The aircraft will not return.

what were the issues? out of curiosity

As I understand it, the developers back then wanted to reduce the scale of everything by 1/8 so that the aircraft had 8 times as much room to work with. They were not able to fix the physics bugs they encountered by doing this.

I haven't seen any internal discussion about restarting development work on aircraft. There are a lot of other issues to work out, and if we did implement aircraft it's likely that all of the maps we have now would require changes or reworks in additional to all of the other art assets and code required to make aircraft work.

Maybe in the Unreal Engine, but it seems development has stopped.