The union representing pilots at Southwest Airlines has asked federal mediators to help break a two-year stalemate in contract negotiations.
Southwest said that it had been optimistic about an agreement, but that the union ended negotiations and filed for mediation.
Southwest has had relatively few labor conflicts over the years, but as the airline's growth has slowed, pilots have complained about a lack of advancement opportunities and stagnant compensation.
On Thursday, the Southwest Airlines Pilots' Association said that it had asked for help from the National Mediation Board.
Southwest Airlines Co. earned $946 million in the first nine months of this year, up 75 percent from the same period last year on higher revenue and lower fuel spending.
Company shares fell 8 cents to $39.19 Thursday.