As the second-quarter reporting season wraps up, carriers have warned again that fuel, which is right up there with personnel as their biggest cost, is headed higher in the third and fourth quarters. That would put overall fuel costs for 2007 as high or even higher than they were in 2006.

The forecasts include gains from locking in fuel prices through hedges, a strategy that gave Southwest Airlines a huge advantage over rivals during the first part of this decade but that can also backfire if prices go lower.

United Airlines' parent UAL, for instance, recorded a loss from fuel hedges in the first quarter after prices for jet fuel slid.

Recent forecasts for higher fuel costs mark a reversal from results in the second quarter, when such costs dropped for many carriers.

"Rising fuel prices are a real concern in the second half," Thomas Horton, chief financial officer at American Airlines' parent AMR, told investors earlier this month. The largest U.S. carrier now anticipates shelling out $2.11 a gallon, on average, for fuel versus an average of $2.01 last year.

At JetBlue Airways, executives had originally budgeted fuel at $1.93 a gallon for the year. They now see average fuel costs rising to $2.07 a gallon, including hedges, from an average $1.99 a gallon last year. In the second quarter, the company paid 2.8% less for every gallon bought.

And on Thursday, US Airways became the latest major carrier to upgrade its fuel cost forecast. The Tempe, Ariz., carrier now sees fuel costs ranging between $2.25 and $2.30 a gallon in the third quarter for its main US Airways carrier, at least 10 cents higher than in the second quarter, and rising further in the fourth quarter.

For the full year, its fuel costs are likely to average $2.17 a gallon to $2.22 a gallon, up from $2.09 a gallon last year, despite hedging 53% of its purchases this year.

These raised forecasts for fuel have muted airlines' sunnier outlook for summer business. Management at JetBlue, US Airways and Southwest said demand so far this quarter seems strong, while revenue should benefit from carriers' efforts to trim domestic capacity.

A steep rise in crude-oil prices over the last two months has depressed shares in major carriers, despite a general trend toward improved profits.

The American Stock Exchange Airline Index, a benchmark tracker that includes legacy, discount and regional carriers' stocks, has fallen 18% since the start of the year after rising 7% last year and 18% in the fourth quarter alone.

Crude-oil prices, meanwhile, have come back strongly from a winter slump that briefly revitalized airline shares. On Friday, crude for September delivery closed over $77 a barrel, the highest closing price for the contract since mid-August.

Jet-fuel prices, which closely track with crude oil, are treading over roughly the same ground they charted last year, according to aviation consultancy Eclat Consulting.

The Washington-area firm estimates jet-fuel prices rose to $2.06 a gallon in July, about 6 cents higher than in June and 25 cents higher than in January. But prices are actually 2 cents lower than in July of last year.

Even if jet-fuel prices don't make big year-over-year gains, they will still be holding to levels of about $2 — or about double their levels at the start of the decade.