The mid-December holiday travel mess that left a dozen Frontier Airlines planes stuck on the tarmac for hours hit the Denver airline in the pocketbook Friday as the U.S. Department of Transportation fined Frontier $1.5 million.

The airline violated the department’s rule prohibiting tarmac delays of more than three hours, according to the agency.

Frontier needs to pay only $600,000 of the fine because it compensated affected passengers on delayed flights, according to a DOT statement. Beyond the fine, the airline was ordered to cease and desist from future similar violations.

It was the third DOT fine that privately held Frontier has faced in as many months. In July, the department ordered the airline to pay $400,000 for violating procedures for bumping passengers from oversold flights and for failing to properly assist disabled passengers.

Frontier last month was fined $40,000 for failing to provide customers with required information about compensation for being bumped and for lost or damaged luggage.

Regarding Friday’s penalty, Frontier spokesman Richard Oliver said the airline revised policies shortly after the December incident, in which a winter storm forced the airline to cancel 275 flights nationwide.

Before its planes waiting on the tarmac reach the three-hour maximum, Frontier will work with Denver International Airport to pull the plane under the A bridge, or closest to the A concourse, and get stairs rolled out for passengers to deplane and get indoors.

“Frontier remains committed to complying with DOT rules and regulations, including those relating to lengthy tarmac delays,” Oliver said. “During last December’s crippling storm, our operation in Denver was faced with a myriad of operational challenges. We have since revised our procedures for irregular winter weather operations and have worked with DIA’s airport authority to implement a drop-and-go deplaning process that will prevent any future occurrences.”

The DOT said that on Dec. 16-18, Frontier let 12 domestic flights sit on the tarmac for more than three hours without giving passengers the chance to deplane.

According to the DOT’s Air Travel Consumer Report for December, 14 of 21 flights that violated the tarmac delay that month were Frontier flights. Two of the Frontier flights were not penalized because those aircraft started to return to the gate within three hours of the main door closing.

But of the remaining 12 Frontier flights, the longest delay — 4 hours, 25 minutes — was Flight 418 from Denver to Atlanta. At the other extreme of delays that were fined, Flight 509 to Denver from New York’s LaGuardia Airport was two minutes over the limit.

Beyond the three-hour maximum, aircraft with 30 or more passengers must also provide food and water and make sure the bathrooms are working during long delays on the tarmac.

Frontier paid out $1.2 million in cash reimbursements and vouchers. The Department of Transportation credited Frontier with $900,000. Still, the department said that the $1.5 million fine was the second highest levied against an airline violating the tarmac delay. American Airlines and Southwest Airlines each received $1.6 million fines in 2015.

Frontier’s problems in December extended beyond planes stuck on the ramp. The storm dumped 8 inches of snow on the airport on the weekend before the busiest travel period of the year, more than double the amount forecast, and left passengers stranded overnight in the airport. About 70 percent of Frontier’s flights that weekend experienced some kind of delay. It took several days to sort through mountains of bags and return them to their owners.

Airline officials explained that 40 percent of the airline’s network goes through Denver. When flights to Denver were canceled, crews and aircraft were left out of place, creating a domino effect for later flights from other cities. Compounding the problem, Frontier workers were unable to get to the airport during the storm, they said.

Frontier officials conceded they should have precanceled more flights in anticipation of the weather event, allowing passengers to rebook on other flights, and they did so prior to a subsequent storm in January.