from period to period.  

 

We have jet fuel inventories at various airport locations, which are used in our airline operations. Also, we acquired the Trainer oil refinery in 2012, which, as part of refinery operations, has refined oil product inventories. Jet fuel and refined oil product inventories are recorded as Fuel Inventory. Fuel Inventory increased during 2012 as a result of the startup of refinery operations in the September quarter and the transition of our fuel supply program. The increase in Fuel Inventory to achieve normal operating levels at our refinery was offset by a similar increase in accounts payable, and therefore, did not have a significant impact on operating cash flows during 2012. The remaining $180 million increase in Fuel Inventory was a result of the transition in Delta's fuel supply program.  



As part of our fuel hedging program, we may be required to pay hedge margin to counterparties when our portfolio is in a loss position. Conversely, if our portfolio with counterparties is in a gain position, we may receive hedge margin. Our future cash flows are impacted depending upon the nature of our derivative contracts and the market price of the commodities underlying our derivative contracts  

 

Timing of SkyMiles Sales. In December 2011, we amended our American Express agreements and agreed to sell $675 million of unrestricted 

SkyMiles to American Express in each December from 2011 through 2014. Under the December 2011 amendment, American Express purchased $675 million of unrestricted SkyMiles in both 2012 and 2011. In 2011, this operating cash flow was largely offset by working capital changes. We anticipate American Express will make additional purchases of $675 million of unrestricted SkyMiles in both 2013 and 2014.  

 

In 2008, we entered into a multi-year extension of our American Express agreements and received $1.0 billion from American Express for an advance purchase of restricted SkyMiles. The 2008 agreement provided that our obligations with respect to the advance purchase would be satisfied as American Express uses the purchased miles over a specified future period (“SkyMiles Usage Period”). During the SkyMiles Usage Period, which commenced in December 2011, American Express draws down SkyMiles valued at $333 million annually over three years beginning 2012 instead of paying cash to Delta for SkyMiles used. This draws down of $333 million in SkyMiles reduced cash flows from operations in 2012 compared to 2011 and 2010.  

 

Pension Contributions. We sponsor defined benefit pension plans for eligible employees and retirees. These plans are closed to new entrants and are frozen for future benefit accruals. Our funding obligations for these plans are governed by the Employee Retirement Income Security Act, as modified by the The Pension Protection Act of 2006. We contributed $697 million and $598 million to our defined benefit pension plans during 2012 and 2011, respectively. We estimate the funding requirements under these plans will total approximately $675 million in 2013.  

 

Undrawn Lines of Credit  

 

Delta has available $1.8 billion in undrawn lines of credit. As described in Note 8 of