payments from us. For additional information about our agreements with American Express, see Note 6 of the Notes to the Consolidated Financial Statements.  

 

Long-Term Debt, Interest Payments. Represents estimated interest payments under our long-term debt based on the interest rates specified in the applicable debt agreements. Interest payments on variable interest rate debt were calculated using LIBOR at December 31, 2011 .  

 

Contract Carrier Obligations. Represents our estimated minimum fixed obligations under capacity purchase agreements with regional carriers (excluding Comair). The reported amounts are based on (1) the required minimum levels of flying by our contract carriers under the applicable agreements and (2) assumptions regarding the costs associated with such minimum levels of flying.  

 

Employee Benefit Obligations. Represents primarily (1) our estimated minimum required funding for our qualified defined benefit pension plans based on actuarially determined estimates and (2) projected future benefit payments from our unfunded postretirement and postemployment plans. For additional information about our employee benefit obligations, see “Critical Accounting Policies and Estimates”.  

 

Aircraft Purchase Commitments. Represents primarily our commitments to purchase 100 B-737-900ER aircraft, 18 B-787-8 aircraft and nine previously owned MD-90 aircraft.  

 

Other Obligations. Represents primarily estimated purchase obligations under which we are required to make minimum payments for goods and services, including but not limited to insurance, marketing, maintenance, technology, sponsorships and other third party services and products.  

 

Critical Accounting Policies and Estimates  

 

Our critical accounting policies and estimates are those that require significant judgments and estimates. Accordingly, the actual results may differ materially from these estimates. For a discussion of these and other accounting policies, see Note 1 of the Notes to the Consolidated Financial Statements.  

 

Frequent Flyer Program  

 

Our frequent flyer program (the “SkyMiles Program”) offers incentives to travel on Delta. This program allows customers to earn mileage credits by flying on Delta, regional air carriers with which we have contract carrier agreements 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 also sell mileage credits to non-airline businesses, customers and other airlines.  

 

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