Anyhow, great explanation. Which bring to other point, if regular airline can cruise in a higher altitude that have no turbulence why not keep the a/c cruise on that altitude all the time? I'm sure the pax will be much happier if the flight is smoother.
Is cost, i.e. need more fuel to cruise higher, the only consideration why a/c not cruise in higher altitude?
Disclaimer: I think I know what I'm talking about, but I may well bollocks it up...
If you want to cruise higher, you run into a phenomenon known as "coffin alley" or "coffin corner". For any given wing, the higher you go, the higher your stall speed (in ground speed). This is due to the fact that as the air gets thinner, airspeed will sink. At a certain altitude, airspeed will meet stall speed. So that's the lower end of the envelope. At the other end, as you go faster, you will encounted mach buffet. Since Mach speed decreases with altitude, the higher you go, the slower you have to go to avoid mach buffet.
Thus you have a situation where the higher you go, the smaller the window between mach buffet speed and stall speed. Big problem. Eventually you can go so high that mach buffet speed=stall speed, at which point your wing will no longer carry you. Of course, even before then any wind shear will take you past some edge of the envelope. You can solve this in two ways. Either go faster than Mach 1 (SR-71 option) or have very specialized wings (U-2/TR-2 option). But for airliners that would not be economically viable.
And I won't even start on engine issues at altitude.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo