variable interest rates. To the extent the interest rates applicable to our floating rate debt increase, our interest expense will increase, in which event we may have difficulties making interest payments and funding our other fixed costs, and our available cash flow for general corporate requirements may be adversely affected. On July 27, 2017, the Financial Conduct Authority (the authority that regulates LIBOR) announced that it intends to stop compelling banks to submit rates for the calculation of LIBOR after 2021. It is unclear whether new methods of calculating LIBOR will be established such that it continues to exist after 2021. The U.S. Federal Reserve, in conjunction with the Alternative Reference Rates Committee, is considering replacing U.S. dollar LIBOR with a newly created index, calculated based on repurchase agreements backed by treasury securities. It is not possible to predict the effect of these changes, other reforms or the establishment of alternative reference rates in the United Kingdom, the United States or elsewhere. See also the discussion of interest rate risk in Part II, Item 7A. Quantitative and Qualitative Disclosures About Market Risk – “Interest.” These obligations also impact our ability to obtain additional financing, if needed, and our flexibility in the conduct of our business, and could materially adversely affect our liquidity, results of operations and financial condition. Our business plan contemplates continued significant investments related to modernizing our fleet, improving the experience of our customers and updating our facilities. Significant capital resources will be required to execute this plan. We estimate that, based on our commitments as of December 31, 2018, our planned aggregate expenditures for aircraft purchase commitments and certain engines on a consolidated basis for calendar years 2019-2023 would be approximately $8.4 billion. Accordingly, we will need substantial financing or other capital resources to finance such aircraft and engines. If we are unable to arrange financing for such aircraft and engines at customary advance rates and on terms and conditions acceptable to us, we may need to use cash from operations or cash on hand to purchase such aircraft and engines or may seek to negotiate deferrals for such aircraft and engines with the applicable aircraft and engine manufacturers. Depending on numerous factors applicable at the time we seek capital, many of which are out of our control, such as the state of the domestic and global economies, the capital and credit markets’ view of our prospects and the airline industry in general, and the general availability of debt and equity capital, the financing or other capital resources that we will need may not be available to us, or may be available only on onerous terms and conditions. There can be no assurance that we will be successful in obtaining financing or other needed sources of capital to operate successfully. An inability to obtain necessary financing on acceptable terms would have a material adverse impact on our business, results of operations and financial condition. Our pension funding obligations are significant. The amount of these obligations will depend on the performance of investments held in trust by the pension plans, interest rates for determining liabilities and actuarial experience. The minimum funding obligation applicable to our pension plans was subject to favorable temporary funding rules that expired at the end of 2017 and, as a result, our minimum pension funding obligations will increase materially beginning in 2019. In addition, we may have significant obligations for other postretirement benefits, retiree medical and other postretirement benefits. We have agreements with companies that process customer credit card transactions for the sale of air travel and other services. These agreements allow these credit card processing companies, under certain conditions (including, with respect to certain agreements, the failure of American to maintain certain levels of liquidity), to hold an amount of our cash (a holdback) equal to some or all of the advance ticket sales that have been processed by that credit card processor, but for which we have not yet provided the air transportation. These credit card processing companies are not currently entitled to maintain any holdbacks pursuant to these requirements. These holdback requirements can be modified at the discretion of the credit card processing companies upon the occurrence of specific events, including material adverse changes in our financial condition. An increase in the current holdbacks, up to and including 100% of relevant advanced ticket sales, would materially reduce our liquidity. Likewise, other of our commercial agreements contain provisions that allow other entities to impose less-favorable terms, including the acceleration of amounts due, in the event of material adverse changes in our financial condition. If our financial condition worsens, provisions in our credit card processing and other commercial agreements may adversely affect our liquidity. We have significant pension and other postretirement benefit funding obligations, which may adversely affect our liquidity, results of operations and financial condition. We will need to obtain sufficient financing or other capital to operate successfully. 20