Southwest Airlines Chief Operating Officer Andrew Watterson plans to tell the U.S. Senate Commerce Committee that the carrier didn't have "enough resiliency" in its operation when harsher-than-expected winter weather slammed "key airports."

Watterson is slated to speak in front of the Senate’s Committee on Commerce, Science, and Transportation during a Thursday hearing titled "Strengthening Airline Operations and Consumer Protections," which will review the causes and effects of recent air travel disruptions, including Southwest's December meltdown that led to nearly 17,000 canceled flights.

The carrier is facing lawsuits from passengers and its own shareholders regarding the disruption, which is being investigated by the U.S. Transportation Department.

In written testimony released ahead of the hearing, Watterson said the Texas-based carrier developed a plan, which included pre-canceling flights, "to reduce activity to an hourly rate that was consistent with our proven capabilities."

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However, the sub-zero temperatures, high winds and frozen precipitation "were worse than forecast" and "had a wide-ranging impact" on operations, especially at its Denver and Chicago Midway airports.

"It became clear that, with the storm severely disrupting our Denver and Chicago Midway stations concurrently, we did not have enough resiliency in our operation for the severe effect this winter event had on us," Watterson said in the written testimony.

Denver and Chicago Midway are two of Southwest's 11 crew bases where flight crews begin and end their duties. The two locations account for 25% of flight crews, Watterson said.

Watterson admitted that the carrier "struggled to keep the operation moving at these key airports" because of numerous factors, including the amount of deicing equipment and related infrastructure, the effect of the extreme cold on jet bridge hydraulics and ground support equipment fuel as well as the location of gates and deicing pads.

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He contended that this disruption began as a weather event on Dec. 21 but turned into a crew scheduling event. However, he said that the carrier's crew scheduling software never stopped working but rather was overwhelmed.

"The pace and volume of close-in Crew and schedule changes over multiple days left our Crew Scheduling professionals unable to efficiently address the state of the operation," Watterson wrote.

As the situation escalated, "Crew Scheduling simply couldn’t keep up with the overwhelming volume of changes, resulting in individual Crew assignments not being updated in a timely manner," Watterson continued.

"Due to these factors, among others, we could not execute the plan we had established for operating during the storm," he added.

Southwest was forced to cancel almost the entirety of its flight schedule in Denver, which is its largest station, between Dec. 21 and Dec. 23. At Chicago Midway, its second-largest station, it did the same thing between Dec. 22 and Dec. 23.

Watterson admitted the airline messed up and, in echoing remarks from CEO Bob Jordan, apologized to those who were affected.

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"Let me be clear: we messed up. In hindsight, we did not have enough winter operational resilience," he said.

Watterson said the carrier has processed all refunds, returned passenger baggage and completed most reimbursement requests. It has also issued 25,000 Rapid Rewards points to passengers who were severely impacted.

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Still, in the written testimony, Watterson said the carrier "is intensely focused on reducing the risk of repeating" this type of operational disruption. 

The executive noted that nearly all of its planned 2023 capacity additions will go to existing markets, which "will help to add depth and greater resiliency by providing better re-accommodation options for customers, crews, and aircraft when we have weather or delays that create irregular operations."