Washington (CNN) -- Flight delays decreased 6 percent from 2007 to 2009, largely because airlines put fewer planes in the sky because of the recession, according to a study by the Government Accountability Office.
But even with the decrease, at least one in four U.S. passenger flights arrived late -- typically an hour late -- at five major airports: Newark Liberty International, LaGuardia and John F. Kennedy in the New York area, Atlanta Hartsfield in Georgia and San Francisco International in California, according to the report.
Those five airports -- along with Chicago O'Hare International in Illinois and Philadelphia International in Pennsylvania -- accounted for almost 80 percent of departure delays at all major airports in the United States.
Flight delays have been a fact of life for air travelers for almost two decades. The only reprieve came after the 2001 terrorist attacks led to a decline in the number of flights.
Air travel was slowly returning to its pre-2001 levels when the industry was hit by the country's economic woes.
Most of the delays are attributable to old and familiar problems: inclement weather, heavy air traffic volumes and airline issues, such as baggage loading, crew issues or maintenance.
The GAO said NextGen, the FAA's modernization of air traffic control systems, could help reduce delays in the next two to three years. "However, the extent to which these actions will reduce delays at individual airports ... is unclear," it said.
Meanwhile, the number of flights experiencing tarmac delays of more than three hours has decreased. Some 903 flights had delays of more than three hours in 2009, compared to 1,654 flights in 2007. In March, the Department of Transportation imposed a rule that requires airlines to allow passengers stuck on planes to get off the plane after three hours.