Its a known mod problem. We can't fix it, Flan has to do this.

Click on the shadow at the point where you placed the plane. The drivers / pilots position is not located at the seat, it's located at models 0 point, which is also the planes turning point. If we would place this point at the pilots seat, the flying ad turning behaivior would become very wired.

You need to craft them with guns, only then its working. If you pick a plane from the inventory in creative, then it is created without guns.

Planes require fuel in survival. Press "R" then place some in the fuel slot.

You have to PLACE the machine gun on a block, then it will automatically load. These are also the MGs used as Weapons in Planes.