SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES

Basis of Presentation

Delta Air Lines, Inc., a Delaware corporation, provides scheduled air transportation for passengers and cargo throughout the United States (“U.S.”) and around the world. Our Consolidated Financial Statements include the accounts of Delta Air Lines, Inc. and our wholly-owned subsidiaries and have been prepared in accordance with accounting principles generally accepted in the U.S. (“GAAP”). We do not consolidate the financial statements of any company in which we have an ownership interest of 50% or less. We are not the primary beneficiary of, nor do we have a controlling financial interest in, any variable interest entity. Accordingly, we have not consolidated any variable interest entity.

We have marketing alliances with other airlines to enhance our access to domestic and international markets. These arrangements may include codesharing, reciprocal frequent flyer program benefits, shared or reciprocal access to passenger lounges, joint promotions, common use of airport gates and ticket counters, ticket office co-location and other marketing agreements. We have received antitrust immunity for certain marketing arrangements, which enables us to offer a more integrated route network and develop common sales, marketing and discount programs for customers. Some of our marketing arrangements provide for the sharing of revenues and expenses. Revenues and expenses associated with collaborative arrangements are presented on a gross basis in the applicable line items on our Consolidated Statements of Operations.

We reclassified certain prior period amounts to conform to the current period presentation. Unless otherwise noted, all amounts disclosed are stated before consideration of income taxes.

Use of Estimates

We are required to make estimates and assumptions when preparing our Consolidated Financial Statements in accordance with GAAP. These estimates and assumptions affect the amounts reported in our Consolidated Financial Statements and the accompanying notes. Actual results could differ materially from those estimates.

Recent Accounting Standards

Revenue from Contracts with Customers

In May 2014, the Financial Accounting Standards Board ("FASB") issued Accounting Standards Update ("ASU") No. 2014-09, "Revenue from Contracts with Customers." Under the new standard, revenue is recognized at the time a good or service is transferred to a customer for the amount of consideration received for that specific good or service. Entities may use a full retrospective approach or report the cumulative effect as of the date of adoption.

In August 2015, the FASB issued ASU No. 2015-14, "Revenue from Contracts with Customers (Topic 606): Deferral of the Effective Date," that deferred the effective date by one year to December 15, 2017 for interim and annual reporting periods beginning after that date. The FASB permitted early adoption of the standard, but not before the original effective date of December 15, 2016. We are currently evaluating how the adoption of this standard will impact our Consolidated Financial Statements.

Presentation of Debt Issuance Costs

During 2015, the FASB issued ASU Nos. 2015-03 and 2015-15, related to simplifying the presentation of debt issuance costs. These standards amend existing guidance to require the presentation of debt issuance costs in the balance sheet as a deduction from the carrying amount of the related debt liability instead of as a deferred charge. It is effective for annual reporting periods beginning after December 15, 2015, but early adoption is permitted. We early adopted these standards in the December 2015 quarter, which we accounted for as a change in accounting principle and applied the guidance retrospectively. This change results in a reclassification of $32 million from prepaid expenses and other to current maturities of long-term debt and capital leases and $84 million from other noncurrent assets to long-term debt and capital leases as of December 31, 2014.

Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share

In May 2015, the FASB issued ASU No. 2015-07, "Disclosures for Investments in Certain Entities That Calculate Net Asset Value per Share (or Its Equivalent)." Under the new standard, investments for which fair value is measured at net asset value per share (or its equivalent) using the practical expedient will no longer be categorized in the fair value hierarchy. It is effective for interim and annual reporting periods beginning after December 15, 2015, but early adoption is permitted. We will adopt this standard effective January 1, 2016. We are currently assessing how the adoption of this standard will impact our Consolidated Financial Statements as our pension plan has a significant number of investments measured at net asset value.

Balance Sheet Classification of Deferred Taxes

In November 2015, the FASB issued ASU No. 2015-17, "Balance Sheet Classification of Deferred Taxes." This standard requires that deferred tax liabilities and assets be classified as noncurrent on the balance sheet. It is effective for interim and annual periods beginning after December 15, 2016, but early adoption is permitted. We early adopted this standard in the December 2015 quarter, which we accounted for as a change in accounting principle and applied the guidance retrospectively. This change results in the reclassification of $3.3 billion from current assets to deferred income taxes, net in other assets on our December 31, 2014 Consolidated Balance Sheet.

