U.S. airlines brace themselves for the annual slow down in September

U.S. airlines' solid, if unspectacular, financial performance in 2012 appears to be faltering just as carriers head into September, typically one of their weakest months of the year.

Higher fuel costs, an uncertain economy and slowing revenue momentum are leading some analysts to trim their earnings estimates for the rest of 2012 and into next year, though the industry's newfound structural improvements suggest it should be able to weather the slowdown without major problems.

Airlines for America, the industry's main trade association, says traffic for seven big U.S. carriers declined by more than 16% in September from August on average over the past five years, and unit revenue slipped by 7% on average.