DALLAS (AP) – Airlines cut fares to get more passengers on planes and salvage the summer travel season, but now their job gets harder heading into the slower fall and winter months.

The nine largest U.S. carriers lost nearly $600 million in the second quarter of this year. Bigger losses are predicted in the third and fourth quarters, and some analysts have raised the possibility of another round of bankruptcies.

America's airlines have been in a defensive crouch for two years.

They've cut flights and fired workers - first to absorb rising fuel prices, then to ride out the recession. But revenue is down one-fifth or more from a year ago at the four largest carriers.

Because they've cut costs, sold new stock and borrowed money, the airlines have plenty of cash for now. But even in good years, airlines build cash during the busy summer travel period, which ends around Labor Day in early September, to get through the slower months.

Airlines need enough cash to pay employees, buy fuel and pay other bills, including payments on the money they've borrowed. If cash falls too low, they can be pushed into bankruptcy protection, as happened earlier this decade with Delta, United, Northwest and US Airways.

United, US Airways and American are often mentioned as the airlines in the most precarious financial positions.

They rely on business travelers who pay hundreds of dollars per ticket to sit in first-class. Many of those people are grounded or flying in cheaper coach seats due to the recession.

Meanwhile fuel costs, although lower than last year's record levels, have been rising. The spot price of jet fuel has jumped about 70 percent since March.

One leading analyst, JPMorgan's Jamie Baker, estimates that by the fourth quarter, American Airlines parent AMR Corp. will burn more than $11 million a day, while United Airlines' parent UAL Corp. will be going through $7 million a day.

As of June 30, AMR had about $2.8 billion in unrestricted cash and short-term investments, UAL had $2.6 billion, and US Airways had about $1.7 billion. If Baker is right about how much cash they'll burn this winter, all three will have a thinner cash cushion than did the carriers who filed for bankruptcy protection in 2004 and 2005.