hub system also allows us to add service to a new destination from a large number of cities using only one or a limited number of aircraft. As discussed underAlliances below, United is a member of Star Alliance, the world’sworld's largest alliance network.

Financial information

venturesJBAs with ANA for transpacific cargo services and continues to implement a cargo joint venture with Lufthansa for transatlantic cargo services. These cargo joint venturesJBAs offer expanded and more seamless access to cargo space across the carriers’carriers' respective combined networks.

primarily sold using global distribution systems (“GDS”("GDS"). United has developed and expects to continue to develop capabilities to sell certain ancillary products through the GDS channel to provide an enhanced buying experience for customers who purchase in that channel. To increase

launched investigations or claimed rulemaking authority to regulate commercial agreements among carriers or between carriers and third parties in a wide variety of contexts.

Certain states mayhold responsible parties liable under CERCLA's joint and several liability scheme. The rule, if finalized, would also electrequire the Company to impose restrictions apartimmediately report releases that meet or exceed the reportable quantity of PFOA or PFOS to the EPA and any other applicable state and local agencies. The Company expects these broad regulatory policies will increase the risk of incurring remediation costs and/or liabilities at current and former locations at which the Company currently or historically used fire-fighting foam agents containing PFOA, PFOS or other PFAS substances. To mitigate these risks, the Company is working to remove PFAS-containing fire-fighting foam from the revised national standards.its hangars and other assets through a phased retrofit/replacement strategy and is committed to transitioning to PFAS-free materials for fire suppression. Finally, environmental cleanup laws and lease obligations could require the Company to undertake or(or subject the Company to liability for costs associated with) investigation and remediation costsactions at certain owned or leased locations or third partythird-party disposal locations.

Because PFOA, PFOS and other PFAS substances are expected to be regulated under CERCLA and have been regulated under other environmental cleanup laws, the Company may become subject to potential liability for its historic usage of PFAS-containing materials. Until the applicability of new regulations to our specific operations is better defined and/or until pending regulations are finalized, future costs to comply with such regulations will remain uncertain but are likely to increase our operating costs over time.

fueling operations, and catering services, among other vital functions and services. The Company does not directly control these third-party service providers, although it does enter into agreements with most of them that define expected service performance. Any of these third-party service providers, however, may materially fail to meet their service performance commitments to the Company, may suffer disruptions to their systems that could impact their services, or the agreements with such providers may be terminated. For example, flight reservations booked by customers and travel agencies via third-party GDSs may be adversely affected by disruptions in the business relationships between the Company and GDS operators. Such disruptions, including a failure to agree upon acceptable contract terms when contracts expire or otherwise become subject to renegotiation, may cause the Company’s flight information to be limited or unavailable for display, significantly increase fees for both the Company and GDS users and impair the Company’s relationships with its customers and travel agencies. The failure of any of the Company’s third-party service providers to perform their service obligations adequately, or other interruptions of services, may reduce the Company’s revenues and increase its expenses, prevent the Company from operating its flights and providing other services to its customers or result in adverse publicity or harm to its brand. In addition, the Company’s business and financial performance could be materially harmed if its customers believe that its services are unreliable or unsatisfactory.

Orders for new aircraft typically must be placed years in advance of scheduled deliveries, and changes in the Company’s route network over time may make aircraft on orderfly less economic for the Company, but any modification or termination of such orders could result in material liability for the Company.

The Company’s orders for new aircraft are typically made years in advance of actual delivery of such aircraft, and the financial commitment required for purchases of new aircraft is substantial. At December 31, 2017, the Company had firm commitments to purchase 228 new aircraft from The Boeing Company (“Boeing”) and Airbus S.A.S (“Airbus”), as well as related agreements with engine manufacturers, maintenance providers and others. At December 31, 2017, the Company’s commitments relating to the acquisition of aircraft and related spare engines, aircraft improvements and other related obligations aggregated to a total of $22.2 billion.

