Summer travelers should pack plenty of patience: More flights are running late this year than in 2012.

The U.S. 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 longer than allowed by federal regulations.

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, which means it can take longer for passengers to get on and off the plane.