An American Airlines Boeing 737 Max 8, on a flight from Miami to New York City, comes in for landing at LaGuardia Airport in New York, March 12, 2019.

The U.S. Federal Aviation Administration on Wednesday joined dozens of other countries' regulators in ordering airlines to ground new Boeing 737 Max planes, citing evidence linking a deadly crash of one of them in Ethiopia over the weekend to a similar fatal flight in Indonesia in October. (You can find more detail on why the planes were grounded here.)

That has left airlines scrambling to rebook passengers and reassign planes. The three U.S. airlines — United, American and Southwest — that have recently added the planes to their fleets, and have more on order, said they will rebook or waive ticket-change fees and fare differences for travelers affected by the FAA's order, which went into immediate effect.

American Airlines, which has 24 Boeing 737 Max planes in its fleet of nearly 1,000 aircraft, said it was ferrying those planes to be parked until the FAA order is lifted. It operates about 85 flights out its 6,700 flights a day using the Max.

Routes with multiple flights each day, where passengers can more easily be rebooked to another time, are likely to take the biggest hit. Travelers who aren't booked on the Max may also be affected as airlines deploy their planes to cover other routes with less frequent service.

United Airlines has 14 of the Boeing 737 Max 9s, a larger model, in its fleet. The airline said it expects minimal disruptions from the issue, but it will work with customers if their flights are canceled.

Southwest Airlines flies 34 Boeing 737 8s that service about 4 percent of its daily flights. The carrier does not charge travelers to change their trips, but said passengers booked on canceled Boeing Max flights won't have to pay the difference in fares to change their dates if it's within two weeks of their original departure.