Thank you trpgpilot. That's a very good start. But at what point does the pressure differential become significant? It's not going to be a specific altitude if only because of high altitude airports (which is a subject on its' own - what happens when you start from somewhere like La Paz Bolivia?). So, at what rate does the cabin altitude increase when the take off is from sea level? Does it just follow the aircraft altitude until somewhere around 8000ft and then stay ay 8000ft or if, for example you've got to 5000ft is the cabin going to be at 5000ft or something lower?