Financial Instruments

In January 2016, the FASB issued ASU No. 2016-01, "Financial Instruments—Overall (Subtopic 825-10)." This standard makes several modifications to Subtopic 825-10 including the elimination of the available-for-sale classification of equity investments, and requires equity investments with readily determinable fair values to be measured at fair value with changes in fair value recognized in net income. It is effective for interim and annual periods beginning after December 15, 2017.

Our investments in the parent companies of Aeroméxico and GOL are currently accounted for as available-for-sale with changes in fair value recognized in other comprehensive income. At the time of adoption, any amounts in accumulated other comprehensive income/(loss) ("AOCI") related to equity investments would be reclassified to non-operating expense/income. As of December 31, 2015, a net unrealized loss of $66 million related to these investments was recorded in AOCI on our Consolidated Balance Sheet.

Cash and Cash Equivalents and Short-Term Investments

Short-term, highly liquid investments with maturities of three months or less when purchased are classified as cash and cash equivalents. Investments with maturities of greater than three months, but not in excess of one year, when purchased are classified as short-term investments. Investments with maturities beyond one year when purchased may be classified as short-term investments if they are expected to be available to support our short-term liquidity needs. All short-term investments are classified as either available-for-sale or held-to-maturity and realized gains and losses are recorded using the specific identification method.

Accounts Receivable

Accounts receivable primarily consist of amounts due from credit card companies from the sale of passenger airline tickets, customers of our aircraft maintenance and cargo transportation services and other companies for the purchase of mileage credits under our frequent flyer program (the "SkyMiles Program"). We provide an allowance for uncollectible accounts equal to the estimated losses expected to be incurred based on historical chargebacks, write-offs, bankruptcies and other specific analyses. Bad debt expense was not material in any period presented.

Inventories

Spare Parts. Inventories of expendable parts related to flight equipment, which cannot be economically repaired, reconditioned or reused after removal from the aircraft, are carried at moving average cost and charged to operations as consumed. An allowance for obsolescence is provided over the remaining useful life of the related fleet. We also provide allowances for parts identified as excess or obsolete to reduce the carrying costs to the lower of cost or net realizable value. These parts are assumed to have an estimated residual value of 5% of the original cost.

Refinery. Refined product, feedstock and blendstock inventories, all of which are finished goods, are carried at recoverable cost. We use jet fuel produced by the refinery and procured through the exchange with third parties of gasoline, diesel and other refined products ("non-jet fuel products") the refinery produces in our airline operations. Cost is determined using the first-in, first-out method. Costs include the raw material consumed plus direct manufacturing costs (such as labor, utilities and supplies) as incurred and an applicable portion of manufacturing overhead.

Accounting for Refinery Related Buy/Sell Agreements

To the extent that we receive jet fuel for non-jet fuel products exchanged under buy/sell agreements, we account for these transactions as nonmonetary exchanges. We have recorded these nonmonetary exchanges at the carrying amount of the non-jet fuel products transferred within aircraft fuel and related taxes on the Consolidated Statements of Operations.

Derivatives

Changes in aircraft fuel prices, interest rates and foreign currency exchange rates impact our results of operations. In an effort to manage our exposure to these risks, we enter into derivative contracts and adjust our derivative portfolio as market conditions change. We recognize derivative contracts at fair value on our Consolidated Balance Sheets.

Not Designated as Accounting Hedges. We do not designate our fuel derivative contracts as accounting hedges. We record changes in the fair value of our fuel hedges in aircraft fuel and related taxes. These changes in fair value include settled gains and losses as well as mark-to-market adjustments ("MTM adjustments"). MTM adjustments are defined as fair value changes recorded in periods other than the settlement period. Such fair value changes are not necessarily indicative of the actual settlement value of the underlying hedge in the contract settlement period.

