FILE PHOTO: United Airlines planes, including a Boeing 737 MAX 9 model, are pictured at George Bush Intercontinental Airport in Houston, Texas, U.S., March 18, 2019. REUTERS/Loren Elliott/File Photo

CHICAGO (Reuters) - United Airlines’ use of larger aircraft on routes previously flown by Boeing Co’s grounded 737 MAX jets is costing the carrier money in the short-term, the company’s President Scott Kirby said in a letter to employees on Tuesday.

Chicago-based United, which owns 14 MAX jets and has dozens more on order, has been using its larger 777 or 787 aircraft to cover routes formerly flown by the suspended MAX planes, without being able to fill the extra last-minute seats.

“Of course, we can’t keep this up forever,” Kirby said.

Still, despite a series of adverse events in the first quarter that also included a U.S. government shutdown and extreme weather, Kirby said the company would pay eligible employees a one-time $100 bonus on April 17.

United failed to meet its operational performance incentive targets in the quarter due to those headwinds, but Kirby said that with warmer weather ahead, he was “confident that we can pull off a spring and summer for the record books.”