I'm sure one of the experts will be along shortly, but here is my understanding:
MCrit is the Mach number at which a shockwave first forms on a machine. It is largely of theoretical interest only. Small shocks form on the upper wing surface and leading faces first. The 'biggie' is the full bow wave shock which breaks greenhouses and frightens horses. Don't I know it!
The BAC Lightning MCrit was around 0.95M (?canopy shock?) in S&L flight but a healthy boom could be dropped at 0.95M by pulling lots of 'g'. I seem to remember the bow shock passed the tip of the pitot probe at around 0.98M. Shockwave effects were virtually undetectable due to its EXCELLENT British design!
A 'flicker' of the Mach indicator and reduced roll rate - that's all! (Oh, and not much endurance.....
)
Hence 'Fat Albert' (737OG), being, well... fat.... has a low MCrit. I suspect 'Crossunder's' 0.615M is not far off the mark? Mmo is well above this, as this is the limit placed by the manufacturers at which shockwaves become a 'nuisance'. Mmo is a
maximum of 0.82M on the 737OG, but adverse Mach effects will be felt well below that, down at 0.78 on a heavy aircraft if significant manoeuvre is made.
I supect MCrit for Concorde would be, at a guess, around 0.9M probably from the engine nacelles, but again I'm sure someone who knows will be along to correct me.