Southwest Airlines, whose customers endured chronic delays for more than a year, is improving its on-time performance after changing its schedule in the summer, according to federal and industry reports.

The airline has ranked at or near the bottom of U.S. airlines for on-time rates since August 2013, when it added flights to its daily schedule. The change squeezed the window between a plane's arrival and its next departure.

Southwest officials have conceded the move backfired, sparking operational difficulties and delays that cascaded through its system. It still enjoys a good reputation with the flying public, however.

Dallas-based Southwest ranked among the worst on-time airlines for more than a year, in some months ranking dead last. In December 2013, for example, Southwest flights nationwide were on time just 57.7 percent of the time. On-time flights are generally defined as arriving within 15 minutes of the scheduled arrival time. All other large airlines that month posted on-time rates of 70-80 percent.

But in recent months, Southwest entered the middle of the pack for on-time rates among U.S. airlines. In August, it added time that it allots for such tasks as flying, taxiing, loading and unloading passengers, and preparing the aircraft for the next flight.

"It's definitely not nearly as horrible as it was before August," said Brett Snyder, a blogger at CrankyFlier.com and operator of a travel concierge service.

Southwest officials, too, say the airline has improved since August, when the majority of its "schedule enhancements" took effect.

"Since that time, we have posted significant on-time performance improvements, especially on a year-over-year basis," said Southwest spokesman Brian Parrish.

The airline met its 80 percent on-time goal for September and October, despite delays nationwide that resulted from sabotage of an air traffic control radar center in a western Chicago suburb. The disruption hit Southwest especially hard: The airline has more departures at Chicago's Midway airport than any other airport.

The radar center was not fully operational from Sept. 26 to Oct. 12.

Other data sources reflect Southwest's improvements.

On-time numbers from masFlight, which has more current data than the U.S. Transportation Department, shows Southwest ranking sixth or seventh since August among 11 airlines, including seventh last month.

"Southwest has vaulted from the bottom of the list to middle of the pack, which would indicate operational improvements," said Edmund Otubuah, managing director of commercial aviation products at masFlight.

FlightStats, which calculates on-time rates slightly differently, ranked Southwest as high as fourth in November, with an on-time rate of nearly 83 percent.

On-time numbers from the Department of Transportation have a lag time, with the most recent data from October showing Southwest ranking eight of 12, its best ranking in eight months.

The operational difficulties stand in stark contrast to Southwest's general reputation. The airline sits atop many customer satisfaction ratings, with such consumer-friendly policies as free checked bags and no fees to change a flight.

In the mid-1990s, Southwest, for five years running, was tops among U.S. airlines for on-time performance, baggage handling and low complaint rates. It bestowed on itself the "Triple Crown" award for excelling in those measures that are important to consumers.

But Southwest, which entered the Minneapolis-St. Paul market in 2009, isn't the same airline anymore. It entered big airports in congested markets — New York's LaGuardia, Washington's Reagan National, Boston's Logan and Los Angeles. It's also flying bigger and more crowded planes. Both developments make it harder to be on time, which means Southwest might not make it back atop the on-time rankings anytime soon.

"Southwest has built an incredible following after years of providing low fares, frequent on-time flights and excellent service," Snyder said. "While the excellent service remains, fares have skyrocketed while on-time performance has fallen.

"It's a different airline than it used to be, but the reputation is so strong that it will take time for it to reflect reality again."