CHICAGO/MONTREAL (Reuters) - U.S. and Canadian airlines that fly the roughly 175-seat aircraft face a logistical challenge: which flights to cancel and which to cover with other planes following the global grounding of Boeing Co’s 737 MAX jets.

Southwest Airlines Co and American Airlines Group Inc, 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.

“We can’t just cancel all of those flights, so the goal is to spread out the cancellations across our entire system to impact the least amount of customers,” American Airlines spokesman Ross Feinstein said.

This means that an American Airlines flight from Miami to the Caribbean initially scheduled on a 737 MAX may now fly on a 737-800 with a similar seat configuration, while that 737-800 flight is canceled.

“It’s a challenge to explain to customers who weren’t previously booked on a MAX why their flight is canceled,” Feinstein said.

The 737 MAX jets were grounded last week following two fatal crashes in the past five months, the causes of which are under investigation.

Southwest, the largest MAX operator in the world with 34 jets representing about 5 percent of its total fleet, is cancelling about 150 flights per day due to the grounding, but not all on MAX routes.

Southwest shares were down 2.3 percent on Wednesday and American shares fell 2.1 percent.

Steve West, Senior Director of Southwest’s Operations Control, said the company is trying to cancel flights five days in advance, while looking at issues such as weather that could free up jets, like last week’s snowstorm in Colorado.

Southwest and American were already grappling with a larger than normal number of out-of-service aircraft, further straining their fleets.

So far United Airlines, with 14 MAX aircraft, has not canceled any flights due to the grounding, but has had to put smaller aircraft on some routes and fly the larger 777 to places like Hawaii.

It is unclear how long the grounding will last. Deliveries are also on hold, meaning an additional hit to airlines due to receive more of the jets this year.

FILE PHOTO: Southwest Airlines Co. Boeing 737 MAX 8 aircraft sit next to the maintenance area after landing at Midway International Airport in Chicago, Illinois, U.S., March 13, 2019. REUTERS/Kamil Kraczynski

Boeing has over 5,000 orders for the MAX, which sold fast thanks to its higher fuel-efficiency and longer range. Now airlines face a dent to 2019 profits.

Calgary-based WestJet said it took steps prior to the MAX grounding to start protecting trans-border flights to sunny destinations that were previously scheduled to fly with the carrier’s 13 MAX planes.

Meanwhile, Air Canada said on Tuesday it would remove its 24 737 MAX aircraft from its schedule until at least July 1, 2019.

“It is easier to put the aircraft back in the schedule than to pull it out,” said a source familiar with the carrier’s thinking, who is not allowed to publicly discuss its strategy.

(GRAPHIC-Boeing 737 MAX deliveries in question link: tmsnrt.rs/2Hv2btC).