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.