620 ILCS 50/18
Each county of this State having less than 500,000 population, in the manner hereinafter provided is hereby authorized to acquire by purchase or otherwise, to construct, improve, repair, maintain and operate an airport or a system of airports to facilitate travel by aircraft between the principal cities and trading points in each county with each other, and also with the principal cities and trading points in other counties and shall have authority to exercise all such powers within its boundaries and in adjacent territory within this state not in excess of five miles from the nearest boundary line thereof, and in any adjoining state, after first having obtained authority if any be necessary, from the United States or such adjoining state or both, to charge reasonable fees and rents for the use of such airports, and to provide the means therefor by levying a tax and borrowing money. Every county operating under this Act in order to secure greater working efficiency may convey the property acquired for airport purposes, or any part thereof, or any interest therein to any other county or municipality adjacent thereto for public airport purposes upon such terms and conditions as the respective governing bodies shall agree by a two-thirds vote of all members of each of such governing bodies. Adjoining counties may jointly exercise all the power and authority conferred upon a county by this Act, in the manner hereinafter provided.
620 ILCS 50/18