Passengers aboard a Denver-bound flight last year spent seven hours stuck on a tarmac, one example of the rapidly increasing “tarmac delays” that air travelers face, according to a new study.
The overall on-time performance of U.S. airlines has reached its worst level since 2014, with nearly a quarter of flights delayed, diverted, or canceled, according to the “Plane Truth 2026” study, which the consumer watchdog group unveiled Tuesday. One in 12 flights in 2025 arrived an hour or more late, the study found.
In Colorado, the on-time arrival rates at (75.4%) and the (73.1%) both lagged below the national average of 76.3% for the largest U.S. airlines. Denver-based Frontier Airlines ranked worst among major airlines for the fourth year in a row in customer complaints and involuntary passenger bumpings.
The study draws on federal data collected by the from airlines. It presents a picture of air travel becoming more of a hassle for travelers, ahead of the busy summer travel months when complaints typically peak.
Travelers this summer can expect “erratic and stressful adventures,” CoPIRG’s Colorado director Danny Katz said after presenting study findings at .
“It is dumbfounding that we are having people sit on their planes for more than three hours and not holding airlines accountable,” Katz said. “Congress should take action. The Trump Administration should take action.”
U.S. airline tarmac delays exceeding three hours increased to 708 in 2025, up by 63% from 435 in 2024 and 2.4 times more than the 289 in 2023, according to the federal data in the 44-page study. Tarmac delays exceeding four hours on international flights increased to 77, up from 64 in 2024.
On July 14, the Denver-bound passengers on United Airlines Flight 2852, who waited on the tarmac at Newark, N.J., endured the longest delay of 2025. Among the domestic flight tarmac delays in 2025, 140 lasted four hours or more, 55 lasted five hours or more, and 23 lasted six hours or more, CoPIRG researchers found.
Tarmac delays have hit the highest level since 2010, when a took effect requiring airlines to let passengers leave the aircraft after three hours. The FAA rule says airlines are liable for fines up to $27,500 per passenger if a domestic flight stays on the tarmac for more than three hours and an international flight for more than four hours.
Flight cancellations and delays increased in 2025, the study found. On the 14 largest airlines, 102,876 flights were canceled in 2025 (1.47% of flights), up by 8% over the percentage in 2024.
Travelers on U.S. airlines saw 1.66 million flights delayed, canceled, or diverted in 2025. Thatap the worst on-time performance since 2014.
Denver-based Frontier Airlines led in complaints with a rate of 19 per 100,000 passengers, based on data from October through December 2025, according to the study, and also ranked worst for involuntarily bumped passengers on overbooked flights. Frontier bumped 8,087, with a rate of 2.51 per 10,000 passengers, ahead of American Airlines, which bumped 14,758 with a rate of .72 per 10,000 passengers.
Frontier is “strongly focused on improving operational reliability,” and officials have “launched a revamped system-wide maintenance strategy, along with enhanced airport operations,” company spokeswoman Jennifer de la Cruz said.
Frontier officials “are seeing positive early results,” she said. “We anticipate further improvements.”
Among the other major airlines at DIA, Southwest, Alaska, and Allegiant had the fewest complaints.