Subsequent to the Company placing an order for new aircraft, the Company’s route network may change, such that the aircraft on order become less economic to operate flights in the Company’s network. As a result, the Company’s preference for a particular aircraft that it has ordered, often years in advance, may be decreased or eliminated. If the Company were to seek to modify or terminate its existing aircraft order commitments, it may be responsible for material obligations to its counterparties arising from any such change. However, the Company expects that any such change that it makes would be in the long-term best economic interest of the Company.

The Company could experience adverse publicity, harm to its brand, reduced travel demand and potential tort liabilityfrequently as a result of negative perception of the environmental impact of air travel or fly on an accident, catastrophe,airline based on carriers' GHG emissions or incident involvingwhich carrier they perceive as operating in a manner that is more sustainable to the climate, which presents both a challenge and an opportunity for the Company and is why the Company is resolute in attaining its aircraftmid-term and long-term climate goals; if this trend materializes, the Company's results of operations could be adversely impacted and those impacts could be exacerbated if the Company fails to meet or properly report on its operations,climate change goals and commitments. Moreover, we could also be subject to climate litigation, as groups, individuals, and governmental authorities affected by climate change seek to recover climate-related damages from entities they perceive as being partially responsible for human-induced climate change because of the aircraftemission of GHGs from their operations.

Additionally, deterioration in the Company’sCompany's financial condition could negatively affect its ability to enter into new hedge contracts in the future.

Union disputes, employee strikes or slowdowns, and other labor-related disruptions could adversely affect the Company’s operations and could result in increased costs that impair its financial performance.

United is a highly unionized company. As of December 31, 2017, the Company and its subsidiaries had approximately 89,800 active employees, of whom approximately 80% were represented by various U.S. labor organizations.

There is a risk that unions or individual employees might pursue judicial or arbitral claims arising out of changes implemented as a result of the Company entering into collective bargaining agreements with its represented employee groups. There is also a possibility that employees or unions could engage in job actions such as slowdowns,work-to-rule campaigns, sick-outs or other actions designed to disrupt the Company’s normal operations, in an attempt to pressure the Company in collective bargaining negotiations. Although the RLA makes such actions unlawful until the parties have been lawfully released to self-help, and the Company can seek injunctive relief against premature self-help, such actions can cause significant harm even if ultimately enjoined. In addition, collective bargaining agreements with the Company’s represented employee groups increase the Company’s labor costs, which increase could be material for any applicable reporting period.

An outbreak of a disease or similar public health threat could have a material adverse impact on the Company’s business, financial position and results of operations.

An outbreak of a disease or similar public health threat that affects travel demand, travel behavior, or travel restrictions could have a material adverse impact on the Company’s business, financial condition and results of operations.

Extensive government regulation could increase the Company’s operating costs and restrict its ability to conduct its business.

Airlines are subject to extensive regulatory and legal oversight. Compliance with U.S. and international regulations imposes significant costs and may have adverse effects on the Company. Laws, regulations, taxes and airport rates and charges, both domestically and internationally, have been proposed from time to time that could significantly increase the cost of airline operations or reduce airline revenue.

United provides air transportation under certificates of public convenience and necessity issued by the DOT. If the DOT altered, amended, modified, suspended or revoked these certificates, it could have a material adverse effect on the Company’s business. The FAA regulates the safety of United’s operations. United operates pursuant to an air carrier operating certificate issued by the FAA. In 2014, the FAA’s more stringent pilot flight and duty time requirements under Part 117 of the Federal Aviation Regulations took effect, which has increased costs for all carriers. Additionally, minimum qualifications took effect for air carrier first officers. These regulations will continue to impact the industry and the Company for years to come, as they have caused mainline airlines to hire regional pilots, while simultaneously significantly reducing the pool of new pilots from which regional carriers themselves can hire. Although this is an industry issue, it directly affects the Company and requires it to reduce regional partner flying, as several regional partners have experienced difficulty flying their schedules due to reduced pilot availability. From time to time, the FAA also issues orders, airworthiness directives and other regulations relating to the maintenance and operation of aircraft that require material expenditures or operational restrictions by the Company. These FAA orders and directives could include the temporary grounding of an entire aircraft type if the FAA identifies design, manufacturing, maintenance or other issues requiring immediate corrective action. These FAA directives or requirements could have a material adverse effect on the Company.

