. A Delta Air Lines flight is pushed put of its gate at the airport in Salt Lake City By Tracy Rucinski CHICAGO Delta Air Lines (N:DAL) pilots are receiving record overtime, straining the airline's labor costs, partly as it adds more flights to take advantage of a gap in demand left by the grounding of the Boeing (NYSE:BA) 737 MAX at rival carriers, union officials told . The surge in overtime highlights a split in the fortunes of U.S. airline workers as airlines like Delta, which does not fly the MAX, scramble to meet demand while staff at rivals like Southwest Airlines (N:LUV) and American Airlines (O:AAL) sit at home on basic pay. Delta issued a record 40,554 "green slips"double-pay for overtime flyingbetween May and August, double the same period in 2018, according to pilot union Delta Master Executive Council, a unit of the Air Line Pilots Association. "There were weekend days that I would get up to 10 calls a day to pick up an extra shift or an extra flight," Delta pilot Dana Dann-Messier, who flies a Boeing 717, told . "There were times when the pace of the summer was so hectic that guys wouldn't be volunteering because they were so wiped out," said Dann-Messier, who is based in Atlanta. Delta, due to publish third-quarter results on Thursday before the market opens, last week raised its estimate for growth in third quarter cost per available seat mile, excluding fuel costs, to about 2.5%, in part due to employee wage increases. "Maintaining the operational reliability customers have come to expect and love about Delta required additional pilot flying this summer, largely due to severe weather impacting our operation and some incremental flying above our previous plan," Delta spokesman Michael Thomas said. He played down the 737 MAX grounding as a major contributing factor. Delta shares gained 1.4 percent on Wednesday to $53.92. Pilots at Southwest, the world's largest Boeing 737 MAX carrier, this week sued Boeing for an alleged over $100 million in lost wages they attribute to reduced flying time because of the grounding, now in its seventh month. American Airlines' pilots have demanded compensation over lost pay but have not filed any lawsuits. The Boeing 737 MAX was grounded worldwide in March following two fatal crashes that killed 346 people within five months.