A Delta jet takes off in view of an Alaska Airlines plane that just landed at Seattle-Tacoma International Airport in Washington state. (Elaine Thompson/Associated Press)

More than 20,000 passengers were unhappy with their airline experience in 2015, an almost 30 percent increase in displeasure over the year before, the U.S. Department of Transportation reported Thursday.

The number of passengers who filed complaints rose from 15,539 received by DOT’s Aviation Consumer Protection Division in 2014. The final month of last year was a big one for passenger complaints, DOT said, after recording 1,565 dissatisfied customers. That was a nearly 20 percent increase from November and an almost 47 percent bump from December 2014.

Complaints are filed with the DOT over all sorts of things connected with flying: flight problems, baggage, reservation and ticketing, refunds, consumer service, disability and discrimination.

Airlines reported an on-time arrival rate of 77.8 percent in December 2015, up from the 75.3 percent on-time rate in December 2014. But that was a decrease from November 2015, when 83.7 percent of flights were on time.

The most on-time airlines in December were Hawaiian Airlines (93 percent on time), Alaska Airlines (85 percent) and Delta Air Lines (83.6 percent). The airlines with the worst on-time performance were Spirit Airlines (68.7 percent), JetBlue Airways (70 percent) and Virgin America (71.1 percent).

For the full year in 2015, airlines recorded an on-time arrival rate of 79.9 percent, up from 76.2 percent in 2014.

In 2015, airlines told DOT they canceled 1.5 percent of their scheduled domestic flights, an improvement over the 2.2 percent cancellation rate in 2014. In 61 instances last year, domestic flights were delayed on the tarmac for more than three hours.

In December, airlines reported that 22.17 percent of their flights were delayed. They told DOT that 5.70 percent of their flights were late because of aviation system delays, compared with 4.84 percent in November; 7.64 percent of delays were caused by late-arriving aircraft, compared with 5.18 percent in November; 6.13 percent by factors, such as maintenance or crew problems, compared with 4.60 percent in November; 0.66 percent by extreme weather, compared with 0.42 percent in November; and 0.06 percent for security reasons, compared with 0.04 percent in November.

Lost baggage has become less of a problem with the advent of bar-coded baggage tags and computer networks that track luggage. But U.S. airlines said baggage was mishandled for four out of every 1,000 passengers in December. For all of 2015, the airlines posted a mishandled baggage rate of 3.24 per 1,000 passengers, down from 2014’s rate of 3.61.

In December, airlines said there were four incidents involving the death, injury or loss of animals while traveling by air, down from the six reports filed in November 2015. For all of 2015, carriers reported 35 animal deaths, injuries to 25 other animals, and three lost animals, for a total of 63 incidents, up from the 45 total incident reports filed for 2014.