Flying may be a hassle, but airlines are making progress on fixing problems such as delays and lost baggage, according to an annual study of airline performance.

AirTran Airways came out on top in the closely watched Airline Quality Rating, while Atlanta-based Delta Air Lines continued to improve. This is the second-consecutive No. 1 ranking for AirTran, which is the second-largest carrier in Atlanta but was acquired by Dallas-based Southwest Airlines last year.

"When you see something good, don't you want it? That's what Southwest saw," said Airline Quality Rating study author Dean Headley, an associate professor of marketing at Wichita State.

The Airline Quality Rating by researchers at Wichita State University and Purdue University gives an overall ranking of airlines by combining publicly available federal data on U.S. airlines' delays, mishandled baggage, customer complaints to the federal government and denied boarding. The rating issued Monday was based on airlines' performance in 2011.

Delta came in sixth out of 15 carriers ranked, edging out Southwest. Delta was the only airline with gains in all four categories of performance, which drove its bump up from No. 7 in 2010.

Delta spokesman Anthony Black said the airline continues to improve, "regardless of which survey, which results, which study comes out." He said the improvements span across operations, service and expansion of product offerings such as in-flight Wi-Fi.

The study's authors said airlines' overall performance has been improving for the past four years. Mostly mild weather last winter helped keep a lid on nightmarish incidents.

AirTran improved its on-time performance, received fewer customer complaints and maintained its top baggage-handling performance. Southwest was ranked seventh.

"We can learn from AirTran and vice versa," said Southwest spokesman Chris Mainz. "As we do fold them into Southwest Airlines' brand, we hope to apply their best practices, especially in regards to baggage handling [where] they're tops in the industry."

Airlines overall made particularly large improvements in reducing bumping, or involuntary denied boardings, which affected fewer than one of every 10,000 passengers in 2011.

"It was an area that needed some improvement," Headley said. "It's a precursor to a potentially very unhappy customer." He said travelers can reduce their risk of being bumped by checking in for flights online up to 24 hours in advance.

As airfares rise along with fuel costs, Headley noted that price is likely a bigger factor in airline ticket purchases than service. But fares can still affect service, he said.

"If your prices don't go up enough, you've got to cut costs to stay in business," Headley said. "If you start cutting costs, at what point does that affect the outcome of the service?"

Annual airline study

Here's how the airlines rank in the Airline Quality Rating, based on their performance in 2011:

1. AirTran

2. Hawaiian

3. JetBlue

4. Frontier

5. Alaska

6. Delta

7. Southwest

8. US Airways

9. SkyWest

10. American

11. Continental

12. United

13. Atlantic Southeast

14. Mesa

15. American Eagle