Every airplane has what is called a service celling this is the max altitude that this plane can fly at safely. But as far as what the max altitude that will display I am not sure. Try any plane press the y key to go into slew mode then press and hold the Q key this will make the plane climb then see how high you can get it to go.
To get back down while still in slew mode just press F1 and hold on to your stomach as this will put you on the ground in a hurry 8)

Yeah, 100,000 feet is maximum, i´ve reached it once, you can´t just get higher, the plane might fly 80° upwards, but the smoke trails are just horizontal.

I think it´s not that good, that they have done it that way, would have been cool with a shuttle otherwise...

thanks

I was just wondering because I have the shutle and the booster
rockets