Some of airlines’ most in-demand flights this summer don’t even leave the ground.

Flight simulators from Atlanta to Dallas to Miami and elsewhere are humming as airlines scramble to get hundreds of pilots trained to meet a surge in bookings that kicked off this spring as vaccinations rolled out and Covid-era restrictions eased.

Domestic leisure travel has recovered to 2019 levels, while business travel is also rebounding, airline executives said this month.

Airlines have received $54 billion in federal aid since March 2020 in exchange for not laying off workers. But voluntary departures, changed fleets and the rapid rise in travel demand have created a need for pilot training that industry experts say is without parallel.