be impaired and the estimated future cash flows generated by those assets are less than their carrying amounts. Factors which could cause impairment include, but are not limited to, (1) deciding to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) operational downsizing, (4) significant changes in the projected cash flows, (5) permanent and significant declines in fleet fair values and (6) changes to the regulatory environment. For long-lived assets held for sale, we record impairment losses when the carrying amount is greater than the fair value less the cost to sell. We discontinue depreciation of long-lived assets when these assets are classified as held for sale.  

     To determine whether impairments exist for aircraft used in operations, we group assets at the fleet-type level (the lowest level for which there are identifiable cash flows) and then estimate future cash flows based on projections of capacity, passenger mile yield, fuel costs, labor costs and other relevant factors. If an impairment occurs, the impairment loss recognized is the amount by which the aircraft’s carrying amount exceeds its estimated fair value. We estimate aircraft fair values using published sources, appraisals and bids received from third parties, as available. For additional information, see Note 1 of the Notes to the Consolidated Financial Statements.  

Income Tax Valuation Allowance and Contingencies  

     We periodically assess whether it is more likely than not that we will generate sufficient taxable income to realize our deferred income tax assets and establish valuation allowances if it is not likely we will realize our deferred income tax assets. In making this determination, we consider all available positive and negative evidence and make certain assumptions. We consider, among other things, our deferred tax liabilities, the overall business environment, our historical financial results, our industry’s historically cyclical financial results and potential, current and future tax planning strategies. We cannot presently determine when we will be able to generate sufficient taxable income to realize our deferred tax assets. Accordingly, we have recorded a full valuation allowance against our net deferred tax assets.  

     Our income tax provisions are based on calculations and assumptions that are subject to examination by the Internal Revenue Service and other taxing authorities. Although the positions we have taken on previously filed tax returns are reasonable, we have established tax and interest reserves in recognition that taxing authorities may challenge these positions, which could result in additional liabilities for taxes and interest. We review and adjust the reserves as circumstances warrant and events occur, such as lapsing of applicable statutes of limitations, conclusion of tax audits, a change in exposure based on current calculations, identification of new issues, release of administrative guidance or the rendering of a court decision affecting a particular issue. We adjust the income tax provision in the period in which the facts that