737 Door problem - why it is not so easy to spot

 So a couple of things to consider. 

There are 4 variants of the 737 that have that extra door cut into the fuselage. 

• 737-900 
• 737Max9 
• 737 Max8-200.
• 737 Max10 (not yet in service) 

The door is required when the passenger capacity exceeds a certain number. However for commonality - Boeing installs the door into the frame for ALL of these variants whether or not the configuration uses them or not. In the case of most of the Max9s and the 900 the plug door is actually not used. 

These images are from seatguru showing the position of the affected door.
 

For reference - on the all coach version of LionAir's 737-900s - the door is at row 31 or on those ships with a Biz class it is on row 29. On the Ryanair Max8 200 the door is at row 28. All other versions have the door deactivated and plugged up and then it is covered over so that the door cannot be seen from the inside to have anything other than a misaligned window. On Delta's 900ERs that it mostly obtained from LionAir - the plug is at row 29/30. On all Alaska and United Max 9s, there is no internal clues.

 

This is how it looks on a Ryanair's different branded 737 Max8-200s, here it can be see on Malta Air's Max8-200, and similarly Ryanair and Buzz.  

 

Here is an example of a 737-900ER older model showing the position of the door without the split scimitar winglets that are distinguishable on the Maxs.  

From the ch-aviation database we can see that there are several airlines operating these aircraft - currently the operators of the 3 in service variants are: 

• Only 2 operators of the Max8-200, Akasa Air (India) and the 3 Ryanair brands. 
• For the Max 9 there are 10 operators, Alaska, Aeromexico, Copa, Correndon, FlyDubai, Icelandair, Lionair, SCAT, Turkish and United.  
• For the Boeing 737-900 there are currently 25 carriers for the standard and ER versions. 

The airframe fuselage barrels are manufactured by Spirit in Wichita KS and then is placed on rail cars that come to the Seattle area. Currently only to the Renton plant. Starting later this year a new FAL will also open up in Everett. The finger has been pointed to Spirit, but as with any of these issues - it is likely that there was a series of events that lead to this lucky escape. 

Disclaimer. I am neither an engineer nor a pilot. However, I have been involved in 737 Max aircraft for more than 5 years and have considerable experience with the craft.