Will Boeing 737 Max 8 Groundings Cause Flight Cancellations or Delays? Here’s What You Should Know

As the United States joins the list of countries grounding the Boeing 737 MAX 8 planes following the deadly Ethiopian Airlines crash, some flights that have a connection to the aircraft have been cancelled or delayed — and the list of impacted flights could grow.

While the Federal Aviation Administration had maintained it had confidence in the 737 MAX 8 aircraft, the U.S. faced growing pressure from other countries to ground the planes after an Ethiopian Airlines Boeing 737 MAX 8 crashed on Sunday, killing all 157 people aboard. In the days since, China, Indonesia, the United Kingdom, Singapore, Australia, Argentina, the European Union and many other countries have all grounded the planes. The crash on Sunday marks the 737 MAX 8’s second deadly incident in less than five months — in October, a Lion Air jet crashed in Indonesia soon after taking off, killing all 189 people onboard.

On Wednesday, President Donald Trump said the U.S. would issue an emergency order to ground all Boeing 737 MAX 8 and MAX 9 aircraft in the wake of the plane crash. According to Trump, any planes currently in the air would be grounded upon reaching their destinations.

Southwest Airlines and American Airlines, two U.S. based carriers that operate the MAX 8 planes, confirmed they were aware of Trump’s order. Southwest, which has the broadest current domestic use of the MAX 8 planes, with 34 in its fleet, did not immediately return a request for comment on how passengers would be affected if their flights are cancelled. The airline does not typically charge a change fee for rebooked flights, though customers are required to pay the cost in fare difference if they schedule a new itinerary.

A statement from Southwest read: “Southwest is aware of media reports stating that the Boeing 737 MAX 8 fleet will be grounded in the United States. We are currently seeking confirmation and additional guidance from the FAA and will respond accordingly in the interest of aviation safety.”

In a statement to TIME, American Airlines said its fleet of 24 aircraft would be impacted by the order to ground all Boeing 737 MAX 8 planes and that it would try to rebook affected customers as soon as it could. While American Airlines usually changes fees for rebooked tickets, the airline has previously waived such costs in times of emergency.

“Earlier today the Federal Aviation Administration (FAA) informed us that based on new information, they are grounding the United States Boeing 737 MAX fleet out of an abundance of caution,” read the statement. “American Airlines has 24 aircraft affected by this directive. We appreciate the FAA’s partnership, and will continue to work closely with them, the Department of Transportation, National Transportation Safety Board and other regulatory authorities, as well as our aircraft and engine manufacturers. Our teams will make every effort to rebook customers as quickly as possible, and we apologize for any inconvenience.”

How the global efforts to ground the MAX 8 aircrafts will affect passengers whose flights are cancelled and delayed will depend on the number of these jetliners each airline has in its fleet, according to Ahmed Abdelghany, a professor of operations management at Embry-Riddle Aeronautical University. Airlines that primarily depend on MAX 8 planes to transport passengers will be more affected than ones that only operate a small number of the aircraft.

“Some airlines will get a big hit,” he says. “Some will not.”

A shortage in aircraft capacity is likely, according to Abdelghany. It’s also unclear how long certain airlines can afford to keep MAX 8 aircraft grounded.

“For every 50, 60 aircraft, you might have one that’s coming out of maintenance,” he says. “Most airlines don’t have spare aircraft.”

Abdelghany says he expects that most passengers scheduled to fly on a MAX 8 plane that might be cancelled will be kept up to date on their airlines’ plans in advance — which should help to quell crowding in airports. What will happen next is largely up to the airline in question. Some may be able to accommodate passengers on a later flight or different aircraft, while others can work with other airlines or airports to find new itineraries for passengers.

China, the world’s biggest market for the aircraft, has grounded 97 jets so far, according to industry publication Flightglobal. China Southern Airlines, which operates at least 22 737 MAX 8 aircraft, has grounded all flights with these planes. FlightAware, which tracks flight cancellations and delays, finds that 30 China Southern Airlines flights have been cancelled as of Tuesday. Most of the cancelled flights are Boeing 737-800 or Boeing 737-700 jets, according to FlightAware.

Norwegian, which has 18 MAX 8s in its fleet, said it would stop flying the aircraft until further notice in a statement. The airline acknowledged that its customers will face cancellations and delays due to grounding the MAX 8 flights. The airline also operates more than 110 Boeing 737-800 aircraft, which were not affected by the temporary grounding.

“We would like to apologize to customers who will be affected by temporary cancellations and delays, but the safety and security of our customers and colleagues will never be compromised, and once authorities advise to cease operations we will of course comply,” read a statement from Norwegian.

Silk Air, which operates out of Singapore, announced that it was withdrawing its fleet of six MAX 8s until further notice. The grounding led to the cancellation of six flights between Singapore and Kuala Lumpur, according to the airline.

The U.K.’s decision to ground MAX 8s also impacted the TUI Group, a holiday airline that flies to different destinations in Europe from Britain. TUI decided to ground all of its Boeing 737 MAX 8 flights across all airlines used by the group.

“TUI will inform customers booked from tomorrow on respective flights about alternative transportation,” the group said in a statement.

Passengers unsure about their aircraft type or flight status should check in with their airlines to see if there are any changes in their itineraries, Abdelghany advises.