Here’s proof US airlines can’t win against customer complaints

Traveler complaints jumped 34% last year, to the highest level since 2000.

Customer complaints were led mostly by concerns over cancellations and delays, which is unchanged in 16 years.

“Everything is getting better, but they are still unhappy about the same things,” says Dean Headley, a marketing professor at Wichita State and co-author of an annual report on airline quality.

So why are customers growing increasingly angry as service actually improves? Headley believes customers have grown to resent the extra fees that many airlines are now charging for checked and carry-on bags, and other services.

Researchers used publicly available information from the US Department of Transportation to rate the airlines for on-time performance, baggage handling, bumping passengers because of oversold flights, and complaints filed with the government.

Here are some of the biggest findings from the report.

On-time flights: The percentage of flights that arrives on time was 79.9% last year. That’s up from 76.2% in 2014.

Lost baggage has improved: The rate of bags being lost, stolen or delayed fell by 10% in 2015.

Less overbooked flights: The number of over booked flights dropped by 17% last year. That number doesn’t count people who voluntarily gave up their seats for money or a travel voucher. Those people are less likely to complain since they typically receive very nice incentives to give up their seat.

Increasing complaints.

Airline customers filed more than 15,000 complaints with the Transportation Department last year. In 2014 about 11,000 complaints were issued.

As expected Spirit Airlines, Frontier Airlines, and other budget carriers led the way with complaints. Budget airlines often have the worst on-time records, charge the most for extras such as baggage fees and drinks, and in general provide poor service compared to higher end airlines.

670 million people flew in the US in 2015 and it was the fifth increased in complaints in the past six years.