Frontier Airlines saw the highest rate of passenger complaints among U.S. carriers in 2022, a year that became one of the most disruptive for flyers on record.

The Denver-based company registered more than 20 complaints for every 100,000 passengers who boarded its aircraft, according to data from the Bureau of Transportation Statistics published earlier this month. That compares with a national average of about six passenger complaints per 100,000.

A Frontier representative did not respond to a request for comment.

The combination of a post-pandemic boom in air travel demand and reduced staffing as a result of pandemic-fueled layoffs and retirements led to a series of delays and cancellations in 2022. The year was capped off by the late December shutdown of Southwest Airlines' flight schedule amid a harsh winter storm and system outages.

The number of complaints against all airlines (both American and foreign) jumped between 2021 and 2022 from 49,991 to 77,656 — with grievances about U.S. airlines more than doubling on the year, from 20,365 to 47,591.

“Air travel is just a mess right now,” said Teresa Murray, director of Consumer Watchdog at the nonprofit U.S. PIRG Education Fund in an accompanying writeup. Murray is also the author of a report on last year's airline troubles.

“Our analysis shows the complaints-to-passengers ratio was more than five times higher in 2022 than in 2019," Murray said.

"Airlines often post unrealistic schedules, they cancel flights and they drag their feet with refunds. Airlines and online ticket agents just don’t face enough consequences when they abuse customers.”

Seattle-based Horizon Air had the lowest complaint rate, at just 1.09; followed by Utah-based SkyWest Airlines at 1.37 and Phoenix-based Mesa Airlines at 1.74. Among the largest carriers, Delta had the lowest complaint rate at 2.66 while Southwest Airlines had the highest at 6.75.

PIRG found consumers filed more complaints against U.S. airlines in 2022 than in any year in at least a quarter century. The 77,656 complaints consumers filed in 2022 was the second-highest number behind 2020 — but that year, most of the 102,550 complaints were against foreign airlines, it found.

The 47,591 complaints that consumers filed last year against U.S. airlines topped any year since 1997, PIRG said.

The data show fewer than 77% of flights arrived on time last year, compared with more than 81% in 2021. Delta had the best rate, at 82.5%, while the ultra-low-cost carrier Allegiant Air had the worst, at 63%.

The national flight cancellation rate was 2.7%, up from approximately 1.8% in 2021. Hawaiian Airlines had the lowest cancellation rate, at 0.92%, while JetBlue had the highest, at 3.74%.