Flight on Southwest or American? Make sure it wasn’t on a Boeing 737 Max.

The FAA may have just messed up your travel plans. 

On Wednesday, the agency announced that it’s grounding all Boeing 737 Max 8 and 9 airplanes after two fatal crashes in the last five months. The two airlines with the most Boeing 737 Max 8 planes? Southwest with 34 and American Airlines with 24. 

The Ethiopian Airlines crash on Sunday prompted the groundings. Just a few months before in October, the same type of plane crashed in Indonesia.

As President Donald Trump made the announcement Wednesday, more than a dozen flights on the affected planes were mid-trip. They will be allowed to land at their final destinations, but once on the ground they have been ordered to stay there as part of the temporary grounding order. 

The planes represent only about 4 percent of Southwest’s fleet of about 750 planes, but in the coming days, those 34 unavailable planes will make rebooking a mess. American has a larger fleet of about 1,000 planes.

In a statement, Southwest said it was complying with the FAA requirement even though it remained confident in the Max 8 after more than 41,000 completed flights. As far as the impact on travelers, “Our goal is to operate our schedule with every available aircraft in our fleet to meet our Customers’ expectations during the busy spring travel season,” the company wrote. 

That means Southwest is offering free rebookings for any customer on a canceled Max 8 flight. They can do so with any other flights with no fees or fare differences. Anyone traveling on Southwest or American flights in the next few days should check their aircraft type to see if they are on a Max 8 plane. 

A Southwest spokesperson noted that its Max 8 planes are distributed across different cities and regions. That means any random flight could be affected, so check your flight info.

The flight tracking site FlightAware reported 440 Southwest cancellations Wednesday — the most of any airline. American had only 196.

American Airlines wrote in a statement that American flies on Max 8 planes 85 times a day, which is only 1.2 percent of its 6,700 daily flights.

The airline said, “Our teams will make every effort to rebook customers as quickly as possible, and we apologize for any inconvenience.”

If customers opt out of booking new flights, they can instead request a full refund.

The last flights were still airborne Wednesday afternoon. A Southwest flight from San Jose to Houston’s William P. Hobby airport was scheduled to arrive around 6:27 p.m. CT, the final Max 8 flight for the airline.