Designated as Cash Flow Hedges. For derivative contracts designated as cash flow hedges (interest rate contracts and foreign currency exchange contracts), the effective portion of the gain or loss on the derivative is reported as a component of AOCI and reclassified into earnings in the same period in which the hedged transaction affects earnings. The effective portion of the derivative represents the change in fair value of the hedge that offsets the change in fair value of the hedged item. To the extent the change in the fair value of the hedge does not perfectly offset the change in the fair value of the hedged item, the ineffective portion of the hedge is immediately recognized in other expense.

Designated as Fair Value Hedges. For derivative contracts designated as fair value hedges (interest rate contracts), the gain or loss on the derivative is reported in earnings and an equivalent amount is reflected as a change in the carrying value of long-term debt and capital leases, with an offsetting loss or gain recognized in current earnings. We include the gain or loss on the hedged item in the same account as the offsetting loss or gain on the related derivative contract, resulting in no impact to our Consolidated Statements of Operations.

The following table summarizes the risk each type of derivative contract is hedging and the classification of related gains and losses on our Consolidated Statements of Operations:



Derivative Type	Hedged Risk	Classification of Gains and Losses
Fuel hedge contracts	Increases in jet fuel prices	Aircraft fuel and related taxes
Interest rate contracts	Increases in interest rates	Interest expense, net
Foreign currency exchange contracts	Fluctuations in foreign currency exchange rates	Passenger revenue

The following table summarizes the accounting treatment of our derivative contracts:



	Impact of Unrealized Gains and Losses
Accounting Designation	Effective Portion	Ineffective Portion
Not designated as hedges	Change in fair value of hedge is recorded in earnings
Designated as cash flow hedges	Market adjustments are recorded in AOCI	Excess, if any, over effective portion of hedge is recorded in other expense
Designated as fair value hedges	Market adjustments are recorded in long-term debt and capital leases	Excess, if any, over effective portion of hedge is recorded in other expense

We perform, at least quarterly, an assessment of the effectiveness of our derivative contracts designated as hedges, including assessing the possibility of counterparty default. If we determine that a derivative is no longer expected to be highly effective, we discontinue hedge accounting prospectively and recognize subsequent changes in the fair value of the hedge in earnings. We believe our derivative contracts that continue to be designated as hedges, consisting of interest rate and foreign currency exchange contracts, will continue to be highly effective in offsetting changes in fair value or cash flow, respectively, attributable to the hedged risk.

Cash flows associated with purchasing and settling hedge contracts generally are classified as operating cash flows. However, if a hedge contract includes a significant financing element at inception, cash flows associated with the hedge contract are recorded as financing cash flows.

Hedge Margin. In accordance with our fuel, interest rate and foreign currency hedge contracts, we may require counterparties to fund the margin associated with our gain position and/or counterparties may require us to fund the margin associated with our loss position on these contracts. The amount of the margin, if any, is periodically adjusted based on the fair value of the hedge contracts. The margin requirements are intended to mitigate a party's exposure to the risk of contracting party default. We do not offset margin funded to counterparties or margin funded to us by counterparties against fair value amounts recorded for our hedge contracts.

The hedge margin we receive from counterparties is recorded in cash and cash equivalents or prepaid expenses and other, with the offsetting obligation in accounts payable. The hedge margin we provide to counterparties is recorded in hedge margin receivable.

Passenger Tickets

We record sales of passenger tickets in air traffic liability. Passenger revenue is recognized when we provide transportation or when the ticket expires unused, reducing the related air traffic liability. We periodically evaluate the estimated air traffic liability and record any adjustments in our Consolidated Statements of Operations. These adjustments relate primarily to refunds, exchanges, transactions with other airlines and other items for which final settlement occurs in periods subsequent to the sale of the related tickets at amounts other than the original sales price.

Passenger Taxes and Fees

We are required to charge certain taxes and fees on our passenger tickets, including U.S. federal transportation taxes, federal security charges, airport passenger facility charges and foreign arrival and departure taxes. These taxes and fees are assessments on the customer for which we act as a collection agent. Because we are not entitled to retain these taxes and fees, we do not include such amounts in passenger revenue. We record a liability when the amounts are collected and reduce the liability when payments are made to the applicable government agency or operating carrier.

