The revenue obtained from the ownership, control and operation of any such airport or other air navigation facility shall be used, first, to finance the maintenance, improvement and operating expenses thereof and, second, to make payments of interest on and current principal requirements of any outstanding bonds or certificates issued for the acquisition or improvement thereof, and to make payment of interest on any mortgage heretofore made. Revenue in excess of the foregoing requirements may be applied to finance the extension or improvement of the airport or other air navigation facilities.