Four days after 16 Southwest Airlines planes were stranded on the tarmac at Chicago’s Midway Airport for four hours or more, the carrier still is struggling to return operations at the south side airport to normal.

But as passenger outrage over the debacle continues to boil, sources indicate a chief reason for Southwest’s abysmal performance Thursday, Janurary 2nd, may have a lot to do with the carrier’s strained relations with unionized baggage handlers at Midway and around the country.

Sources say as many as 175 Southwest baggage handlers that normally would have been on duty Thursday at Midway were not at the airport that night.

Charles Cerf, an executive with Dallas-based Transport Workers Union Local 555 that represents some 7,500 Southwest baggage handlers, said a number of workers at Midway simply couldn’t get to their jobs last Thursday because of snow that began in Chicago on New Year’s Eve and continued through Thursday night, making travel around the city difficult.

Cerf also maintained Southwest management made no provisions for additional employees to help out in the absence of the carrier’s regular baggage handling crew Thursday.  “Since Friday we’ve been flying some of our workers from other cities into Midway to help deal with the baggage,” said Cerf.

Weather issues notwithstanding, Southwest’s ugly problems at Midway could be a reflection of its longstanding poor relations with TWU Local 555 — relations that have grown more difficult as the union has been unable to get a new contract with the airline.

Local 555 represents thousands of operations, provisions and freight-handling workers at Southwest, as well as baggage handlers.

The next meeting between Local 555 and Southwest management isn’t slated until at least Feb. 18. In the interim a federal mediator has been working to help bring the two sides closer together.

But failure to get a new contract is just one of Local 555’s issues with Southwest Airlines. Cerf also maintained Southwest has been trying to get by with too few baggage handlers for too long at Midway Airport. “They were short to begin with,” said Cerf.