Southwest Airlines’ struggles with on-time reliability continued in January and dragged down results at Chicago’s Midway Airport, its largest hub, federal data show.

The low-cost carrier is hiring workers and adding three departure gates to improve results at Midway, which again had the lowest rate of on-time departures among major airports.

Southwest is straining to keep pace with skyrocketing numbers of passengers and checked bags resulting from its “Bags Fly Free” marketing campaign, Southwest executives told the Tribune last month.

The crunch is greatest at Midway, which is poised to become the largest airport in Southwest’s system this summer if the carrier adds more flights there, as planned.

About 60 percent of Midway’s flights left on-time, compared to 74 percent at Chicago’s O’Hare International Airport, according to the Bureau of Transportation Statistics.

The on-time results mirror those of the largest carriers operating at both.

O’Hare’s largest carrier, United Airlines, finished third overall for the month, with an 84.5 percent on-time rate, federal data show. Those results do not include contract carriers flying under the United Express brand.

Texas-based Southwest finished 13th among the 16 carriers reporting January results, with a 74.4 percent on-time rate. Southwest accounts for 87 percent of Midway’s passengers, according to BTS.

For decades, Southwest’s on-time performance bested its peers because it flew mainly to smaller, uncongested airports and operated a point-to-point network that emphasized getting passengers to their destinations in a single aircraft.

But that has changed with the carrier’s recent expansion into hotspots including New York City and Boston, and as it uses Midway as a hub that funnels passengers to destinations around the country.

Greg Wells, Southwest’s senior vice president for operations, describes the resulting hybrid network as a “maze of an operation” that has made it difficult to reverse slipping on-time results in the last year.

“Most airlines, including legacies, can learn from each other,” Wells said. “There is no one to learn from at Southwest. We have blazed the trail. We are inventing this stuff as we go.”

Though the vast majority of Southwest’s passengers still fly directly to their destinations, about half of Southwest’s Midway traffic is now connecting, said Ned Laird, a Seattle-based aviation consultant.

Midway is also a focal point of Southwest’s ambitious growth strategy, which will add Newark, N.J.; Charleston, S.C.; and Greenville-Spartanburg, S.C. to the airline’s network this month.

“At Midway, they operate more like an operator at O’Hare,” Laird said of Southwest. “It makes the transport of the passengers more complex. As a result, the operation has not functioned as efficiently.”

Adding to Southwest’s growing pains is the market share it grabbed from other airlines by waiving fees for checked luggage. Southwest earlier reported that the total number of passengers it carried in January increased nearly 10 percent, from 8.3 million people a year earlier.

Ground workers says they are also dealing with record amounts of baggage, one of the most complicated aspects of airport operations.

“The planes are coming in with more bags, period, because people check more bags,” said Charles Cerf, president of TWU local 555, which represents all the ground workers at Southwest and is pushing the airline to boost hiring.

“The agents are very concerned about delivering that legendary customer service that we’ve all had at Southwest,” Cerf told the Tribune last week. “It’s a pride thing and a culture issue with our members. They’re concerned that they may not be able to capture that service like they used to.”

Responded Brandy King, a Southwest spokeswoman: “We are currently hiring in Chicago, and we continue to evaluate our operational needs – making necessary staffing adjustments, as needed.”

The carrier has several initiatives under way to improve its performance, Wells said. They include bringing in a team of airport managers to dig into Midway’s bottlenecks and brainstorm possible solutions.

This team has master-minded several “high-impact” Southwest initiatives, including “Express Bag Drop,” kiosks designed to shorten check-in lines by tending to customers who have boarding passes and simply need to check their luggage. The baggage drops are in place in Oakland, Denver, Phoenix, Chicago, Baltimore, Orlando, Los Angeles, Dallas, Nashville, Houston and San Diego with Tampa, St. Louis and San Jose coming soon, King said.

jjohnsson@tribune.com