Southwest Airlines, the nation's third-largest carrier, is struggling with a surge in planes being taken out of service for maintenance. The airline blames the mechanics union, but the union claims it's a matter of safety.

Photos obtained exclusively by CBS News show what mechanics describe as landing gear tires so worn that Southwest had to pull a 737 out of service on Tuesday. More than 40 airplanes per day have been grounded for maintenance issues since last Tuesday. That's unusually high, more than double a normal day.

Southwest has declared an "operational emergency" and its chief operating officer Mike Van de Ven is blaming the mechanics union, claiming it has a 'history of work disruptions' and pointing the finger at a "subset" of mechanics primarily at four maintenance facilities.

The union said management is "scapegoating" technicians and "the airline's self-described 'operational emergency' is simply an attempt to divert attention away from the airline's safety issues."

Southwest acknowledges the situation has caused "extremely long delays" and Southwest cancels more than 400 flights amid fight with mechanics, including about 100 on Tuesday alone. They include the planes in pictures obtained by CBS News.

Mechanics said the company has since restricted access to an out of service list.

"This kind of stress on the operation is not good. It doesn't bode well for the safety of the airline," said John Goglia, a former National Transportation Safety Board member.

More than a dozen Southwest mechanics said co-workers felt emboldened to write up problems following a CBS News investigation earlier this month into allegations of undue pressure on mechanics to keep Southwest planes in service.

Southwest declined an interview request on Wednesday, but senior director of safety management, captain Dave Hunt spoke last month.

"Our priorities are always first and foremost safety. Safety first and that is always paramount," he said.

Southwest and the union have been locked in tense contract negotiations for more than six years. The Federal Aviation Administration said it has heightened its oversight of Southwest which is standard procedure during labor issues.