Sorry for the late response. We’re currently developing a way to make planes affected by altitude, because in aviation, aircraft become much more harder and unstable to handle the further they climb. This is why planes have service ceilings. Not only is handling affected, but engines that use combustion are affected, as air density decreases. As of now, they can go infinitely high without there being a barrier. If we put a barrier that is made of an invisible part, it may cause the aircraft to bug out, which is why we are avoiding the use of those physical boundaries.
Looking good bro! I don’t see any clouds though 
Overall everything looks amazing! I’d suggest improving the details in the cockpit to stand out with realism, preferably a mesh-based cockpit or higher-resolution decals. The atmosphere and environment look gorgeous for Roblox though!
Are you using mesh for the terrain? or are they decals?