Following the global grounding of Boeing's 737 Max jets, U.S. and Canadian airlines that fly the roughly 175-seat aircraft face a fresh logistical challenge every day: which flights to cancel and which to cover with other planes.

Southwest Airlines and American Airlines Group, the two largest MAX operators in the United States, said they have bolstered their reservation and operations teams to figure out how to spread flight cancellations across their networks, not just on Max flights.

American Airlines, for example, had most of its 24 Max jets flying in and out of Miami, where load factors have been full during the spring break season.