New York (CNN) — United Airlines is asking its pilots to take voluntary unpaid leave in May because of delays in Boeing deliveries, according to a memo sent by the union representing pilots.

Because of the delivery delay, “our forecasted block hours for 2024 have been reduced and we are offering our pilots voluntary programs for the month of May to reduce excess staffing,” United Airlines said in a statement Monday. It also comes as the industry warns of a mass pilot shortage, as the workforce begins to age out after widespread furloughs during Covid-19.

The voluntary programs could possibly extend into summer and fall, the memo said. The offer means the company will still pay some benefits while the pilot is out.

Monday’s development signals the fallout from Boeing’s production crisis is spilling over into airline operations, especially United, where 81% of its mainline operations use Boeing jets. The Federal Aviation Administration is taking a closer look at United after a series of almost a dozen incidents at the airline, United said in a March memo.

United had already announced it halted the hiring of new pilots “due to continued new aircraft certification and manufacturing delays at Boeing.” Southwest Airlines, which flies nothing but Boeing 737 jets, also will hire about 50% fewer pilots and 60% fewer flight attendants than planned this year due to Boeing cutting deliveries to the airline by about 40%. It will also trim capacity from earlier plans, offering about 1 percentage point fewer seats than it had planned.

United CEO Scott Kirby told investors in March that his company is looking at possibly buying more jets from Boeing competitor Airbus, and he said earlier this year that an Alaska Air incident — during which a Boeing 737 Max 9 plane lost a door plug mid-flight on January 5, leaving a hole in the side of the plane — was the “straw that broke the camel’s back” on its plans to get deliveries of the Max 10 any time in the foreseeable future.

United uses far more Boeings than its rivals American Airlines and Delta Air Lines which have only about half their fleets coming from Boeing. The union said Monday that the delivery issues pertain to the 787 and 737 fleets.

On March 15, a United Boeing 737-800 landed in Medford, Oregon, missing an external panel, though no passengers were injured. Also in March, another United Boeing plane spewed flames from an engine after taking off, one slid off the runway, one lost a wheel during takeoff, and yet another trailed hydraulic fluid.

“While they are all unrelated, I want you to know that these incidents have our attention and have sharpened our focus,” Kirby said in a March message to customers.

Boeing has been in the spotlight since the Alaska Airlines Boeing 737 Max 9 incident.

It is not the first time the airline has tried to cut pilots. During the pandemic, airlines offered tens of thousands of furloughs and voluntary buyouts due to lack of flying.

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