WASHINGTON - Southwest Airlines’ Chief Operations Officer Andrew Watterson told senators Thursday the carrier “messed up” when its holiday meltdown in December left millions of passengers scrambling.

A “cascade” of factors, including a winter storm and outdated crew-tracking technology, caused the airline to cancel more than 16,700 flights over the Christmas week, Capt. Casey Murray, president of the Southwest Airlines Pilots Association, said during a hearing by the Senate Commerce, Science, and Transportation Committee.

Baltimore-Washington International Thurgood Marshall Airport, one of Southwest’s major bases, saw hundreds of canceled flights at the height of the disruptions. Nationwide, some 2 million passengers were stranded.

“Let me be clear, we messed up,” Watterson said. “In hindsight, we did not have enough winter operations this Christmas week.”

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Frontline workers saw this meltdown coming and have been “sounding alarm bells” for decades, seeing similar disruptions happen more frequently with increasing severity, Murray said.

“Our pilots have been sounding the alarm about Southwest’s inadequate crew scheduling technology and outdated operational processes,” Murray said. “Unfortunately, those warnings have been summarily ignored by Southwest leaders.”

Sen. Ed Markey, D-Massachusetts, said that Southwest’s mismanagement of the situation “absolutely led to real pain and conflict” for his constituents.

A software update that could have mitigated many of the cancellations will go live on Friday, Watterson said, but the overall scheduling technology will remain while the company conducts its investigation and looks for a replacement.

“Right now we are doing a top to bottom review of our winter operations and when it comes out it may be in the millions and millions of dollars,” Watterson told senators. “But it won’t be until probably in March that we’ll finish the assessment of exactly how much and where.”

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The “root cause” of the disruptions was insufficient winter weather operations, Watterson said, specifically at airports in Denver and Chicago, where the wave of cancellations started when Southwest could not de-ice enough planes to meet demand.

Southwest plans to invest $1.3 billion to modernize its operations across all departments in response to the incident, Watterson said, and will “fund what’s necessary to proceed” after a review.

The airline has reimbursed 96% of requests from affected passengers, Watterson told reporters after the hearing, automatically approving any costs under $4,000.

Senators at the hearing told stories from constituents of unfulfilled requests, cross-country drives in expensive rental cars, and losses that could not be regained with money.

“They can’t really be reimbursed for the time and frustration of being in that technical situation,” said Sen. Tammy Baldwin, D-Wisconsin. “There’s so many other factors [affecting delayed passengers] and because of Southwest cancellations.”

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Thursday’s committee hearing is part of the panel’s consideration of policy recommendations in the Federal Aviation Administration reauthorization bill to improve protections for passengers and build more resilient airline operations.

“It is my hope that we use the reauthorization opportunity to push the FAA safety and technology into the 21st century, to protect competition and to resist the temptation to get into the business of regulating prices, which will only make air traffic air travel unaffordable,” said Sen. Ted Cruz, R-Texas, ranking member of the committee.

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