Frequent Flyer Program

Our SkyMiles Program offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, Delta Connection and airlines that participate in the SkyMiles Program, as well as through participating companies such as credit card companies, hotels and car rental agencies. We sell mileage credits to non-airline businesses, customers and other airlines. Effective January 1, 2015, the SkyMiles program was modified from a model in which customers earn redeemable mileage credits based on distance traveled to a model based on ticket price. This award change did not affect the way we account for the program.

The SkyMiles Program includes two types of transactions that are considered revenue arrangements with multiple deliverables. As discussed below, these are (1) passenger ticket sales earning mileage credits and (2) the sale of mileage credits to participating companies with which we have marketing agreements. Mileage credits are a separate unit of accounting as they can be redeemed by customers in future periods for air travel on Delta and participating airlines, membership in our Sky Club and other program awards.

Passenger Ticket Sales Earning Mileage Credits. Passenger ticket sales earning mileage credits under our SkyMiles Program provide customers with two deliverables: (1) mileage credits earned and (2) air transportation. We value each deliverable on a standalone basis. Our estimate of the selling price of a mileage credit is based on an analysis of our sales of mileage credits to other airlines and customers, which is re-evaluated at least annually. We use established ticket prices to determine the estimated selling price of air transportation. We allocate the total amount collected from passenger ticket sales between the deliverables based on their relative selling prices.

We defer revenue for the mileage credits related to passenger ticket sales when the credits are earned and recognize it as passenger revenue when miles are redeemed and services are provided. We record the air transportation portion of the passenger ticket sales in air traffic liability and recognize these amounts in passenger revenue when we provide transportation or when the ticket expires unused.

Sale of Mileage Credits. Customers may earn mileage credits through participating companies such as credit card companies, hotels and car rental agencies with which we have marketing agreements to sell mileage credits. Our contracts to sell mileage credits under these marketing agreements have multiple deliverables, as defined below.

Our most significant contract to sell mileage credits relates to our co-brand credit card relationship with American Express. Our agreements with American Express provide for joint marketing, grant certain benefits to Delta-American Express co-branded credit card holders ("Cardholders") and American Express Membership Rewards Program participants and allow American Express to market using our customer database. Cardholders earn mileage credits for making purchases using co-branded cards, may check their first bag for free, are granted discounted access to Delta Sky Club lounges and receive other benefits while traveling on Delta. These benefits that we provide in the form of separate products and services under the SkyMiles agreements are referred to as "deliverables." Additionally, participants in the American Express Membership Rewards program may exchange their points for mileage credits under the SkyMiles Program. As a result, we sell mileage credits at agreed-upon rates to American Express for provision to their customers under the co-brand credit card program and the Membership Rewards program.

In December 2014, we amended our marketing agreements with American Express which increased the value we receive under the agreements and extended the term to 2022. The amended agreements became effective January 1, 2015. The deliverables under the amended agreements are substantially similar to the previous agreement. We account for the amended agreements consistent with the accounting method adopted in September 2013 that allocates the consideration received to the individual products and services delivered based on their relative selling prices (discussed below). The increased value received under the amended agreements increases the amount of deferred revenue for the travel component and increases the value of the other deliverables, which are recognized in other revenue as they are provided.

In September 2013, we modified our marketing agreements with American Express, which required us to change the accounting method for recording SkyMiles sold. Under the previous method, the embedded premium or discount was allocated to the residual products or services in a combined transaction. The new method allocates consideration received based on the relative selling price of each product or service. We determined our best estimate of the selling prices by considering discounted cash flow analysis using multiple inputs and assumptions, including: (1) the expected number of miles awarded and number of miles redeemed, (2) the rate at which we sell mileage credits to other airlines, (3) published rates on our website for baggage fees, discounted access to Delta Sky Club lounges and other benefits while traveling on Delta and (4) brand value. The impact of adopting the relative selling price method re-allocated a portion of the embedded discount to the travel component, lowering the deferral rate we use to record miles sold under the agreements and increasing revenue recognized on the remaining deliverables.

We recognize revenue as we deliver each sales element. We defer the travel deliverable (mileage credits) as part of frequent flyer deferred revenue and recognize passenger revenue as the mileage credits are used for travel. The revenue allocated to the remaining deliverables is recorded in other revenue. We recognize the revenue for these services as they are performed.

