U.S. airlines experienced an 18 percent decline in passenger revenue in 2009 compared to 2008, according to the Air Transport Association of American (ATA), which announced the drop this week and said it's the largest on record, exceeding the 14 percent decline reported from 2000 to 2001.ATA blames the decline on a 6 percent drop in passenger volume and a 13 percent in the average price paid to fly one mile—both results of the weak global economy."The global recession, accompanied by high levels of unemployment, hit air travel demand especially hard in 2009, but the decline appears to be bottoming out," ATA President and CEO James C. May said in a statement. "Anecdotal evidence suggests a positive revenue trajectory in 2010."Meanwhile, ATA reported that passenger revenue fell 4 percent in December 2009 versus December 2008, marking the 14th consecutive month in which passenger revenue declined year over year.At the same time, cargo traffic on U.S. airlines actually grew in November 2009, up 7 percent year over year, driven primarily by increased international trade. Although December cargo data is not yet available, it was the first observed increase in cargo traffic since July 2008.