Summer travelers should pack plenty of patience: More flights are running late this year than in 2012.
The federal Department of Transportation says that only 79.4 percent of domestic flights arrived on time in May, down from 83.4 percent in the same month last year.
Hawaiian Airlines was most likely to arrive on time, while American Eagle, the regional-flying affiliate of American Airlines, had the worst rate.
Five planes were stuck on airport tarmacs for more than three hours, which is the limit allowed by federal regulations. Cancellations ticked up slightly from a year ago.
In the last five years, mergers have reduced the number of U.S. airlines and flights, which should help the carriers stay on schedule. But flights are extremely full -- average occupancy on all the major U.S. airlines topped 85 percent in June -- which means it can take longer for passengers to get on and off the plane.
The May results were still better than April's, when airlines blamed furloughs of federal air traffic controllers for causing nearly one in four flights to be late and nearly one in 50 to be canceled.
Consumers were most likely to complain about United Airlines or American Airlines and least likely to kvetch about Alaska or Southwest.
Hawaiian, which benefits from favorable flying weather, held its usual spot at the top of the on-time rankings, with 92.4 percent of its flights arriving within 14 minutes of schedule -- that's the government's definition of on time. Alaska Airlines and Delta Air Lines were next.