More flights are running late this year than in 2012.
The U.S. Department of Transportation says 79.4 percent of domestic flights arrived on time in May, down from 83.4 percent in the same month last year.
Five planes were stuck on airport tarmacs for more than three hours, which is longer than allowed by federal regulations. Cancellations ticked up slightly.
The May results are still better than April, when airlines blamed furloughs of federal air traffic controllers for causing nearly one in four flights to be late.