Whenever revenue bonds are issued under section 10 of this Act, all revenue derived from the operation of such airport, landing field, and facilities or appurtenances thereof, shall be set aside as collected and shall be deposited in a separate fund designated as the “Airport Fund” of the county. This fund shall be used only in paying the cost of operation and maintenance of the airport or landing field, in providing an adequate depreciation fund, and in paying the principal of and interest upon the revenue bonds issued under section 10 of this Act.