Three days before the busiest day of the holiday travel season, Denver International Airport’s third largest carrier still isn’t back to normal.

Frontier Airlines completed about 80 percent of its 300 scheduled flights on Tuesday, but mountains of baggage need to be sorted and hundreds of passengers still are frustrated as the ultra-low-cost carrier struggles to recover from the aftermath of heavier-than-forecast snow over the weekend.

In Denver, the airline canceled 21 of 124 scheduled flights.

Airline Weekly managing partner Seth Kaplan said Frontier’s slow recovery is likely because the airline typically flies to a destination only once a day, and has outsourced jobs such as ticketing and baggage handling, which can make it hard to add staff in a crisis.

“Passengers, yes, want to be on time and not have their bags lost, but also want very low fares,” Kaplan said. “It’s a tricky balancing act.”

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Frontier has 64 planes serving 59 cities, Kaplan said, citing data from airline schedule provider Diio and airline intelligence provider ch-aviation. By comparison, Southwest has 712 airplanes serving 101 cities. If one Southwest flight is canceled, passengers have a better chance of getting on another flight to the same destination that day.

“If a Frontier flight to a certain destination gets canceled or from a certain origin gets canceled, there’s less of a chance of another flight that day,” he said.

Frontier spokesman Richard Oliver said the airline is still trying to figure out what went wrong over the weekend, but said it was not the schedule. Only 10 of the destinations the airline flies from Denver operate only a few days a week, he said.

The real trouble, he said, is that 40 percent of the airline’s network routes 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. Tuesday morning, eight planes were still in the wrong location, but that number dropped to one by noon, he said.

Another issue was the inability of workers to get to the airport on Saturday, putting an increased workload on a smaller staff still at DIA when the snow ended, he said.

“We do believe that had all our employees been able to make it on Saturday morning, we wouldn’t have the same backlog that we did,” Oliver said.

Frontier should have pre-canceled more flights in anticipation of the storm, said Jim Faulkner, another airline spokesman. This would have allowed passengers to rebook in advance of the storm that dumped more snow than was forecast.

The carrier is sorting through piles of checked baggage at DIA, much of it left behind by passengers after hours of delays, Oliver said. The airline delivered more than 500 bags to Colorado travelers Tuesday, Faulkner said.

All bags that belong to travelers who do not live in Denver were expected to be sent to the proper airports by Tuesday night so they could be delivered to passengers.

“We are coming out of this and returning to normal operations,” Oliver said, adding that the airline will be ready to handle the holiday rush.

Friday is expected to be the busiest day of the two-week holiday travel season, with 24 percent more passengers passing through DIA than on an average day.

Frontier’s issues have not snowballed into the rest of the airport operations, DIA spokesman Heath Montgomery said.

“We have seen more people in their ticket counter areas and baggage claim areas and customer service areas,” Montgomery said. “Yes there have been some impacts, but they’ve really been contained to the Frontier operations.”

Holiday traveler counts at DIA are expected to be up about 10 percent from last year, Montgomery said.

Multiple airlines canceled more than 300 flights at the Denver International Airport Saturday after 8 inches of snow dumped on Denver, more than double what was forecast.

Frontier had the third worst on-time arrival rates in October, the most recent month available, with 77.8 percent of flights arriving on time, according to the Bureau of Transportation Statistics. That same month, it also had the third highest rate of canceled flights, at 1.4 percent.

Staff writer Emilie Rusch contributed to this story.