DALLAS — Pilots for Southwest Airlines say the grounding of Boeing’s troubled 737 Max aircraft is costing them about $9 million a month in lost wages.

The Southwest Airlines Pilots Association, which represents 10,000 pilots, said Wednesday that it’ll seek reimbursement from Boeing for lost wages and legal costs incurred by the union for cooperating with the U.S. Department of Justice. It’s the first labor group to join carriers in pushing to recover costs from Boeing.

The union told The Dallas Morning News that the $9 million figure is “a conservative estimate.” Southwest pilots are paid based on the number of trips they fly. With the Max aircraft out of service since mid-March, fewer trips are being flown, the union said.

“SWAPA will be seeking compensation and reimbursement from Boeing for every dollar legally available to be challenged when the Max issues are resolved,” the union said in a statement.

Southwest flies the most Max jets of any airline — 34 — in its famously all-Boeing 737 fleet. The nation’s largest domestic carrier has taken the Max off its flight schedules through Labor Day weekend, resulting in about 100 cancellations a day.

The planes, also used by American and United airlines, will be out of commission for three of the nation’s four major carriers throughout the busy summer travel season. There’s no firm timetable for when the Max aircraft will be allowed to fly again.