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.” We may in the future pursue amendments to our LIBOR-based debt transactions to provide for a transaction mechanism or other reference rate in anticipation of LIBOR’s discontinuation, but we may not be able to reach agreement with our lenders on any such amendments. As of December 31, 2019, we had $9.6 billion of borrowings based on LIBOR. The replacement of LIBOR with a comparable or successor rate could cause the amount of interest payable on our long-term debt to be different or higher than expected. 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, 2019, our planned aggregate expenditures for aircraft purchase commitments and certain engines on a consolidated basis for calendar years 2020-2024 would be approximately $8.1 billion. We may also require financing to refinance maturing obligations and to provide liquidity to fund other corporate requirements. Accordingly, we will need substantial financing or other capital resources to finance such aircraft and engines and meet such other liquidity needs. If we are unable to arrange such financing 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 or otherwise defer corporate obligations. 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. We believe that our future success will depend in large part on our ability to retain or attract highly qualified management, technical and other personnel. We may not be successful in retaining key personnel or in attracting other highly qualified personnel. Any inability to retain or attract significant numbers of qualified management and other personnel would have a material adverse effect on our business, results of operations and financial condition. Relations between air carriers and labor unions in the U.S. are governed by the RLA. Under the RLA, CBAs generally contain “amendable dates” rather than expiration dates, and the RLA requires that a carrier maintain the existing terms and conditions of employment following the amendable date through a multi-stage and usually lengthy series of bargaining processes overseen by the NMB. For the dates that the CBAs with our major work groups become amendable under the RLA, see Part I, Item 1. Business –  “Employees and Labor Relations .” In the case of a CBA that is amendable under the RLA, if no agreement is reached during direct negotiations between the parties, either party may request that the NMB appoint a federal mediator. The RLA prescribes no timetable for the direct negotiation and mediation processes, and it is not unusual for those processes to last for many months or even several years. If no agreement is reached in mediation, the NMB in its discretion may declare that an impasse exists and proffer binding arbitration to the parties. Either party may decline to submit to arbitration, and if arbitration is rejected by either party, a 30-day “cooling off” period commences. During or after that period, a PEB may be established, which examines the parties’ positions and recommends a solution. The PEB process lasts for 30 days and is followed by another 30-day “cooling off” period. At the end of this “cooling off” period, unless an agreement is reached or action is taken by Congress, the labor organization may exercise “self-help,” such as a strike, which could materially adversely affect our business, results of operations and financial condition. None of the unions representing our employees presently may lawfully engage in concerted slowdowns or refusals to work, such as strikes, sick- outs or other similar activity, against us. Nonetheless, there is a risk that employees, either with or without union involvement, could engage in one or more concerted refusals to work that could individually or collectively harm the operation of our airline and impair our financial performance. For example, last year we initiated litigation and obtained a permanent injunction after the unions representing our mechanics and other ground workers 20 Union disputes, employee strikes and other labor-related disruptions, or our inability to otherwise maintain labor costs at competitive levels may adversely affect our operations and financial performance. The loss of key personnel upon whom we depend to operate our business or the inability to attract additional qualified personnel could adversely affect our business. We will need to obtain sufficient financing or other capital to operate successfully.