The airline's sales estimate is frequently cited as a benchmark for the industry, USA Today reported Tuesday. Normally growth in the revenue per seat per mile category reaches "low to mid-teens," the newspaper said.

Tammy Romo, vice president of financial planning at Southwest Airlines, also said revenue projections were down. "We have seen signs of weakness in our recent booking and revenue trends," she said.

Passengers may see further flight reductions in 2009, as the economy slows and airlines adjust schedules to keep planes as full as possible, the newspaper said.

"Managing your capacity is critical to controlling your revenues," Delta Airlines spokeswoman Betsy Talton told USA Today.