Southwest Airlines Chief Operating Officer Andrew Watterson will apologize on Thursday before a U.S. Senate committee over the holiday meltdown that led to the cancellation of 16,700 flights and pledge changes to ensure that there will be no repeats. "Let me be clear: we messed up. In hindsight, we did not have enough winter operational resilience," Watterson's written testimony for a U.S. Senate Commerce Committee hearing seen by Reuters says. In other written testimony seen by Reuters, Southwest Airlines Pilots Association (SWAPA) President Casey Murray will tell the committee that the low-cost carrier's "overconfidence" in planning and a "systemic failure to provide modern tools" were responsible for the December meltdown that the union said stranded 2 million passengers and is estimated to have cost it more than $1 billion. Murray will tell the committee that pilots "have been sounding the alarm about (Southwest's) inadequate crew scheduling technology and outdated operational processes for years. Unfortunately, those warnings were summarily ignored."