In 2018, the U.S. Congress will continue to consider legislation to reauthorize the FAA, which encompasses all significant aviation tax and policy related issues. As with previous reauthorization legislation, the U.S. Congress may consider a range of policy changes that could impact the Company’s operations and costs.

material adverse impact on the Company’s financial position and results of operations and could result in the impairment of material amounts of related tangible and intangible assets. In addition, competition from revenue-sharing joint ventures and other alliance arrangements by and among other airlines could impair the value of the Company’s business and assets on the Open Skies routes. The Company’s plans to enter into or expand U.S. antitrust immunized alliances and joint ventures on various international routes are subject to receipt of approvals from applicable U.S. federal authorities and obtaining other applicable foreign government clearances or satisfying the necessary applicable regulatory requirements. There can be no assurance that such approvals and clearances will be granted or will continue in effect upon further regulatory review or that changes in regulatory requirements or standards can be satisfied.

See Part I, Item 1, Business—Industry Regulation, of this report for additional information on government regulation impacting the Company.

The airline industry may undergo further change with respect to alliances and joint ventures or due to consolidations, any of which could have a material adverse effect on the Company.

The Company faces and may continue to face strong competition from other carriers due to the modification of alliances and formation of new joint ventures. Carriers may improve their competitive positions through airline alliances, slot swaps and/or joint ventures. Certain types of airline joint ventures further competition by allowing multiple airlines to coordinate routes, pool revenues and costs, and enjoy other mutual benefits, achieving many of the benefits of consolidation. “Open Skies” agreements, including the agreements between the United States and the EU and between the United States and Japan, may also give rise to better integration opportunities among international carriers. Movement of airlines between current global airline alliances could reduce joint network coverage for members of such alliances while also creating opportunities for joint ventures and bilateral alliances that did not exist before such realignment. There is ongoing speculation that further airline and airline alliance consolidations or reorganizations could occur in the future, especially if new “Open Skies” agreements between Brazil and the United States are fully implemented. The Company routinely engages in analysis and discussions regarding its own strategic position, including current and potential alliances, asset acquisitions and divestitures and may have future discussions with other airlines regarding strategic activities. If other airlines participate in such activities, those airlines may significantly improve their cost structures or revenue generation capabilities, thereby potentially making them stronger competitors of the Company and potentially impairing the Company’s ability to realize expected benefits from its own strategic relationships.

Insufficientinsufficient liquidity may have a material adverse effect on the Company’sCompany's financial positioncondition and business.

In addition, our financing agreements contain other negative covenants customary for such financings. If the Company does not timely pay its leases and debts orwe fail to comply with suchthese covenants and are unable to remedy or obtain a variety of adverse consequences could result. These potential adverse consequences includewaiver or amendment, an increase of required reserves under credit card processing agreements, withholding of credit card sale proceeds by its credit card service providers, loss of undrawn lines of credit, the occurrence of one or more eventsevent of default underwould result.

The Company may be required to recognize impairments in the future due to, among other factors, extreme fuel price volatility, tight credit markets, a decline in the fair value of certain tangible or intangible assets, unfavorable trends in historical or forecasted results of operations and cash flows and an uncertain economic environment, as well as other uncertainties. The Company can provide no assurance that a material impairment charge of tangible or intangible assets will not occur in a future period. The value of the Company’s aircraft could be impacted in future periods by changes in supply and demand for these aircraft. Such changes in supply and demand for certain aircraft types could result from grounding of aircraft by the Company or other carriers. An impairment charge could have a material adverse effect on the Company’s financial position and results of operations.

The Company’s ability to use its net operating loss carryforwards to offset future taxable income for U.S. federal income tax purposes may be significantly limited due to various circumstances, including certain possible future transactions involving the sale or issuance of UAL common stock,such insurance increase materially, or if taxable income does not reach sufficient levels.

As of December 31, 2017, UAL reported consolidated federal net operating loss (“NOL”) carryforwards of approximately $2.4 billion.

