Standard & Poor's Ratings Services has lowered its ratings on Southwest Airlines Co. (LUV), lowering the debt ratings to 'BBB' from 'BBB+’ and placing it with negative implications. However, Southwest has retained its investment grade rating. The downgrade reflects lower operating cash flow caused by record high fuel prices, low global demand for air travel and weak prices. The travel downturn has eroded the airline's flow of cash at a time when the airline faces substantial fixed obligations and declining cash balances. Southwest's cash balance fell to $902 million in the third quarter from $1.4 billion at the end of 2008. Recently, Southwest borrowed $124 million under a new term loan agreement secured by five Boeing 737-700 aircrafts. It also replaced its previous $600 million unsecured revolving credit facility with a new $600 million unsecured revolving credit facility that will expire in Oct. 2012. The rating ...