United Airlines Holdings Inc. asked pilots to take unpaid time off next month, blaming late deliveries of new aircraft from Boeing Co.

The delays have reduced the number of hours United had planned that its pilots would fly this year, and the additional time off will reduce excess staffing as a result, a spokeswoman for the carrier said Monday. The company didn’t specify how many hours would be cut. 

The leave program could also be extended into the summer and potentially the fall, CNBC reported earlier on Monday, citing a memo from the Air Line Pilots Association union that represents United flight crews. 

The request for voluntary time off is United’s latest move to contend with a difficult year. Boeing delays have upended the carrier’s flying plans while a series of safety incidents have drawn the attention of travelers and US aviation regulators.

The US Federal Aviation Administration is conducting a review of United’s operations and considering actions that could curb the airline’s growth plans, Bloomberg reported last month. The carrier in January pulled the long-delayed Boeing 737 Max 10 from its fleet plans for the year, and has since begun to slow pilot hiring and training classes for new aviators due to the plane’s absence. 

United was to have received 80 Max 10s this year, part of an outstanding order with Boeing for 277 of the planes. 

A spokesman for the Air Lines Pilots Association didn’t immediately respond to a request for comment.