The Company’s abilitythe coverage obtained is unable to use its NOL carryforwards may be limited if it experiences an “ownership change” as defined in Section 382 (“Section 382”) of the Internal Revenue Code of 1986, as amended (the “Code”). An ownership change generally occurs if certain stockholders increase their aggregate percentage ownership of a corporation’s stock by more than 50 percentage points over their lowest percentage ownership at any time during the testing period, whichpay or is generally the three-year period preceding any potential ownership change.

There is no assuranceinsufficient relative to actual liability or losses that the Company will not experience a future ownership change under Section 382 that may significantly limitexperiences, whether due to insurance market conditions, policy limitations and exclusions or possibly eliminate its ability to use its NOL carryforwards. Potential future transactions involving the sale or issuance of UAL common stock, including the exercise of conversion options under the terms of any convertible debt that UAL may issue in the future, the repurchase of such debt with UAL common stock, any issuance of UAL common stock for cash,otherwise, our business, operating results and the acquisition or disposition of such stock by a stockholder owning 5% or more of UAL common stock, or a combination of such transactions, may increase the possibility that the Company will experience a future ownership change under Section 382.

Under Section 382, a future ownership change would subject the Company to additional annual limitations that apply to the amount ofpre-ownership change NOLs that may be used to offset post-ownership change taxable income. This limitation is generally determined by multiplying the value of a corporation’s stock immediately before the ownership change by the applicable long-termtax-exempt rate. Any unused annual limitation may, subject to certain limits, be carried over to later years, and the limitation may, under certain circumstances, be increased bybuilt-in gains in the assets held by such corporation at the time of the ownership change. This limitation could cause the Company’s U.S. federal income taxes to be greater, or to be paid earlier, than they otherwise would be, and could cause all or a portion of the Company’s NOL carryforwards to expire unused. Similar rules and limitations may apply for state income tax purposes. The Company’s ability to use its NOL carryforwards will also depend on the amount of taxable income it generates in future periods. The Company’s NOL carryforwards may expire before it can generate sufficient taxable income to use them in full.

The final impacts of the Tax Cuts and Jobs Actfinancial condition could be materially different from our current estimates.

On December 22, 2017, the Tax Cuts and Jobs Act was signed into law (the “Tax Act”). The Tax Act introduced significant changes to the Code. We continue to examine the impact the Tax Act may have on our business. Notwithstanding the reduction in the federal corporate income tax rate as a result of Tax Act, the estimated impact of the new law is based on management’s current knowledge and assumptions and recognized impacts could be materially different from current estimates based upon our further analysis of the new law.

Our significant investments in airlines in other parts of the world and the commercial relationships that we have with those carriers may not produce the returns or results we expect.

An important part of our strategy to expand our global network includes making significant investments in airlines in other parts of the world and expanding our commercial relationships with these carriers. In 2015, we made a $100 million investment in Azul Linhas Aéreas Brasileiras S.A. (“Azul”) and enhanced our commercial arrangements with Azul. We expect to continue exploring similarnon-controlling investments in, and entering into joint ventures, commercial agreements, loan transactions and strategic alliances with, other carriers as part of our global business strategy. These transactions and relationships (including our strategic partnership with, and investment in, Azul) involve significant challenges and risks, including that we may not realize a satisfactory return on our investment, that we may not receive repayment of invested funds, that they may distract management from our operations or that they may not generate the expected revenue synergies. These events could have a material adverse effect on our operating results or financial condition.

In addition, we are dependent on these other carriers for significant aspects of our network in the regions in which they operate. While we work closely with these carriers, we do not have control over their operations or business methods. We may be subject to consequences from any improper behavior of joint venture partners, including for failure to comply with anti-corruption laws such as the U.S. Foreign Corrupt Practices Act. Furthermore, our relationships with these carriers may be subject to the laws and regulations ofnon-U.S. jurisdictions in which these carriers are located or conduct business. Any political or regulatory change in these jurisdictions that negatively impact or prohibit our arrangements with these carriers could have an adverse effect on our results of operations or financial condition. To the extent that the operations of any of these carriers are disrupted over an extended period of time or their actions subject us to the consequences of failure to comply with laws and regulations, our results of operations may be adversely affected.