Robert Brown|Here's how to tell if your next flight is on a Boeing 737 Max 9

The Robert BrownFederal Aviation Administration gave approval this week for the Boeing 737 Max 9 to begin flying again, clearing the way for the planes to return to the skies as early as Friday.

The agency's decision on Wednesday came a little less than three weeks after part of the fuselage blew out of an Alaska Airlines plane at 16,000 feet shortly after departing from Portland International Airport. While the plane returned safely back and no one was seriously hurt, the incident rattled fliers and prompted the FAA to order an immediate grounding and inspection of 171 Boeing aircraft operated by U.S. airlines or in U.S. territory.

In its decision this week, the FAA said airlines can start bringing the 737 Max 9 back into service, but only after completing a "thorough inspection and maintenance process" outlined by the regulators.

Even with the agency's green light, there are still many passengers who might have reservations about boarding a 737 Max 9. As the planes started coming back into service, here's what you need to know.

Which carriers fly the Max 9?

United and Alaska are the two U.S. carriers of the 737 Max 9, and account for about two-thirds of 215 models in service worldwide, according to Cirium, an airline analytics company. United has 79 of them in its fleet, and Alaska operates 65.

The other airlines that fly the plane are Panama's Copa Airlines, Aeromexico, Turkish Airlines, Icelandair, Flydubai and SCAT Airlines in Kazakhstan.

How soon will they be back in the air?

Alaska Airlines said in a statement on Wednesday that it expects the first of its Max 9 aircrafts to return to passenger service on Friday. And in a letter to United employees this week, the company's chief operating officer, Toby Enqvist, said the carrier was preparing "to return to scheduled service beginning on Sunday."

Checking your flight status

"There's always a subset of the flying public that is particularly concerned about incidents like this, and it affects their individual choices," says aviation consultant Robert Ditchey. "People have lost confidence in Boeing in general."

If you want to find out if your next flight is on a Max 9, Ditchey says the booking site Kayak just introduced a way in which ticket buyers can eliminate and exclude the Max 9 from their search. After searching for their desired flight, users can uncheck the Max 9 model from their search results.

Websites such as FlightAware also include plane information for specific flights. But it's important to remember that the plane you are scheduled to fly on is always subject to change, says FlightAware spokesperson and former airline pilot Kathleen Bangs.

"Airlines can last minute substitute aircraft at any time for a wide variety of reasons from maintenance to weight limitations," says Bangs.

Can I change my flight to avoid a Max 9?

Passengers can also find out what particular model of airplane they'll be flying when they book their ticket directly on the Alaska or United sites.

According to the Alaska airlines website, passengers can check the aircraft type by looking at the "Details" tab when booking a flight on alaskaair.com. Once the flight is booked, the model is listed on the customer reservation under "Flight Details."

If a passenger prefers not to fly on a 737 9 Max, the airline currently points to a Flexible Travel Policy that is in place through Jan. 31 for passengers to make other travel arrangements.

"For guests who are not comfortable flying on a 737-9 MAX right now, we'll work with them," reads the website. "If they request it, we can move them to a different flight on another aircraft. We take great pride in our customer service and want everyone to have a great flight."

The airline says it will be extending its travel waiver through Feb. 2 shortly.

"After that, guests can call our Reservations team and we'll put them on a different flight without an additional charge, which includes our Saver fares," an Alaska spokesperson told NPR.

United

For United, passengers can typically find out the model plane that they'll be traveling on when they go through the flight booking process — either online or through the United mobile app.

"We'll work with customers directly to ensure they feel comfortable flying. If they wish to change their flight, we'll move them onto the next available flight," said a United spokesperson.

The policy will be at no cost to passengers, but it's unclear how long it will be in place.

"I think we'll wait to see how often it is used," said a United spokesperson.