U.S. airline traffic fell in 2008 for only the fifth time since the government began tracking the data 35 years ago, as the global economy weakened and carriers slashed schedules.

Traffic, measured in miles flown by paying passengers, slid 2.3 percent for the seven biggest U.S. carriers, led by a 6.5 percent drop for United Airlines. Results pending from smaller airlines, including SkyWest Inc., won't be enough to make up the shortfall and prevent 2008 from snapping a streak of five annual gains.

Air travel likely will slump again this year because Delta Air Lines Inc. and other carriers plan to chop seating capacity by as much as 8 percent on top of their 2008 pullbacks. Traffic has fallen in back-to-back years only once, in 2001-2002, based on Transportation Department records dating to 1974.

"It's going to be another tough year for airlines, and they're all trying to figure out just how tough," Hunter Keay, an analyst at Stifel Nicolaus & Co. in Baltimore, said last week.

Keay estimates U.S. carriers will cut about 10 percent more flying capacity in 2009, with traffic slipping by 5 percent to 10 percent.

Record jet-fuel prices in mid-2008 prompted the biggest airlines to ground more than 460 planes, slash 26,000 jobs and reduce capacity by 10 percent or more.