Southwest Airlines is testing a software fix it developed after the Christmas travel meltdown

    (CNN) -- Southwest Airlines said it is testing software fixes that the company developed after its Christmas travel meltdown, as the airline faces multiple federal investigations.

The software fixes are an "upgrade," rather than a replacement of the crew scheduling system, Southwest executives said on a conference call with reporters Thursday. The airline and its employees have said the scheduling software left the company unable to recover from winter storms on some of the busiest travel days of the year and caused it to cancel more than 16,700 flights between December 21 and 29, roughly half its schedule during that period.

The company decided to keep the underlying software system because it "generally worked as designed" even during the meltdown, CEO Bob Jordan said. The software's shortcoming, he said, is "solving past problems."

The company is currently testing the software and expects to begin using it "in a few weeks' time."

Southwest's cancellations dwarfed other airlines during the Christmas storm because crew members had to call in to the airline, rather than notify the company electronically, to let them know of their availability.

"That was a problem," said Andrew Watterston, Southwest's chief operating officer Thursday. "It wasn't the problem for the situation. It was a symptom of the problem."

Switching to electronic notification would require a change in the labor contracts with pilots and flight attendants, said Jordan. Negotiations are now taking place on replacing the existing contracts covering all issues, including pay and benefits.

Other changes stemming from the company's review of its winter meltdown include a new team in its command center, telephone system improvements, and better preparedness for bitterly cold weather.

"We're looking at de-icing procedures top to bottom, we're buying more engine covers for extremely cold weather, we're looking at fuel mixes for ground equipment when you have sub-zero temperatures," Jordan said.

The company said it doesn't have a cost estimate for the fix.

"We haven't even talked cost, so I don't know if it's going to cost us anything or not," said Southwest Chief Operations Officer Andrew Watterson.

Investigations

The airline's executives also pushed back on the Department of Transportation's announcement late Wednesday that it is investigating whether Southwest "engaged in unrealistic scheduling of flights" by selling more tickets than it could handle.

If that were the case, "then you'd expect to see poor on time performance, poor reliability" even on good weather days, Watterson told reporters on a conference call Thursday.

"You don't see the signs of a schedule that is out of whack with the resources' ability to operate, given our strong operating performance over the last three months," Watterson said.

In addition to the DOT investigation, the ongoing reviews include an internal probe, one led by its board of directors, and an external inquiry conducted by a consultancy firm. That external report should be delivered in the coming weeks and "we will attack it with a sense of urgency," Jordan said.

-- CNN's Chris Isidore contributed to this report