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. Cancelations 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, 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.

“On-time performance was still pretty solid at nearly 80 percent as airlines navigated (stormy) summer weather,” said Jean Medina, a spokeswoman for Airlines for America, a trade group of the largest U.S. carriers. “It’s also important to note that customer satisfaction, as ranked by lower complaint rate, is improving.”

Look at Twitter and you’ll see that many travelers gripe about airlines, but few of them bother to file a formal complaint with the Transportation Department. In May, there were 561 complaints against U.S. carriers, or one for every 100,000 people who got on a plane. That was down by about one-third from the rate in May 2012.

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.

At the other extreme, just 69.9 percent of American Eagle flights were on-time. ExpressJet and Mesa, which both operate regional flights for bigger airlines, and Frontier Airlines all scored below 75 percent.

Among the five largest U.S. airlines, Delta had the best on-time record, followed by US Airways, United, Southwest and American.

Two of the five flights that sat on the tarmac for at least three hours were operated by American. The others were Alaska, Shuttle America and ExpressJet.

Another 345 flights sat on the ground for at least two hours, including 93 operated by American, 59 by American Eagle, 49 by United, and 35 by US Airways.

(AP)