U.S. airlines are having trouble keeping flights on time this winter, and they are recording a sharp increase in long delays.

The Transportation Department said Tuesday that 42 flights in January were stuck on the ground so long that the airlines could face fines. That is the highest number of long ground delays in one month since February 2010, shortly before the rule allowing fines took effect.

Only 76 percent of flights on leading airlines arrived on time in January, down sharply from 81.3 percent a year earlier. The government defines on time as arriving within 14 minutes of schedule.

That followed a similar pattern in December, when delays were more common than a year earlier.

Hawaiian, Delta and American had the best on-time ratings. Virgin America had the worst.

The Transportation Department said 30 domestic flights were delayed on the ground at least three hours and 12 international flights were stuck at least four hours. Those flights appeared to violate federal rules requiring airlines to give long-delayed passengers a chance to get off the plane. The department said it was investigating all 42 incidents.

The department said that 2 percent of flights on the leading 12 U.S. airlines were canceled in January, down from 2.6 percent in January 2016 but worse than December.

Consumers filed 20 percent fewer complaints about U.S. airlines than they did a year earlier. Few consumers bother to file a report with the Transportation Department; many more complain directly to the airline instead.

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The full report can be found here.