Breakage. For mileage credits that we estimate are not likely to be redeemed (“breakage”), we recognize the associated value proportionally during the period in which the remaining mileage credits are expected to be redeemed. Management uses statistical models to estimate breakage based on historical redemption patterns. A change in assumptions as to the period over which mileage credits are expected to be redeemed, the actual redemption activity for mileage credits or the estimated fair value of mileage credits expected to be redeemed could have a material impact on our revenue in the year in which the change occurs and in future years.

Regional Carriers Revenue

Our regional carriers include both our contract carrier agreements with third-party regional carriers ("contract carriers") and our wholly-owned subsidiary, Endeavor. In May 2013, Endeavor emerged from bankruptcy and we became its sole owner pursuant to a confirmed plan of reorganization.

Our contract carrier agreements are structured as either (1) capacity purchase agreements where we purchase all or a portion of the contract carrier's capacity and are responsible for selling the seat inventory we purchase or (2) revenue proration agreements, which are based on a fixed dollar or percentage division of revenues for tickets sold to passengers traveling on connecting flight itineraries. We record revenue related to our contract carriers and Endeavor in regional carriers passenger revenue and the related expenses in regional carriers expense.

Cargo Revenue

Cargo revenue is recognized when we provide the transportation.

Other Revenue

Other revenue is primarily comprised of (1) the non-travel components of the sale of mileage credits discussed above, (2) baggage fee revenue, (3) other miscellaneous service revenue, including ticket change fees, (4) revenue from ancillary businesses, such as the aircraft maintenance and repair and staffing services we provide to third parties and (5) sales of non-jet fuel products to third parties by our oil refinery.

Long-Lived Assets

The following table summarizes our property and equipment:



		December 31,
(in millions, except for estimated useful life)	Estimated Useful Life	2015			2014
Flight equipment	20-32 years	$	26,057		$	24,313
Ground property and equipment	3-40 years	5,862			5,198
Flight and ground equipment under capital leases	Shorter of lease term or estimated useful life	1,112			1,141
Advance payments for equipment		879			617
Less: accumulated depreciation and amortization⁽¹⁾		(10,871		)	(9,340		)
Total property and equipment, net		$	23,039		$	21,929


⁽¹⁾	Includes accumulated amortization for flight and ground equipment under capital leases in the amount of $782 million and $767 million at December 31, 2015 and 2014, respectively.

We record property and equipment at cost and depreciate or amortize these assets on a straight-line basis to their estimated residual values over their estimated useful lives. The estimated useful life for leasehold improvements is the shorter of lease term or estimated useful life. Depreciation and amortization expense related to our property and equipment was $1.8 billion, $1.7 billion and $1.6 billion for each of the years ended December 31, 2015, 2014 and 2013, respectively. Residual values for owned aircraft, engines, spare parts and simulators are generally 5% to 10% of cost.

We capitalize certain internal and external costs incurred to develop and implement software, and amortize those costs over an estimated useful life of three to seven years. Included in the depreciation and amortization expense discussed above, we recorded $148 million, $129 million and $110 million for amortization of capitalized software for the years ended December 31, 2015, 2014 and 2013, respectively. The net book value of these assets totaled $420 million and $411 million at December 31, 2015 and 2014, respectively.

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 fleet'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.

Goodwill and Other Intangible Assets

Our goodwill and identifiable intangible assets relate to the airline segment. We apply a fair value-based impairment test to the carrying value of goodwill and indefinite-lived intangible assets on an annual basis (as of October 1) and, if certain events or circumstances indicate that an impairment loss may have been incurred, on an interim basis. We assess the value of our goodwill and indefinite-lived assets under either a qualitative or quantitative approach. Under a qualitative approach, we consider various market factors, including the key assumptions listed below. We analyze these factors to determine if events and circumstances have affected the fair value of goodwill and indefinite-lived intangible assets. If we determine that it is more likely than not that the asset may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. Under a quantitative approach, we calculate the fair value of the asset using the key assumptions listed below. If the asset's carrying value exceeds its fair value calculated using the quantitative approach, we will record an impairment charge for the difference in fair value and carrying value.

