I've waited a few days to get the salient information on the latest Boeing 737 issue, the blow-out in flight of a "plug door" on an Alaska Airlines Max 737-9.
The plug door in question is not actually a door. It's a plug in a door-shaped hole. The Max-9 has different variants with different seating arrangements, but to save assembly complexity and cost the same fuselage. The Fuselage has the same number of openings, but not all of them are used.
The Max-9 can seat a maximum of 230 passengers. On higher density versions with over 189 seats the door just behind the wings is actually a fully working emergency exit. On variants with seating for under 189 passengers, the emergency door is not required and instead the door shaped hole is plugged by... a plug.
The plug panel has a window in it and it is firmly held in place. On the inside of the aircraft the plug is covered by a panel with just a window in it. The only thing you'd notice inside the aircraft would may be a difference in the spacing of the window compared to others.
The plug is not supposed to be removed, although it can be removed with difficulty for maintenance. To open the plug, you need to remove four locking bolts that firmly stop any action of the door to open. Once the bolts are removed The bolts are torqued to spec and have castellated nuts. The nut then have cotter or split pins inserted so the nut cannot loosen.
Anyone that knows castellated nuts and split pins, know it a pretty secure method for holding a nut on a bolt. The main failure mode of split pins is metal fatigue by reusing old pins. But everyone knows you ALWAYS fit a brand new pin....right?
What seems to have happened is all 4 bolts have removed themselves from the door so it was able to release itself from the fuselage in flight.
The FAA have instructed Mazx-9 operators to check the plug door installations. It's already been reported that loose bolts have been found. Not just the locking bolts that prevent the plug from opening, but also bolts holding the hardware to the door and/or fuselage.
Now, the issue has been identified, the issue needs to be tracked back to where the bolts failed to be tightened and inspected correctly. Was it a Boeing after the fuselage assemblies had been delivered, or was it at Spirit where the fuselages were manufactured?
With two variants with two different door assemblies, were the assembly instructions up to scratch covering both options? Was there an issue with the instructions that allowed a plug door to be fitted incorrectly? For instance was a check sheet not followed for the locking bolts? For instance the emergency exit option doesn't have them, so was there enough clarity in the assembly instructions that on the plug option that the locking bolts should be fitted, torqued and split pins fitted and the installation checked before the cabin trim was installed over the plug?
Was there a oversight in the instructions that allowed a plug door to not be assembled?
I can imagine a scenario where due to assembly pressures the interior trim was installed before the plug door bolts were properly installed and checked, but the assembly instructions said once the trim was installed there was no need to check the door, because it was assumed that at that point the door had been checked and inspected. I just wonder if there was a gaping hole in the plug door inspection sign-off process that allowed the trim to be installed before the door inspection, unless someone falsified the inspection records..... That would be a pretty severe lapse in safety though.
All speculation at the moment, but having experience of manufacturing processes, I can imagine a few scenarios where assembly instructions didn't clearly indicate the hierarchy of processes i.e. that the interior trim should not be installed until the door was checked and signed off.
It there even was a process to check and sign off the door installation. Maybe it wasn't classed as a safety issue because it wasn't a safety exit? Maybe it didn't receive the appropriate scrutiny?
Another issue is were aviation experts involved in the writing of the assembly instructions? Were they aware of the severity of an in-flight door loss? Did they have enough experience, instruction or training to understand the repercussions of an insecure door plug?
The final aspect could be that the door didn't fit the frame correctly. That's a slim possibility because it's already been established the locking bolts are missing on the Alsaka Airplanes aircraft, but not impossible. There may not have been instructions concerning the fit of the door in the frame. It may have just been assumed it would be an accurate fit and there were no instructions regarding clearances between the plug and the frame.
Who knows. Only time will tell, but I get the impression so far that Boeing and Spirit will be let off once again with a slap on the wrist and no meaningful sanctions will be applied for this lapse in safety.
UPDATE 25/03/2024:
Looks like I was pretty close to the mark on this one. The plug in question was opened for remedial work and then closed again. Somehow the plug was closed without the important locking bolts being fitted at all. Even worse, an employee took a picture of the plug without the bolts in place, not realising the importance of the locking bolts to the safety of the aircraft!
So was the procedure to do the repair faulty? Was the step to replace the locking bolts not included? The employee looks to be happy to take a picture without the important bolts in place, so were they a member of the team that fitted the interior trim and weren't aware of the importance of the bolts? Did they even know of the existence of the bolts?
Boeing have allegedly failed to produce sufficient information regarding the closure of the door plug to the FAA. What appears to signal is either a lock of instruction for a procedure that wasn't planned for or a malicious disregard of instructions and a "just wing it" attitude at the factory.
I was wrong on the consequences for Boeing employees though. The CEO has just gone and board positions are being reshuffled. We'll see how that impacts aircraft safety. I suspect the effect will be minimal.
Stephanie Pope becomes the new CEO with immediate effect. With her coming from the financial and services sector, sadly it appears to confirm the emphasis on the board at Boeing is not engineering.
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