738 will go up to 51,000 ft if the climb is extremely slow and weights are at a
Minimum. Tried it in
You guys have a lot of time to kill in your sims

Had a look at your pressurization differential by any chance?interested..
Concerning the root stall thingy,boeing prefers it that way..so much so they added stall strips there,i dont blame them..i rather stall wings level than stalling a tip.
