the actual results may vary materially from the amounts presented in the table.  



Long-Term Debt, Principal Amount. Represents scheduled principal payments on long-term debt.  

 

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, 2014 .  

 

Operating Lease Payments, Future Aircraft Leases . Represents estimates of lease payments on the remaining 36 B-717-200 aircraft still to be delivered in 2015 pursuant to our lease agreement with Southwest Airlines and on the subsequent lease agreement with The Boeing Company on our B-717-200 fleet.  

 

Aircraft Purchase Commitments. Represents our commitments to purchase 69 B-737-900ER, 45 A321-200, 25 A330-900neo, 25 A350-900, 18 B787-8, 10 A330-300 and two CRJ-900 aircraft. Our purchase commitment for the 18 B-787-8 aircraft provides for certain aircraft substitution rights.  

 

Contract Carrier Obligations. Represents our estimated minimum fixed obligations under capacity purchase agreements with regional carriers. 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.”  

 

Other Obligations. Represents 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 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