They correspond roughly to small municipal, medium-sized metropolitan and major metropolitan airports respectively, and encompass an increasingly rigorous set of regulations.
It was far from the centres of action in Egypt and Mesopotamia, where people were building the world's first cities, writing down its first epics, and waging its first organised wars.
An oil windfall since the main export pipeline began operations in 2006 has transformed an unloved Soviet city into a glitzy capital, often with locals evicted from their houses.