Impairment of payment.

Any law which authorizes the pledge of any or all of the net income and revenues to the payment of any bonds issued pursuant to the Municipal Airport Law or which affects any of the net income or revenues pledged to such bonds, or any law supplemental thereto or otherwise appertaining thereto, shall not be repealed or amended or otherwise directly or indirectly modified in such a manner as to impair adversely any such outstanding bonds unless such outstanding bonds have been discharged in full or provision has been fully made therefor.