The government received 20,170 complaints from consumers, up from the total of 15,539 complaints received in 2014. While grievances filed with the DOT represent only a fraction of overall complaints, they are thought to be a reliable barometer of airline customer service.

The last time complaints exceeded the 20,000-level was in 2000, when the DOT recorded 23,381 grievances. The most complaints ever received by DOT in a year happened in 1987, when regulators recorded 44,813 complaints.

Passengers’ dissatisfaction with American Airlines

American Airlines dominated the rankings in almost every category for 2015. It had the most complaints about flight problems (1,094), oversales (152), reservations (506), fares (479), refunds (415), baggage (601), customer service (426) and disability issues (183). In only one category — discrimination — American lost its top ranking to rival United Airlines.

American completed its merger with US Airways in 2015. When combined with US Airways’ complaints, the the merged airline has no rival.

Spirit generates many complaints

Discount airline, Spirit, is roughly 12 times smaller than American, but it still managed to draw 2,069 complaints, making it the third most complained-about airline in the United States.

“We are confident that our performance will continue to improve throughout 2016,” says American spokesman Ross Feinstein. Spirit didn’t immediately respond to a request for comment. This story will be updated when Spirit responds.

The DOT reports complaints so that consumers can compare the overall complaint records of individual airlines to determine which airlines offer the best service “and select airlines based on that knowledge,” says spokeswoman Namrata Kolachalam “The complaints also serve as a basis for investigations, rule-making, legislation, and research.”

The government data also revealed other key performance metrics for the U.S. airline industry. Among the key numbers:

• As a group, airlines were more on-time in 2015. The reporting carriers posted an on-time arrival rate of 79.9 percent, up from 76.2 percent in 2014.
• They canceled fewer flights for the year. Airlines canceled only 1.5 percent of their scheduled domestic flights, an improvement over the 2.2 percent cancellation rate in 2014, according to the government.
• Airlines lost slightly fewer bags. For all of last year, the carriers posted a mishandled baggage rate of 3.24 per 1,000 passengers, down from 2014’s rate of 3.61.

The DOT numbers point to a growing divide between airlines and passengers. They suggest that it’s possible to run an airline that performs well operationally but alienates its customers with draconian rules and unfriendly service. Domestic airlines may be safe, efficient and run on time, but these numbers suggest that passengers can’t stand to fly.

And they’re making their voices heard, too. Interestingly, a new survey of social media sentiment found that American Airlines, Frontier and Spirit had among the most customer complaints on Twitter.