A few thoughts:

- That door design is common to several of the larger and higher-density seating versions of the 737, including the MAX 9 and the 900ER. There are hundreds of these aircraft in service.

- It's likely that this was a production or maintenance issue rather than a design issue. There will undoubtedly be widespread inspections to ensure this isn’t a common problem. Regardless of the root cause, this is very concerning to the industry and traveling public.

- Since this incident occurred over land, in an area with significant population, the investigating authorities will search for (and likely find) the missing door. That will be a key to the investigation. I’ve supported past aviation investigations where military radars detected debris falling from aircraft and localized the ground impact points.

- ALWAYS wear your seatbelt when at your seat! Make sure it’s tight enough that you can’t move from beneath the belt, either up or down. You never know when turbulence or other incidents might occur.

- Copied from another forum I frequent, written by a Boeing retiree:

"The part that opened and fell off is called a mid exit door (Boeing calls it a MED for short). The door is hinged at the bottom, and opens from top to bottom. The door appears as a plug, and since it is intended to be an "open only" door or hatch, and when installed looks like just another part of the interior sidewall of the airplane, nobody would guess its a door except for the small lever that is covered by a small cover to open said door/hatch.

This mid exit door is installed on all 737-900ER and 737-9 Max airplanes, along with the high density version of the airplane for RyanAir and others for the model 737-8200 Max. The door is installed by Spirit on the fuselage prior to being shipped to Boeing. Typically this door is only opened only for maintenance or an emergency. I won't speculate on why this one opened, however prior to delivery these doors are checked by Boeing and the airline before delivery."

Greg