American Airlines has told the remaining 3,300 flight attendants on voluntary leave to come back by November or December and that it will start recruiting new cabin crew members, according to a note to staff sent Thursday, which was reviewed by CNBC.

It is the latest sign the carrier is planning for sustained travel demand beyond this summer’s surge.

“Increasing customer demand and new routes starting later this year mean we need more flight attendants to operate the airline,” said Brady Byrnes, vice president of flight service, in a memo.

The airline is planning to hire 800 flight attendants by March 2022, the note said.

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American offered extended voluntary leaves of up to two years during the pandemic to help reduce costs as travel demand was devastated by the virus. It also encouraged staff to take early retirement packages. Other carriers urged employees to take similar packages.

But bookings, led by domestic leisure travel, surged over the late spring and early summer, and now carriers are scrambling to increase staffing.

Getting employees who have been on leave or haven’t been actively working can be challenging and time-consuming.

Two-thirds of American flight attendants on leave will need to be retrained before they can fly, Byrnes said.

“The return of up to 3,300 flight attendants by December is no easy task for our teams working to support you behind the scenes,” Byrnes wrote.

The Dallas Morning News previously reported that the company was planning to cancel flight attendants’ voluntary leaves.

American, Southwest Airlines, Delta Air Lines, and other carriers have been busy training hundreds of pilots who have been called back to work, a process that can take more than a month in some cases.

Those carriers as well as United Airlines and Spirit have announced plans to hire additional pilots to keep up with what executives expect to be robust demand next year.