We value goodwill and indefinite-lived intangible assets primarily using market capitalization and income approach valuation techniques. These measurements include the following key assumptions: (1) forecasted revenues, expenses and cash flows, (2) terminal period revenue growth and cash flows, (3) an estimated weighted average cost of capital, (4) assumed discount rates depending on the asset and (5) a tax rate. These assumptions are consistent with those hypothetical market participants would use. Since we are required to make estimates and assumptions when evaluating goodwill and indefinite-lived intangible assets for impairment, actual transaction amounts may differ materially from these estimates.

Changes in certain events and circumstances could result in impairment. Factors which could cause impairment include, but are not limited to, (1) negative trends in our market capitalization, (2) reduced profitability resulting from lower passenger mile yields or higher input costs (primarily related to fuel and employees), (3) lower passenger demand as a result of weakened U.S. and global economies, (4) interruption to our operations due to a prolonged employee strike, terrorist attack, or other reasons, (5) changes to the regulatory environment (e.g., diminished slot restrictions or additional Open Skies agreements), (6) competitive changes by other airlines and (7) strategic changes to our operations leading to diminished utilization of the intangible assets.

Goodwill. When we evaluate goodwill for impairment using a quantitative approach, we estimate the fair value of the reporting unit by considering both market capitalization and projected discounted future cash flows (an income approach). If the reporting unit's fair value exceeds its carrying value, no further testing is required. If, however, the reporting unit's carrying value exceeds its fair value, we then determine the amount of the impairment charge, if any. We recognize an impairment charge if the carrying value of the reporting unit's goodwill exceeds its estimated fair value.

Identifiable Intangible Assets. Indefinite-lived assets are not amortized and consist of routes, slots, the Delta tradename and assets related to SkyTeam. Definite-lived intangible assets consist primarily of marketing agreements and are amortized on a straight-line basis or under the undiscounted cash flows method over the estimated economic life of the respective agreements. Costs incurred to renew or extend the term of an intangible asset are expensed as incurred.

We assess our indefinite-lived assets under a qualitative or quantitative approach. We analyze market factors to determine if events and circumstances have affected the fair value of the indefinite-lived intangible assets. If we determine that it is more likely than not that the asset value may be impaired, we use the quantitative approach to assess the asset's fair value and the amount of the impairment. We perform the quantitative impairment test for indefinite-lived intangible assets by comparing the asset's fair value to its carrying value. Fair value is estimated based on (1) recent market transactions, where available, (2) the royalty method for the Delta tradename (which assumes hypothetical royalties generated from using our tradename) or (3) projected discounted future cash flows (an income approach). We recognize an impairment charge if the asset's carrying value exceeds its estimated fair value.

Income Taxes

We account for deferred income taxes under the liability method. We recognize deferred tax assets and liabilities based on the tax effects of temporary differences between the financial statement and tax basis of assets and liabilities, as measured by current enacted tax rates. Deferred tax assets and liabilities are recorded net as noncurrent deferred income taxes. A valuation allowance is recorded to reduce deferred tax assets when necessary.

Manufacturers' Credits

We periodically receive credits in connection with the acquisition of aircraft and engines. These credits are deferred until the aircraft and engines are delivered, and then applied as a reduction to the cost of the related equipment.

Maintenance Costs

We record maintenance costs to aircraft maintenance materials and outside repairs. Maintenance costs are expensed as incurred, except for costs incurred under power-by-the-hour contracts, which are expensed based on actual hours flown. Power-by-the-hour contracts transfer certain risk to third-party service providers and fix the amount we pay per flight hour to the service provider in exchange for maintenance and repairs under a predefined maintenance program. Modifications that enhance the operating performance or extend the useful lives of airframes or engines are capitalized and amortized over the remaining estimated useful life of the asset or the remaining lease term, whichever is shorter.

Advertising Costs

We expense advertising costs in passenger commissions and other selling expenses in the year incurred. Advertising expense was approximately $225 million for the years ended December 31, 2015, 2014 and 2013, respectively.

Commissions

Passenger sales commissions are recognized in operating expense when the related revenue is recognized.