FAA’s 20-year industry forecast, released March 6, for the second consecutive year projects a slowdown in U.S. airline traffic, citing changes in the global economy and industry constraint on capacity growth.

That discipline has enabled airlines to keep yields steady, which lessens demand stimulation, says Nan Shellabarger, FAA executive director for aviation policy and plans. Domestic capacity increased just 0.1% in 2012, while domestic mainline carrier yield rose 3.5%.

The new forecast says U.S. carrier traffic will grow about 2.2% a year in 2013-2033, down from 2.6% in last year’s forecast. After another year of slow or no growth in 2013, the FAA says growth over the next five years will be slightly higher than the long-running rate of 2.6%, assuming the U.S. economy grows faster than the current rate.

The agency also believes U.S. commercial carrier annual enplanements will not reach 1 billion until 2027. Two years ago, it predicted that milestone would be reached in 2021, and last year, citing the poor economy, high fuel prices and industry consolidation, it changed the prediction to 2024.

Another factor in the new forecast, Shellabarger says, is a shift in the measuring stick the FAA is using to determine the rate of traffic growth. Starting this year, it is using the projected change in disposable personal income as the primary driver instead of GDP.

“Ultimately, it’s people that fly, “Shellabarger explains, and there can be disconnects between GDP and disposable personal income (an after-tax measure) because of factors such as tax policy, business inventory accumulation and the balance between exports and imports.

This year’s 20-year forecast does not take into account any impact from the sequester federal budget cuts. The economic forecasts from IHS Global Insight, which form a large part of the basis for the FAA’s projections, assumed the sequester would be avoided, Shellabarger says, and it is too soon to determine the long-term impact in any case.