ATLANTA (CNN) — For the fourth year in a row, Virgin America is tops in airline quality among the biggest U.S. airlines.

That's according to the 26th annual national Airline Quality Rating report, which was released Monday.

The report comes the same day as Alaska Airlines announced that it will buy Virgin America for $2.6 billion.

Although the No. 1 airline's overall score declined slightly from 2014, Virgin America still managed to retain its top ranking for 2015. JetBlue finished second, followed by No. 3-ranked Delta Air Lines.

But things are not so rosy for many airline passengers. Despite overall industry improvement in three of the four categories tracked, customer complaints rose by 38 percent from 2014 to 2015.

"This is the worst complaints have been for 15 years, as a rate. The last time it got this high was in 2001," said report co-author Dean Headley, a researcher at Wichita State University's business school.

Why the dissatisfaction?

Headley suspects that passengers are annoyed by ticket change fees of up to $200 and a lack of improvement in airline amenities, despite the fact that four of the nation's top airlines saved more than $11 billion in fuel costs in the first nine months of 2015.

"There's $11 billion that they just walked into, so to speak.

"And have they done anything to help the consumer experience? Some food here and there and maybe a new airplane, but as an industry, it's not overly noticeable," he said.

The report, which looks at the 13 largest airlines in the United States over the 2015 calendar year, is the work of Headley and Brent Bowen, dean of the College of Aviation at Embry-Riddle Aeronautical University.

"These results clearly show that the air traveling public is not happy," said Bowen.

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How performance shakes out

The report uses performance data gathered from the U.S. Department of Transportation's monthly Air Travel Consumer Report to see whether public impressions of airline quality actually line up with airline performance.

The rating report looks at four performance categories: on-time performance, mishandled bags, involuntary denied boardings and customer complaints.

Overall airline performance improved slightly in 2015.

The industry showed overall improvement from 2014 to 2015 in on-time performance, mishandled bags and involuntary denied boardings.

In the fourth category, the rate of complaints jumped from 1.38 complaints per 100,000 passengers in 2014 to 1.9 in 2015.

Six of the 13 airlines evaluated, JetBlue, Delta, ExpressJet, SkyWest, Southwest and United, improved their overall scores.

Six others, including Virgin America, declined from 2014, and Spirit Airlines was new to the ratings.

Best and worst by category

Hawaiian Airlines ranked No. 1 in the on-time performance category, coming in on-time 88.4 percent of the time in 2015.

Spirit Airlines, which ranked last overall, had the worst on-time performance, at just 69 percent.

Spirit also had the highest complaint rate (11.73 complaints per 100,000 passengers) of all the airlines ranked.

Alaska Airlines fared far better in that category with the lowest rate of complaints (0.5 per 100,000 passengers).

Virgin America had the best record for baggage handling, while Envoy Air ranked last. Envoy also had the highest rate of involuntary denied boardings, with JetBlue ranking first in that category.

Overall airline ranking

2014 rankings in parentheses

1. Virgin America (1)

2. JetBlue (4)

3. Delta (3)

4. Hawaiian (2)

5. Alaska (5)

6. Southwest (6)

7. SkyWest (10)

8. United (9)

9. ExpressJet (11)

10. American (7)

11. Frontier (8)

12. Envoy Air (12)

13. Spirit (new to rating in 2015)

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