Griping about the airlines has become such par for the (con)course that Spirit Airlines launched a "State of the Hate" campaign last year asking fliers to share their biggest complaints, generating 30,000 responses. Now, the U.S. Department of Transportation has an even more precise assessment of Americans' disdain for the airline industry: The number of airline service complaints rose 17.9 percent in 2014 from the previous year, the DOT said in a report released on Tuesday

The total number of complaints filed with the DOT -- which most experts agree is likely only a fraction of the amount of complaints filed with airlines themselves -- was a whopping 15,532. American consumers haven't been this enraged with the airlines since 2001, when they filed 16,508 such complaints with DOT. 

The report also found that baggage mishandling, in which luggage is lost, damaged, delayed or stolen, went up in 2014 (3.62 incidents per 1,000 passengers, up from 3.22 in 2013). United Airlines was the biggest offender in this category, averaging 5.20 incidents per 1,000 passengers, while Virgin America did the best, averaging just 1.18 incidents per 1,000 passengers. 

Baggage wasn't the only area in which airlines declined in performance. More passengers with confirmed reservations were denied boarding due to overbooking in 2014 than the previous year: 0.92 per 10,000 passengers, up from 0.90 in 2013. 

But airlines are getting better at curbing tarmac delays. There were only 30 domestic flights with tarmac delays of more than three hours, and just nine international flights with tarmac delays longer than four hours. This is likely due to 2010 regulations that state an airline can be fined up to $27,500 per person stuck on a plane on the ground for more than three hours. Before the new rules, there were 868 domestic flights with tarmac delays in 2009.