Airlines may fly more this spring than in six…

U.S. airlines are expected to carry more passengers this spring than they have in six years, and more international fliers during that season than ever before, according to a forecast released Wednesday by the industry's trade group.

Airlines for America, which represents most of the major carriers in the U.S., projects that roughly 129.5 million fliers will be on board U.S. carriers in March and April. That's the largest number since 133.7 million travelers took to the air during those months in 2008, and a 1% bump over the 128.2 million who flew last spring.

A growing chunk of those fliers will be traveling to and from the United States, with a record-setting 17.1 million passengers flying internationally.

"We attribute the increase in spring air travel to rising U.S. household net worth, an improving economy, and the affordability of air travel, which remains one of the best bargains for consumers," John Heimlich, the trade group's vice president and chief economist, said in a statement.

The bump in spring time travel follows a year in which an industry that's been streamlined by a series of mega-mergers, and become more disciplined in matching available seats to demand that continued a profit-making streak.

Based on the full-year results of nine carriers, the U.S. industry last year reaped an $11.6 billion profit counting special items, equal to 7.8% of revenue, A4A says. Not counting special items, the industry-wide tally was $7.4 billion.

"The U.S. airline industry continued its upward climb in 2013, recording a fourth consecutive year of modest profitability, despite incurring more than $50 billion in fuel costs for the third straight year as well as increases in every single non-fuel expense," Heimlich said.

International travel has been a key growth area for the industry. In 2013, a record 185.4 million passengers flew to and from the U.S., a 4.4% hike over the previous year. And 53.1% of those passengers flew on a U.S. airline.