Alaska Airlines Plane Had Bolts Missing

 

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.

The Door Plug Blowout on a 737 Max 9: Another Headache for Boeing

 

When Alaska Airlines flight 1282 took off around 5 PM Friday, Jan. 5 from Portland, Oregon, few if any of the 171 passengers suspected that anything unusual was going to happen.  But at 5:11 PM, as the Boeing 737 Max 9 was climbing through 16,000 feet, passengers heard a loud bang followed by a roaring wind noise that made conversation impossible.  Where a normal porthole window had once been, a gaping two-by-four-foot hole had appeared next to a row of seats on the left side of the plane.   One passenger, Kelly Bartlett, didn't realize what had happened until a teenage boy moved into an empty seat next to her.  He was sitting in the row next to the hole, two empty seats away from the window, and the blast sucked the shirt off his back.  If he hadn't been wearing his seatbelt, he might have gone with it.

 

Oxygen masks deployed all over the plane, and the passengers remained calm amid the chaotic noise.  The pilot immediately returned to Portland and landed the plane safely.  No one other than the boy next to the hole was injured. 

 

The Boeing 737 Max 9 can be configured for various numbers of seats, and for more than 200, an emergency exit is required in the location where the hole appeared.  But for smaller capacities, the emergency exit is replaced by a panel, basically a plug the shape of the exit door, that blocks the exit opening.  From inside the plane, the window and trim make this plug almost invisible.  But all that was holding it against the differential pressure of over a ton as the plane rose through 16,000 feet were four bolts, at least if the plug had been installed correctly.  This particular Max 9 was delivered to Alaska Airlines only last October, so the problem may have existed since it was built.

 

A Portland high-school teacher found the door intact in his back yard, so investigators are looking at it closely to determine the cause of the failure. 

           

In the meantime, the U. S. Federal Aviation Administration (FAA) has grounded all Boeing 737 Max 9 planes, which affects some 171 aircraft.  It has ordered inspections of the bolts and other structures around the door plugs, and United Airlines has already found that some bolts on its door plugs are loose. 

 

Only last week, we described in this space how all passengers on a commercial flight involved in a runway collision in Japan survived with only minor injuries, and we can fortunately say the same about this accident in Portland.  But things could have been much worse.  In similar incidents involving sudden holes in fuselages, passengers or flight attendants have been sucked out bodily.  If the boy sitting next to the hole hadn't been wearing his seatbelt, that probably would have been his fate.  And if the plug had waited to fail at a higher altitude, the pressure differential would have been greater, possibly tearing a seat off its mounts. 

 

Although we can rejoice that nobody was seriously injured, the big question now is why the plug blew out.  The fact that at least one other plane has been found with loose bolts holding the plug says that this may not have been an isolated incident.  That is why the FAA has wisely grounded the Max 9s until a thorough investigation shows exactly what the problem was.

 

We can only speculate at this point, but already some things are fairly clear.  The fact that the plane suffering the accident was so new points to a possible manufacturing problem.  Nuts on airframes must be torqued to a specific tension, because the proper amount of torque represents a compromise between not enough tension on the bolt, which might leave it subject to vibration loosening or fatigue in some cases, and too much tension, which could lead to bolt failure.  Many bolts on aircraft have locking cables, cotter pins, or other means by which the nut on the bolt is prevented from turning.  It's not clear whether the four bolts that hold the door plug in place had such provisions, but even locking devices can fail, or be improperly installed.

 

Another 737 Max series, the Max 8, was the subject of an extensive and expensive investigation involving defective software that intentionally crashed the plane when it received faulty data from attitude sensors.  This problem cost Boeing billions of dollars and lost prestige, and the last thing the company needs right now is another expensive and embarrassing safety problem.

 

In a statement to employees that was also released to the public, Boeing CEO Dave Calhoun promised "100% and complete transparency every step of the way" during the investigation.  He can hardly promise less, because Boeing's reputation is on the line with every accident that points to a manufacturing cause. 

 

As airlines which own 737 Max 9s wait impatiently to begin using their millions of dollars of investment again, both the FAA and Boeing have big incentives to figure out why the plug blew out and how to make sure it doesn't happen again.  The recovery of the intact door will be very helpful in the investigation, and I expect we will know something definite within 60 to 90 days. 

 

In the meantime, air flight remains a safe mode of travel for the vast majority of passengers.  The incredible number of things that all have to work flawlessly for a typical flight to be completed goes completely unnoticed by most passengers, but it is the product of the efforts of thousands of engineers, technicians, service people, pilots, crew members, air traffic controllers, and others who do their jobs without public recognition. 

 

We are fortunate that the last two attention-grabbing commercial aircraft accidents have resulted in relatively few casualties.  But grounding the Max 9s was the right thing to do, and everyone looks forward to the time when we can know what happened, why it happened, and how to keep it from happening again.

 

Sources:  I referred to the FAA website at https://www.faa.gov/newsroom/updates-grounding-boeing-737-max-9-aircraft and the following news reports:  https://abcnews.go.com/US/alaska-airlines-emergency-fittings-top-door-plug-fractured/story?id=106218951, https://www.npr.org/2024/01/08/1223517098/door-plug-boeing-737-max-portland-ntsb-faa. and https://abc7ny.com/alaska-airlines-flight-emergency-boeing-door-plug/14298712/.