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By Halimah Abdullah

Virgin America claimed the top spot for overall U.S. airline performance in 2015 for the fourth year in a row—a ranking that comes on a day in which Alaska Air announced a $2.6 billion deal to acquire the airline.

The results are part of the 26th annual Airline Quality Rating released Monday at the National Press Club in Washington.

Alaska Air Group Inc. announced Monday that they are buying Virgin America in a more than $2 billion deal that would create a behemoth airline with beefed up West Coast presence.

Related: Alaska Air Buys Virgin For $2.6B, Creates America's Fifth-Largest Airline

According to the Airline Quality Rating, JetBlue airlines improved their ranking and finish a close second. Delta ranked third.

The study found that overall the airline industry improved its on-time performance, rate of involuntary denied boardings and the rate of mishandled baggage.The ranking is a joint project of Wichita State University and Embry-Riddle Aeronautical University.

The improved airline performance means customers are benefiting from the recent round of mergers, Dean Headley, associate professor of marketing at the W. Frank Barton School of Business at Wichita State University said in a press release.

“Bigger has produced some operating gains for the airlines, but customer perceptions of poor outcomes are still reflected in a higher number of customer complaints for the year,” Headley said in the release.

JetBlue, Delta, ExpressJet, SkyWest, Southwest and United all performed better in 2015. However, Alaska, American, Envoy, Frontier, Hawaiian saw their scores decline from 2014.

Virgin's score declined as well, but it was still higher than other airlines.

However, customer complaints among all airlines is now at its highest level in 15 years.

“These results clearly show that the air traveling public is not happy,” study co-researcher, Brent Bowen, dean of the College of Aviation at Embry-Riddle Aeronautical University said in a press release. “Passengers are reaching out and letting us know exactly that, based on the number of complaints filed with the Department of Transportation. The human element of air travel is obviously deteriorating, and passengers are fed up.”