Ask Southwest Airlines employees about their company's technology, and one word keeps coming up: "antiquated."While Southwest grew from a scrappy, Texas-based discount airline operating three planes into one of the nation's largest -- union officials representing Southwest workers say the company did not keep pace with technology changes. And they say they've been raising concerns for years."We've been harping on them since 2015-ish every year," Mike Santoro, a captain and vice president of the Southwest Airlines Pilots Association, told CNN.The result: A massive Christmas travel meltdown that scuttled holiday plans for hundreds of thousands of passengers. Nearly 16,000 flights canceled. Orphaned baggage piling up at airports and travelers told to give a shipping address.Southwest is digging itself out of the mess and announced plans to "return to normal operations with minimal disruptions on Friday."Leaders of multiple labor unions also told CNN that management briefed them about a more normal flight schedule heading into the holiday weekend.But as extreme winter conditions swept much of the country last week -- including important airports in Southwest's network -- the airline's plan for "irregular operations" passed the breaking point, according to multiple people familiar with the situation.They and the airline itself described an internal process that requires multiple departments to manually redesign the airline's schedule -- a system that works "the vast majority of the time," Southwest said in a statement.Manual labor"The magnitude and scale of this disruption stressed our technology and processes, forcing a great deal of manual processing," Southwest said. "Our crews are showing up in every way throughout this challenge."Other airlines managed to recover by early this week; at Southwest, the cancellations only increased.While Southwest does have major connecting airports, much of its schedule involves planes and crews crisscrossing the country -- a network that aviation watchers say is more vulnerable than legacy carriers' hub-and-spoke model that can contain a disruption to particular geographic regions.When something goes wrong, the Southwest software -- including the crew scheduling system tool -- leaves much of the work of rebuilding that delicate network to be done manually."It can't see the best way to fix anything when flights are canceled," said Brian Brown, president of Transport Workers Union Local 550, representing Southwest dispatchers and meteorologists. "It requires a lot more human intervention and human eyesight or brainpower, and can only handle so much."Where are the crews? The planes?The result is that airline officials "don't necessarily know where our crews are, where our planes are," Brown said.Crew schedulers in another department are manually checking which pilots and flight attendants meet strict federal rules on work hours -- rules meant to keep inflight safety professionals from excessive fatigue."You end up with thousands of crew members having to call in and their wait times were hours just to talk to someone," Brown said. Software enhancements would make the process more efficient, he added.The manual work meant crew members who could be working were instead stuck in lengthy phone queues waiting for instructions or for a hotel assignment to get their federally mandated rest."The phone systems that the company uses is just not working," Lyn Montgomery, who represents Southwest flight attendants at TWU Local 556, told CNN."They're just not manned with enough manpower in order to give the scheduling changes to flight attendants and that's created a ripple effect that is creating chaos throughout the nation."Tech improvements have been spottySouthwest has made some software improvements. Last year, the airline connected its reservation system with major booking software used by corporate travelers. The reservation system was overhauled in 2017 -- a process overseen by current CEO Bob Jordan.In recent years, the airline also introduced barcodes and scanners that replaced the pencil and paper method for counting checked baggage. But other tools remained rooted in 1990s technology, union officials said.As the storm swept through, Southwest employees who load baggage, marshal planes into the gates, push back departing aircraft and remove ice from aircraft surfaces were racing to keep up in the frigid outdoor conditions."We've had experienced people who were trying to de-ice aircraft, and they were literally not able to," said Randy Barnes, president of TWU 555. Some wind gusts were so dangerously strong that employees suspended from the air in bucket trucks were at risk of falling, and sprayed de-icing fluid was not reaching aircraft."They were trying to convey that to their local management team," Barnes told CNN. "They were saying you just can't do it -- it does not work."Brown said he and local union agents also advocated for special pay for employees working in these conditions and for opportunities to come inside for a break from the bitter cold.Video apologyJordan, who took the reins earlier this year and began at Southwest as a programmer, apologized in a video message and pledged "to double down on our already existing plans to upgrade systems for these extreme circumstances so that we never again face what's happening right now."As the airline prepared to run a full schedule on Friday, Jordan wrote in an internal message to employees -- that was obtained by CNN -- that the airline is "in good shape.""We have all hands on deck and tested solutions in place to support the restored operation," he wrote. "I'm confident, but I'm also cautious."