providing a revolving credit facility of $ 7.0 billion and expiring on october 17 , 2008 . interest on any amounts we borrow under these facilities would be charged at 90-day libor plus 15 basis points . at december 31 , 2007 , there were no outstanding borrowings under these facilities . our existing debt instruments and credit facilities do not have cross-default or ratings triggers , however these debt instruments and credit facilities do subject us to certain financial covenants . covenants in our credit facilities generally require us to maintain a $ 3.0 billion minimum net worth and limit the amount of secured indebtedness that may be incurred by the company . the notes issued in january 2008 include limitations on secured indebtedness and on sale-leaseback transactions . these covenants are not considered material to the overall financial condition of the company , and all applicable covenant tests were satisfied as of december 31 , commitments we have contractual obligations and commitments in the form of capital leases , operating leases , debt obligations , purchase commitments , and certain other liabilities . we intend to satisfy these obligations through the use of cash flow from operations . the following table summarizes the expected cash outflow to satisfy our contractual obligations and commitments as of december 31 , 2007 ( in millions ) : capital leases operating leases principal interest purchase commitments pension fundings liabilities .
what portion of total expected cash outflow for capital leases is due in 2018?
our capital lease obligations relate primarily to leases on aircraft . capital leases , operating leases , and purchase commitments , as well as our debt principal obligations , are discussed further in note 8 to our consolidated financial statements . the amount of interest on our debt was calculated as the contractual interest payments due on our fixed-rate debt , in addition to interest on variable rate debt that was calculated based on interest rates as of december 31 , 2007 . the calculations of debt interest do not take into account the effect of interest rate swap agreements . the maturities of debt principal and interest include the effect of the january 2008 issuance of $ 4.0 billion in senior notes that were used to reduce the commercial paper balance . purchase commitments represent contractual agreements to purchase goods or services that are legally binding , the largest of which are orders for aircraft , engines , and parts . in february 2007 , we announced an order for 27 boeing 767-300er freighters to be delivered between 2009 and 2012 . we also have firm commitments to purchase nine boeing 747-400f aircraft scheduled for delivery between 2008 and 2010 , and two boeing 747-400bcf aircraft scheduled for delivery during 2008 . these aircraft purchase orders will provide for the replacement of existing capacity and anticipated future growth . in july 2007 , we formally cancelled our previous order for ten airbus a380-800 freighter aircraft , pursuant to the provisions of an agreement signed with airbus in february 2007 . as a result of our cancellation of the airbus a380-800 order , we received cash in july 2007 representing the return of amounts previously paid to airbus as purchase contract deposits and accrued interest on those balances . additionally , we received a credit memorandum to be used by ups for the purchase of parts and services from airbus . the cancellation of the airbus order did not have a material impact on our financial condition , results of operations , or liquidity. .