United Airlines is asking pilots to take unpaid time off next month, citing late-arriving aircraft from Boeing, according to a note sent to pilots.

It’s another example of how Boeing’s customers say the manufacturer’s production problems and safety crisis are impacting their growth plans. The offer comes after United and other airlines in recent years have clamored for more pilots when the Covid-19 pandemic travel slump ended and demand surged.

“Due to recent changes to our Boeing deliveries, the remaining 2024 forecast block hours for United have been significantly reduced,” the United chapter of the Air Line Pilots Association, the pilots’ union, said in a note to members Friday. “While the delivery issues surround our 787 and 737 fleets, the impact will affect other fleets as well.”

United confirmed the request for voluntary unpaid time off. The airline previously said it would pause pilot hiring this spring because of aircraft arriving late from Boeing, CNBC reported last month.

The union said it expects United to offer more time off “for the summer bid periods and potentially into the fall.”

United was contracted to receive 43 Boeing 737 Max 8 planes and 34 Max 9 models this year, but now expects to receive 37 and 19, respectively, according to a company filing in February. It had expected Boeing would also hand over 80 Max 10s this year and 71 next year. That model hasn’t yet been certified by the Federal Aviation Administration, and the airline removed them from the delivery schedule because it is “unable to accurately forecast the expected delivery period,” it said in the filing.

United CEO Scott Kirby has been among the most vocal about the production problems and delivery delays at Boeing, including most recently the crisis stemming from a door plug that blew out of a nearly new Boeing 737 Max 9 operated by an Alaska Airlines flight that was at about 16,000 feet.

Boeing CEO Dave Calhoun last week announced he would leave at the end of the year as part of a broad leadership shake-up, which included the departures of the board chairman and the head of Boeing’s commercial airplanes unit.