Subject to such restrictions as may be contained in the ordinance authorizing the issuance of the bonds issued under this subchapter or other contract with the bondholders, as well as restrictions contained on contracts with the bondholders of other prior issues, all revenues derived from the operation and the lease of buildings, structures, or other improvements in excess of those required for payment of and interest upon the bonds, plus any reserve for contingencies, plus those funds required for operation and maintenance expenses for the buildings, structures, or other improvements, shall be used by the airport commission only for the purpose of expansion, maintenance, or operation of the airport or any of its facilities, buildings, or other improvements. This may include the payment of principal of and interest upon other bonds issued under this subchapter, or of other prior issues, and for no other purpose.