(May 13, 2008)—Southwest Airlines says that it borrowed $600 million from Citibank and seven European banks to bolster its cash position because of uncertainty about the economy.

The airline disclosed in a regulatory filing Monday that it had tapped the full amount of the loans on Friday.

The loans were secured by the first-lien mortgages on 21 of Southwest's Boeing 737-700 aircraft.

The filing didn't give a purpose for the debt, but spokeswoman Beth Harbin said, "It's due to uncertainty about the economy and the credit market and the high price of fuel."

"We thought it would be wise to bolster our cash position," she said.

The loans mature in 2020 and are repayable in quarterly installments beginning Aug. 9. They bear an interest of the London Interbank Offered Rate plus 0.95 percent.

Southwest said in the filing that there are no financial covenants to the loans.

Lenders holding more than 50 percent of the outstanding principal could accelerate the unpaid principal under certain circumstances, the company said in the filing.