It is now reported that the reason the door plug blew out on that 737 MAX 9 is that Boeing workers at the factory failed to install the necessary bolts to hold it in place. This permitted the plug to gradually move upward out of its slot and then ultimately blow out. This also is the probable reason why that plane had a number of pressure warnings in preceding days, because air would have likely been leaking past the plug as it worked loose.
@lauren And they didn't ground it after pressure warnings?
Geezus. I'm looking forward for their justification of that decision being "Well, those pressure sensors are known-unreliable, so..."
I'd've been really surprised if the warning were about cabin pressure itself. I rather expect that they were of the form "the pressurization system has to work way to hard to maintain this pressure difference, something is likely leaking there". I'll be curious to read the NTSB report when it comes out.
To give my favorite anomaly example: A "[serious incident][https://assets.publishing.service.gov.uk/media/604f423be90e077fdf88493f/Boeing_737-8K5_G-TAWG_04-21.pdf]" was logged on a Tui flight in 2020 because the plane required more thrust to take off than anticipated. That's all: more thrust to take off, and more fuel burned in-flight than modeled. At no time was the plane anywhere near outside a safe operational envelope.
... but this kicked off an investigation because it was an anomaly in a well-controlled, well-understood portion of the flight system, and the conclusion was the airline booking system had inadvertently mis-classified every unmarried woman as a child (due to a cultural-skew error between the authors of the booking system and the users; "Miss" in the home country of the developers means 'young girl,' and in the UK it can be synonymous with 'Ms.')... resulting in the system mis-estimating the weight of the passengers by about 1.2 metric tonnes.
This is the kind of attention to the unexpected we want out of our commercial carriers.
(Bonus: an earlier variant of this report I can't lay hands on right now also cited pilot error, because apparently the captain, checking the passenger manifest, failed to around, look into the cabin, and note that the plane was not, in fact, half full of adult men and half full of girls under the age of thirteen 😁 ).
@robryk @mark That is by definition a pressure warning! Problems in the pressurization system. It doesn't only mean pressure is currently too low.