values. We record impairment losses on flight equipment and other long-lived assets used in operations when events and circumstances indicate the assets may 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) a decision to permanently remove flight equipment or other long-lived assets from operations, (2) significant changes in the estimated useful life, (3) significant changes in projected cash flows, (4) permanent and significant declines in fleet fair values and (5) changes to the regulatory environment. For long-lived assets held for sale, we discontinue depreciation and record impairment losses when the carrying amount of these assets is greater than the fair value less the cost to sell.  

 

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.  

 

Income Tax Valuation Allowance  

 

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 totaling $10.7 billion against our net deferred tax assets. If we determine that it is more likely than not that we will generate sufficient taxable income to realize our deferred income tax assets, we will reverse our valuation allowance (in full or in part), resulting in a significant income tax benefit in the period such a determination is made.  

 

 

Defined Benefit Pension Plans  

 

We sponsor defined benefit pension plans for eligible employees and retirees. These plans are closed to new entrants and frozen for future benefit accruals. As of December 31, 2011 , the unfunded benefit obligation for these plans recorded on our Consolidated Balance Sheet was $11.5 billion . During 2011 , we contributed $598 million to these plans and recorded $300 million of expense in salaries and related costs on our Consolidated Statement of Operations. In 2012 , we estimate we will contribute approximately $700 million to these