Airlines are doing a better job of sticking to their schedules, and consumer complaints are falling.

The Department of Transportation said Monday that 84.5 percent of flights on the largest 12 U.S. airlines arrived on time during April, better than the previous month and up from 81.8 percent in April 2015.

Hawaiian Airlines and Delta Air Lines were the best at staying on schedule, while Spirit Airlines was last, arriving late more than one-fourth of the time.

Consumer complaints about U.S. airlines fell 20 percent to 870 from 1,083 in April 2015.

April flights on Hawaiian, which operates on routes that are usually blessed with good weather, arrived on time 94.1 percent of the time. Delta had a 90.3 percent on-time rating. Discount carrier Spirit arrived on time just 73.8 percent of the time.

The government counts flights as on-time if they arrive within 14 minutes of schedule.

Passengers on two domestic flights, one by American Airlines and another by Alaska Airlines, and two international flights operated by United Airlines were stuck on the ground longer than allowed by federal regulations. The Transportation Department said it was investigating; violations can bring fines.

The department said it received 870 consumer complaints about U.S. airlines, down from 1,083 in April 2015. That was a tiny fraction of the roughly 60 million people who boarded planes during the month, although many consumers who protest to airlines don’t bother to file a complaint with the government.

Southwest Airlines had the lowest rate of complaints, while Spirit had the highest rate by a wide margin.