DALLAS – American Airlines is increasing by $5 each the amount passengers pay to check a first and second bag, helping the world’s second-largest carrier boost revenue from sources other than tickets.

The change to $20 for the first bag and $30 for the second begins for tickets purchased on or after Aug. 14, the Fort Worth-based unit of AMR Corp. said yesterday in a statement.

AMR’s second-quarter revenue slipped 21 percent from a year earlier. The drop occurred during the start of historically strong summer travel. The company posted a quarterly loss of $319 million, excluding one-time expenses.

“Our fees and charges have to cover our operating costs if the airline is to remain viable in the long term,’’ Tim Smith, an American spokesman, said in an interview.