Twenty days ago a door plug blew out of a Boeing 737 MAX 9. The Federal Aviation Administration grounded the aircraft. It approved a return to service plan then revoked that. Airlines conducted additional inspections on 40 planes and that data fed a revised inspection plan that saw the type return to service today*. Yet one massive question remains unanswered: Why just some planes?
Investigators almost immediately began to focus on four retaining bolts which prevent the door plug from sliding out of position. Multiple accounts now suggest that they were removed as part of a maintenance event during the aircraft’s assembly and not reinstalled, a massive failure of both the build process and associated quality assurance systems meant to track such things. Moreover, additional inspections on the MAX 9s suggest there were several other aircraft where fasteners were loose. In that context it makes sense that similarly configured MAX 9s should be grounded until the planes can be inspected and remediated. But that just scratches the surface of the problem.
The insert mounts against 12 pads integral to the fuselage. It is then secured by four bolts to prevent it from slipping. This design has been discussed significantly with respect to the “door plug” model that blew out on the Alaska Airlines plane. That same design exists well beyond just these MAX 9 planes, however.
When Boeing brought the MAX 9 to the Paris Air Show in 2017 it did not have the same “door plug” design installed. Instead it flew with a non-activated exit plug on board. This plug mounts the same way as the door plug, but with a smaller window and without an exit slide.
This appears similar to the plug installed on Copa’s first eight deliveries, as well as the MAX 9s flying for Icelandair and Turkish Airlines.
And, while I cannot find images to prove it, I’m pretty sure the remaining MAX 9s with exit slides installed are also mounted and secured the same way. After all, the point of the design is to have a common fuselage built by Spirit AeroSystems, with the modular plug installed.
If it were a design problem with the plug or a question around manufacture or materials the smaller subset of affected planes makes sense. But if the problem is with the bolts, then why are these other MAX 9 planes not also affected?
A decades-old design
As with many things MAX-related, this plug design is not new. It entered service on the 737-900 and 737-900ER more than 20 years ago. The FAA confirmed this in a Safety Alert for Operators (SAFO) issued on 21 January 2024:
The Boeing 737-900ER mid-exit door plugs have an identical door plug design to the 737-9 MAX. As part of their Safety Management Systems, some operators have conducted additional inspections on the 737-900ER mid-exit door plugs and have noted findings with bolts during the maintenance inspections.
The agency declined to elaborate on what “noted finding with bolts” is supposed to mean, because apparently it is important to have ambiguity when it comes to safety critical details.
The SAFO further recommends that airlines visually inspect the bolts on all their 900/ERs, “regardless of if this inspection has been conducted under the existing maintenance program” in recent months.
Despite having an identical design, however, the 900/ERs are not grounded. The additional inspection is not even mandatory (though I cannot imagine any airline not performing it). Why not?
The FAA says it is because the 900/ERs have flown for so long without problems while the MAX showed trouble much earlier in its program life:
The Boeing 737-900ER has over 11 million hours of operation and 3.9 million flight cycles. The door plug has not been an issue with this model. By contrast, the 737 MAX-9 aircraft that experienced the door-plug issue had a low number of flights when the incident occurred.
Given how much attention is now focused on the failure during the final assembly perhaps that makes some sense. But if the problem is with the bolts and that design, why would it not apply universally to all aircraft with those bolts and that design?
Mixed messages
The FAA (mostly) deserves credit for taking quick and decisive action. Yes, there was some confusion around which planes needed to be grounded and what level of inspection was necessary at the initial announcement. And some confusion about the inspection plan, which was issued then revoked then reissued. But the agency did get the planes on the ground and eventually validated a plan for returning them to service.
But the mixed messages around what needs to be inspected and why remains confusing. Especially given the agency’s history of requiring comparable inspections across all aircraft affected by any design flaw.
Another question to ask
We will almost certainly never get an answer, or course. And given that, there’s another question where I know answers will not be forthcoming that I’d love to ask.
On Saturday, January 6th both United and Alaska Airlines issued statements noting that some of their planes would be grounded based on FAA guidance while others – notably those that recently went through a heavy maintenance check would remain flying. Presumably the heavy check would include making sure the bolts and fasteners are all in place and secure. Several hours later those planes were also grounded. What informed that change? Why did the FAA tighten its grip, so to speak? Or did the airlines misinterpret something from the early communications?
*Copa announced early on Thursday it expected its grounded MAX 9 fleet to resume operations later in the day, and HP-9919CMP operated as Copa 701 from Panama City to Sao Paulo. Alaska Airlines expects its planes to be back in service beginning on Friday, with the fleet fully recovered in a week. United also expects to ramp up, with its planes fully flying by Sunday.
A favor to ask while you're here...
Did you enjoy the content? Or learn something useful? Or generally just think this is the type of story you'd like to see more of? Consider supporting the site through a donation (any amount helps). It helps keep me independent and avoiding the credit card schlock.
Leave a Reply