United Airlines is asking its pilots to take time off in May because of delays in receiving new planes that the airline ordered from Boeing, which is struggling with production due to manufacturing problems.

A United spokesperson said today that the offer is voluntary.

“Due to the recent delays in Boeing deliveries, our forecasted (flight hours) have been reduced and we are offering our pilots voluntary programs for the month of May to reduce excess staffing,” spokesperson Leslie Scott said.

In a note to pilots obtained by The Associated Press, United said it expects to make similar requests during the summer and possibly into the fall.

The Air Line Pilots Association said United is offering short-term leaves and unpaid time off, but they are not mandatory.

Boeing declined to comment.

United doesn’t expect to get all the Boeing jetliners it ordered and was due to receive this year or next year. A month ago, United said it was contractually due 191 planes this year and 127 next year but expects to receive only 88 this year and 64 in 2025.

Almost all of the shortfall consists of Boeing 737 Max planes, including a new, larger model. United had planned to begin flying 80 Max 10 jets this year. The Federal Aviation Administration has not yet certified the Max 10, however, and FAA approval is likely to be further delayed by increased scrutiny of Boeing since a panel blew out of an Alaska Airlines Max 9 in January.

United is considering options to replace orders for the Max 10.

Since the Alaska Airlines accident, federal regulators have been investigating production quality issues at Boeing, and the FAA has barred Boeing from increasing production of 737 Max jets.

United previously said that it expects to lose money in the first quarter because its Max 9s were grounded for inspections for three weeks after the blowout on the Alaska plane. The Chicago-based airline is scheduled to release financial results April 16.

United CEO Scott Kirby is one of several airline executives who have called out problems at Boeing and sought a meeting with Boeing directors.

Boeing announced last week that CEO David Calhoun will step down at the end of the year as part of a leadership shakeup at the company. The head of Boeing’s commercial-airplanes unit has already been replaced, and the chairman of the board will not stand for reelection in May.

Shares of Boeing Co. fell 1.5%, while United Airlines Holdings was up less than 1% in afternoon trading today.