Nearly one in four U.S. flights were late in August, and some planes sat on the ground so long that the airlines could get fined.
The U.S. Department of Transportation said Friday that 77.6 percent of domestic flights arrived on time in August. That was down from 80.3 percent in the same month of 2015.
Hawaiian Airlines, helped by favorable weather, had the best on-time rate, while low-fare carriers Frontier and Spirit had the worst rates. Among the four biggest airlines, Delta narrowly beat Southwest for top honors, followed by United and American.
In 20 cases, planes holding passengers sat on the ground longer than federal rules allow - three hours for domestic flights, four for international ones. The Transportation Department said it was investigating the incidents.
At least 11 of those delayed flights were operated by American and five by United or United Express.
Those excessive on-ground delays have dropped sharply since federal rules allowed the department to fine the airlines.
Passengers lodged 1,602 complaints with the department against U.S. airlines in August, compared with 1,639 in August 2015.