improvements, including right-sizing capacity to better meet customer demand and the continued restructuring of our route network to reduce less productive short haul domestic flights and reallocate widebody aircraft to international routes and (3) a $476 million decrease in short-term investments primarily from sales of auction rate securities.

Cash flows provided by operating activities in 2006 reflect an increase of $401 million in working capital compared to 2005. This increase was primarily a result of revenue and network productivity improvements, restructuring initiatives and labor cost reductions implemented during our Chapter 11 proceedings and an improved revenue environment. Our 2006 cash flows from operating activities also include a $116 million decrease in our restricted cash balance primarily due to a release of cash from restricted to operating under agreements we reached with certain vendors. In 2005, our restricted cash balance increased significantly primarily due to cash holdbacks under our Visa/MasterCard credit card processing agreement.

Cash flows from investing activities

Cash used in investing activities totaled $625 million and $361 million for 2007 and 2006, respectively. Cash used in investing activities in 2007 reflects an increase in investment of $545 million for flight equipment and advanced payments for aircraft commitments and $78 million for ground property and equipment. During 2007, restricted cash decreased by $185 million. In addition, we received $34 million and $83 million from the sale of our investments in priceline.com Incorporated and ARINC Incorporated, respectively.

Cash used in investing activities totaled $361 million for 2006, compared to cash provided by investing activities of $22 million for 2005. This change reflects a $401 million decrease in cash used for the purchase of flight and ground equipment in 2006. Our 2005 cash flows from investing activities include $842 million in proceeds from our sale of ASA and certain flight equipment.

Cash flows from financing activities

Cash used in financing activities totaled $120 million and $606 million for 2007 and 2006, respectively. Cash used in financing activities in 2007 primarily reflects (1) the repayment of the DIP Facility with a portion of the proceeds available under the Exit Facilities and existing cash, (2) the prepayment of $863 million of secured debt with a portion of the proceeds from the sale of the 2007-1 Certificates and (3) scheduled principal payments on long-term debt and capital lease obligations. During 2007, we also received proceeds from an amendment to the Spare Parts Loan, as discussed above.

Cash used in financing activities totaled $606 million for 2006, compared to cash provided by financing activities of $830 million for 2005. This change is primarily due to the net proceeds we received under the DIP Facility shortly after our Chapter 11 filing in 2005. While in Chapter 11, we ceased making payments on our unsecured debt.

 Contractual Obligations

The following table summarizes our contractual obligations as of December 31, 2007 that relate to debt, operating leases, aircraft order commitments, capital