State. The fuselage of the plane in question arrived in Washington in August of 2023.
At the Renton plant, it was found that five rivets near the port-side door plug were damaged and had to be replaced. To access the rivets, it was necessary to remove the door plug. Except for the fact that it has no handle and other fittings that would make it a usable door, the door plug fits in the fuselage like a regular door. There are twelve "stop pads" which engage with fittings on the plug, but in order for it to move like a door, the plug must be free to move away from these pads. A regular door has a separate locking mechanism to keep it attached to the plane, but in the door plug, it appears that instead of a locking mechanism, four bolts retain it in place. Without these bolts, the only thing keeping the door plug in place is the mechanical integrity of the stop-pad pins and other machinery that is not designed to keep it there, but to let it move when needed.
After the defective rivets were replaced by Spirit personnel at the Boeing plant, a photo was taken of the completed work. This is the photo that shows three out of the four door-plug bolts were definitely missing (a fourth location was concealed by insulation, but that one was probably missing too, based on evidence from the recovered door plug).
These events took place in September of 2003. The aircraft was delivered to Alaska Airlines on Halloween of 2023, the end of October. Somehow the door plug managed to stay in place for a number of flights through November and December, but by January 5, the stop pads and associated parts had fatigued with repeated pressurizations enough to fail at 16,000 feet. If the plane had been at a cruising altitude of 35,000 feet when the plug blew, the depressurization could have sucked many passengers out and possibly crashed the plane. So this incident was an extremely close call.
As a teacher, I am continually impressed with the need for an ability that is unique to humans: the ability to pay attention. I impress this need upon my students, but every time I grade exams, I discover what happens when attention is not properly directed, or directed on the wrong things. Boeing and Spirit obviously have extensive procedures in place to manufacture, assemble, and inspect aircraft. And nearly all the time, these procedures work. But every procedure is useless if the human minds carrying them out do not perform them according to the rules.
Clearly, it was someone's duty to document with a photograph the rework of the five damaged rivets. But it is so easy to see how someone, even an inspector whose main job was to certify the correctnesss of a repair, would have his or her attention focused on the rivets, and not on the door plug only a few feet to the rear of the rivets. The NTSB inspectors, focused as they were on the door plug, saw immediately from that photo that someone had forgotten to install the retaining bolts before the insulation and interior finish materials were installed. And probably the first time the bolts were put in, before the rework procedure, somebody checked to make sure they were there. But this time, because things were slightly out of the ordinary during the rivet rework, that small but critical act of looking to see if the bolts were in place was omitted. And once everything was buttoned up, nobody could tell from outside that the bolts were missing.
This raises a question that occurs to a person who has disassembled and reassembled many pieces of equipment over the years. When a technician removed the bolts to take out the door plug and gain access to the rivets, where did those bolts go? On a workbench? In a pile of similar bolts? It seems like if they were just sitting around after the job was done, that would get somebody thinking about where they belonged. This is the kind of seemingly unimportant detail that suddenly becomes significant, and I'm sure that some NTSB personnel are asking similar questions of the people involved in the rework. I would not want to be one of the technicians who get grilled.
Modern technological means of documenting manufacturing processes have made it easier to trace actions such as the ones the NTSB is investigating. In the old pre-digital-camera and pre-email days, investigators would have had to rely only on recollections of mechanics, and it's very unlikely anyone would have taken pictures at every step of the process or produced documents with as much detail as electronic data can include these days.
Still, it's not robots who assemble airplanes, it's people. And people (and robots) can make mistakes, especially when they are doing something out of the ordinary such as rework, where it is impossible to write procedures for every contingency and people are trusted simply to do the common-sense good thing. The only problem here is, that wasn't quite good enough. Fortunately, the consequences were a lot more benign than they could have been, and the accident can serve as a warning, or encouragement if you like, that no matter how trivial an inspector's work may seem, it can save lives—or lose them.
Sources: I referred to an Associated Press report on the NTSB findings which appeared Feb. 6 at https://apnews.com/article/boeing-emergency-landing-report-alaska-airlines-8543c90b68b4d932a700cf57ff8f1b8e. The preliminary NTSB report itself is at https://www.ntsb.gov/investigations/Documents/DCA24MA063%20Preliminary%20report.pdf. I also referred to the Wikipedia article on Spirit AeroSystems.
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