Is the state of air travel improving? Not according to the DOT, which just released its Air Travel Consumer Report for 2015.

Airline passengers filed 20,170 complaints with the DOT in 2015, up 30 percent over 2014. That was for all airlines, foreign and domestic, and travel agents. For U.S. airlines, the picture was even grimmer. Complaints increased by 34 percent, from 11,365 in 2014 to 15,260 in 2015.

American received the most complaints, with 3,983, followed by United with 2,721, Spirit with 2,069, Frontier with 1,037, and Delta with 1,025.

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When the numbers of complaints are contextualized, against the numbers of passengers served, U.S. carriers ranked as follows, from worst to best:

1. Spirit – 11.73 complaints per 100,000 passengers
2. Frontier – 7.86 complaints per 100,000 passengers
3. American – 3.36 complaints per 100,000 passengers
4. United – 2.85 complaints per 100,000 passengers
5. Virgin America – 1.66 complaints per 100,000 passengers
6. Envoy Air – 1.45 complaints per 100,000 passengers
7. Hawaiian – 1.06 complaints per 100,000 passengers
8. JetBlue – 0.86 complaints per 100,000 passengers
9. Delta – 0.74 complaints per 100,000 passengers
10. SkyWest – 0.65 complaints per 100,000 passengers
11. ExpressJet – 0.62 complaints per 100,000 passengers
12. Southwest – 0.52 complaints per 100,000 passengers
13. Alaska – 0.50 complaints per 100,000 passengers

Spirit isn’t only the worst, it’s the worst by far. The company began the new year by replacing its CEO with Robert Fornaro, who has vowed to improve the airline’s public image and its operational efficiency. However, reversing such a deep trust deficit won’t be quick or easy, and the expected merger with Frontier, the second-worst-ranked carrier, will make the job even more challenging in the short term.

At the other end of the complaint spectrum, Alaska, Southwest, Delta, and JetBlue all drew fewer than one complaint per 100,000 passengers. In an industry that consistently underperforms its customers’ expectations, that’s as good as it gets.

Make Your Voice Heard

Airline-related complaints may be registered on the DOT’s complaint website, or by phone, at 202-366-2220.

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After 20 years working in the travel industry, and almost that long writing about it, Tim Winship knows a thing or two about travel. Follow him on Twitter @twinship.

This article first appeared on SmarterTravel.com, where Tim Winship is Editor-at-Large.