There's nothing more irritating for an air traveler than to arrive at the airport and learn that your flight is delayed. Every airline experiences flight delays sometimes, but some are a lot worse than others.  Here's a look at the four worst offenders for the 12-month period ending in August, according to the U.S. Department of Transportation.

An American tradition

AMR's (UNKNOWN:AAMRQ.DL) American Airlines and American Eagle subsidiaries were nearly tied for the fourth spot among the most-delayed U.S. carriers. Only 74.6% of American's mainline operations arrived on time, while 74.2% of American Eagle's regional flights arrived on time. (It's also worth noting that the DOT only counts a flight as delayed if it arrives at the gate more than 14 minutes late.)

American Airlines and regional subsidiary American Eagle have been particularly delay-prone.

American Eagle is consistently one of the most-delayed airlines in the country. In part, it suffers from having a large base at O'Hare Airport in Chicago, one of the worst airports for delays. Moreover, as an operator of small regional jets, American Eagle is the first to get hit when bad weather comes. If delays start piling up, airlines will always give priority to large mainline jets, delaying or canceling the smaller regional flights.

American's mainline operations have also been historically delay-prone. However, the carrier took a big step forward over the summer, improving upon the industry average for delays in July and August. If American can sustain this performance, it will have much happier customers in the future.

No new frontier

Frontier Airlines is third from the bottom in terms of on-time arrivals. Frontier's main problem is simple: Nearly all of its flights arrive at or depart from Denver International Airport. While Denver is not in the top tier of delay-prone airports, it's worse than average because of its high elevation and propensity for bad weather in the winter.

Frontier Airlines gets hit with delays whenever there's bad weather in Denver (Photo: Frontier Airlines)

When the weather in Denver cooperates, Frontier can be reliable. In June, Frontier was third in the nation in terms of punctuality, with its flights arriving on time 77.9% of the time in what was a bad month for most of the airline industry. However, for the full 12-month period ending in August, just 74% of Frontier's flights arrived on time.

Feeling blue

JetBlue Airways (NASDAQ:JBLU) is another airline that frequently runs behind schedule. JetBlue operates its largest hub at New York's JFK Airport, one of the worst airports for flight delays. It also has significant operations at the other New York-area airports and a large focus city in Boston. In other words, its flights are heavily concentrated in the busy Northeast airspace.

JetBlue's big presence in the Northeast is a curse in terms of flight delays (Photo: JetBlue Airways)

As a result, JetBlue is the second most-delayed carrier in the country, with only 73.7% of its flights arriving on time for the 12 months ending in August. In busy months, its performance can be even worse. For example, JetBlue had the worst on-time performance in the country in July, with just 63.9% of its flights arriving on time!

And the worst...

SkyWest (NASDAQ:SKYW) subsidiary ExpressJet takes the prize as the most-delayed airline in the U.S. With 73.4% of its flights arriving on time for the 12 months ending in August, ExpressJet barely edged out JetBlue for this "honor".

ExpressJet primarily flies small regional jets for United Continental (NASDAQ:UAL), although it also does some flying for Delta Air Lines (NYSE:DAL), and (to a lesser extent) American Airlines. Like American Eagle, ExpressJet gets the short end of the stick when bad weather or other operational problems cause delays to stack up; its flights are usually the first to get delayed or canceled.

It doesn't help that many of ExpressJet's flights are into delay-prone United hubs, particularly Newark Airport and O'Hare Airport. The carrier also operates its biggest base at Delta's Atlanta megahub, where regional carriers tend to bear the brunt of delays on busy days.

Foolish bottom line

There's a sizable gap between the best and worst airlines for on-time performance. In many cases, the delays are not the fault of the airlines so much as the airports where they operate. Still, that's not much consolation for passengers -- and no airline wants to become known for causing its customers the most aggravation!

Which airlines have given you the most trouble with delays? Let me know in the comment box below!