WASHINGTON — The U.S. Department of Transportation received 18,148 complaints about air travel in 2017.

It was a year when airline incidents were in the headlines, including a United passenger dragged from a plane and a Delta passenger mauled by an emotional support dog.

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The number of complaints about air travel to the federal government last year — which included complaints about airlines, tour operators and other travel industry companies — was up 1.3 percent from 2016, according to statistics for the year released recently by the U.S. Bureau of Transportation Statistics.

The DOT logged 851 complaints about treatment of disabled passengers in 2017 and 98 complaints about discrimination, according to the department's air travel consumer report.

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Spirit Airlines, an ultra low-cost carrier, had the highest rate of complaints. A total of 11,570 of the complaints were about U.S. airlines, down slightly from 2016, while more than 6,000 complaints in 2017 were about foreign airlines.

Here's the ranking of U.S. airlines based on the rate of complaints received by the DOT in 2017:

Airline — Complaints per 100,000 passengers boarding planes

1. Southwest — 0.47
2. SkyWest — 0.53
3. Alaska — 0.57
4. ExpressJet — 0.73
5. Delta — 0.92
6. Hawaiian — 0.95
7. JetBlue — 1.14
8. United — 1.89
9. Virgin America — 1.92
10. American — 1.96
11. Frontier — 2.78
12. Spirit — 5.59

Source: U.S. Bureau of Transportation Statistics