From mistake fares to tarmac delays, passengers responded with everything but silence as customer service-related complaints against U.S. airlines rose 33% during the past year.

Overall complaints are up nearly 60% year-over-year for U.S. airlines, and customer service specific complaints, 1,770 to be exact, represent 11.3 percent of the nearly 12,000 complaints U.S. carriers received between January and September, according to data released this week from the U.S. Department of Transportation.

That’s a 10% year-over-year increase in terms of customer service’s share of all complaints logged and American Airlines, as the largest U.S. carrier, received the most customer service complaints in absolute terms (315). But Spirit Airlines’ customer service performance (142 related complaints) was the worst in relative terms considering it had the most complaints, 11.3 per 100,000 passengers versus Frontier Airlines’ 9.35 and American’s 3.51.

Complaining is essentially a cornerstone of Spirit’s brand and the carrier actually embraced it as part of its Bare Fare campaign rolled out earlier this year. Between January and September the carrier also received 601 complaints for flight problems, 193 for ticketing/reservations/boarding, 190 for baggage, and 173 for fares–the latter its business’ backbone.

Fare- and fee-related complaints are up 62% year-over-year for U.S. carriers, coinciding with mistake fare mishaps from both United and American Airlines in 2015, with the Department of Transportation siding with United but requiring American to honor $0 fares from the U.S. to China.

Skift recently looked at what it takes to successfully manage an airline customer service Twitter account, and following are two charts breaking down complaints against U.S. carriers by sub-categories so far this year:

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Source: U.S. Department of Transportation

Top 10 U.S. Airlines Receiving Most Complaints, January-September 2015

AirlineFlight ProblemsOversalesRes/Tkng/BoardingFaresRefundsBaggageCustomer ServiceOther ComplaintsTotal
American Airlines7751053673862934393152152,895
United Airlines680872271831303422861882,123
Spirit Airlines60142193173149190142681,558
Frontier Airlines30715146637713410942893
Delta Air Lines29841915112102103102800
US Airways236521155059938264751
Southwest Airlines204135012291327573588
Allegiant Air16741384458335634534
JetBlue Airways805202019344419241
SkyWest Airlines12945002155160

Source: U.S. Department of Transportation

Photo Credit: Spirit Air CEO Ben Baldanza (L) and a puppet chatting about the carrier's Bare Fare campaign earlier this year. Despite trying to be clear about its offerings and advertisements, its passengers are still complaining. Spirit Airlines