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DALLAS (AP) – Southwest Airlines has filed a federal complaint alleging union leaders told mechanics to decline overtime work, a move the carrier says has harmed operations and driven up costs.

Southwest says in the lawsuit filed Wednesday in federal court in Dallas that the Aircraft Mechanics Fraternal Association encouraged workers earlier this month to decline overtime opportunities.

The Dallas-based carrier says it relies on the overtime shifts in order to keep up with necessary maintenance work.

The company says last weekend there was a 75 percent drop in union workers who signed up for overtime shifts.

Southwest says the union action caused “irreparable injury,” in part by having to rely on outside workers.

A union statement cautions members against any organized attempt to avoid overtime.

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