UPDATE 2-Southwest to tell U.S. lawmakers 'we messed up' during holiday meltdown

(Adds more details from testimony, background, comments from union)

By David Shepardson and Rajesh Kumar Singh

WASHINGTON/CHICAGO, Feb 8 (Reuters) - 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the committee that pilots "have been sounding the alarm about (Southwest's) inadequate crew scheduling technology and outdated operational processes for years.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."

Watterson reiterates the company's view that its crew scheduling software didn’t stop working but that it was overwhelmed.

"We are doing a system-wide review of our preparedness for winter operations and will implement any measures necessary to mitigate the risk of an event like this occurring in the future," his testimony says.

Watterson's written testimony offers a new apology: "It caused a tremendous amount of anguish, inconvenience, and missed opportunities for our customers and employees."

Murray blames the meltdown on "poor planning, systemic under-investment in crew scheduling technology and processes, and a failure to collaborate with frontline employees."

Murray says Southwest's mismanagement forced the airline during the crisis to fly more than 500 flights without passengers to reposition crew or planes and more than 10,000 flights where pilots sat in the back rather than flying.

Murray says Congress should demand "a firm timeline for when crew scheduling technology and associated crew management processes will be fixed."

Southwest is facing an investigation by the U.S. Transportation Department over its flight scheduling during the crisis.

The committee had asked Chief Executive Bob Jordan to testify but Southwest told Reuters that he had a prior commitment. Jordan is in Baltimore today for an employee rally.

Murray says Jordan and Watterson "inherited a massive, complex operation held together by duct tape and baling wire. It will take time to tear down and rebuild." (Reporting by David Shepardson; Editing by Mark Porter)