The recent failure of a plug door in an Alaska Airlines Boeing 737 generated lots of speculation about the cause of this unusual failure. As is frequently the case when components fall off aircraft, fasteners were immediately brought into question. Whether or not the plugs were positively fastened to the airframe is still in question, but the mainstream media refers to all aviation fasters as “bolts,” and frequently implies that fasteners are the primary restraining force against cabin air pressure. While missing fasteners could definitely allow the plug to shift, screws don’t hold back the pressure; the design of the plug does.
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Episode Transcript:
By now, everyone has seen the dramatic images: an Alaska Airlines 737 with a gaping hole in the middle of the fuselage, and the missing door plug lying in a suburban Portland backyard.
Any cabin depressurization in an airliner is dramatic, and this one fortunately ended well with no injuries. As I’m talking to you now, the FAA and aviation pundits everywhere are speculating about the fasteners that hold the fuselage door plug in place—or more specifically, whether they were installed.
The mass media, of course, gets the story largely wrong. So, I want to clear up a couple of misconceptions.
First of all, the lightest and most efficient way to close any pressure vessel is with an inward opening plug seal. This means that the primary clamping force is provided by the pressure differential between the inside and outside of the vessel—in this case, an airplane fuselage.
Plug type doors and hatches are used everywhere, from the chemical process industry to spacecraft. There are lots of ways to describe this technology, but for our purposes, there are three basic variants: the first is the plug that is designed to remain in place for the life expectancy of the pressure vessel. These are usually openings that are provided to aid in assembly or fabrication of the pressure vessel, but which aren’t expected to be used ever again.
The second is the hatch or door that is designed to be opened infrequently, usually for maintenance or inspection purposes. The third is the true door or hatch, designed to be opened and closed frequently.
Engineers design each type differently, but the fuselage plug on the Boeing 737 is an interesting case. Depending on the number of seats installed in the fuselage—which is determined by the airline, not Boeing—the number of emergency exits is dictated by FAA regulations. Cabin doors and emergency exits are heavy, expensive to build and are also maintenance heavy. So, Boeing and their airline customers would like to use as few of them as possible.
Alaska Airlines’ specification for the aircraft in question carried fewer than the maximum number of passengers possible, and as a result, Boeing sold them an airplane with a plug replacing the unneeded emergency exit. The fasteners in question are for the missing plug, and the media likes to call them “bolts” although they’re more accurately called cap screws.
In aviation, there are lots of specialty fasteners. My favourite are “pins and collars.” But like all plug doors and pressure applications, the fasteners don’t resist the pressure; instead, they locate the plug against its mating frame seal. There should be no shearing or tensile forces on those bolts to break them, so in this case, I strongly suspect that the bolts were never installed in the first place.
There will be witness marks on the components if fasteners were there, but this now looks highly unlikely. So how can this happen? Well, several media reports claim that the plugs are removed during the fit out of the fuselage interior, then replaced. They may also be removed for some maintenance operations. It is very unusual in aviation to completely forget a fastener, which is an obvious checklist item. But it looks like it happened.
How can this be prevented? In the future, likely by machine vision, which is already used in some mass production assembly processes to check for missing fasteners. A simple system for some is to simply require replacement with new ones, which makes it easy to track the workflow since new parts are accounted for in the maintenance log.
There are other techniques, too, such as captive fasteners or the use of torque wrenches that have a recording function. I like this approach, because it would be possible to know how many screws or nuts were torqued in a given repair operation.
There are lots of ways to check this kind of thing, but mistakes do happen. But we can’t blame the fasteners, if the technician forgets to put them in.