During the first six months of the year, passengers filed 9,542 complaints, up from the 7,935 during the same period a year earlier.
Cancellations, delays and missed connections were the top causes, garnering 3,107 passenger complaints. Another 1,417 complaints dealt with baggage, 1,282 ticketing and boarding problems, and 1,121 customer service.
Two of the largest airlines, American and United, had the most complaints, with 1,540 and 1,271, respectively, for the January-through-June period. (US Airways, which has become part of American, had 608 complaints.)
But the much smaller Spirit Airlines ranked third during the six-month period with 937 complaints and Frontier Airlines followed with 622 complaints. While American and United each had less than three complaints per 100,000 passengers, Frontier's rate was more than 10 per 100,000 passengers and Spirit's more than 11.
Overall, nearly 78% of flights arrived on time during the first six months of the year. The department counts on-time flights as those arriving within 15 minutes of their scheduled departure or arrival.
Among the 14 airlines that report to the department, low-cost carriers had some of the worst delays.
Spirit had less than half its flights arrive on time during June. The percentage of the airline's flights arriving late dropped to 34% when including the figures for the first six months of the year.
Frontier ranked third-worst in June, with 1 out of every 3 flights arriving late. For the six-month period, about 32% of its flights arrived late.
United ranked second-worst in June for delays, also with about a third of its flights arriving late. But the carrier had the fifth-worst mark for the first half of the year, with about a quarter of its flights arriving late.
“While our performance in June didn’t meet our expectations, we are committed to doing what it takes to run a more reliable airline," Hart said.
The best airlines for on-time arrivals in June were Hawaiian at more than 90%, Alaska at 87% and Delta at 82%. The same trifecta led the half-year statistics, as well.
Delays in June were blamed on a variety of reasons. About 6.3% of delays among the reporting airlines were blamed on the aviation system, up nearly 1 point 6.29% from a month earlier. Nearly 7% were blamed on factors beyond the airline's control, up from 5.3% a month earlier. Weather delayed a small share of flights, put the portion ticked up to 0.87% from 0.73% a month earlier.
The six-month consumer report also shows that airlines handled bags properly for 99.6% of passengers while completing 98% of their flights with 78% of the flights arriving on time.
Airlines are committed to continually improving, with the 10 publicly traded airlines investing $1.4 billion each month during the first half of the year, said Melanie Hinton, spokeswoman for Airlines for America, a trade group for the largest airlines.
"With the investments that airlines are making in the flight experience and with their employees, customers are benefiting and flying more than ever before," Hinton said. "Flying remains a bargain, as evidenced by the record number of people traveling this summer."
The rate of mishandled bags and bumping travelers from flights each declined.
Airlines mishandled bags at a rate of 3.63 per 1,000 passengers in June, down from a rate of 3.77 a year earlier, said the report. And involuntary bumping dropped to 0.79 per 10,000 passengers during April, May and June, down from 1.05 rate during the same period a year earlier.