For the first time since Boeing 737 Max 9 jetliners were grounded after a mid-air blowout earlier this month, the aircraft are again carrying passengers — a prospect that might prompt questions among some travelers.

Alaska Airlines resumed a limited number of flights with its Max 9s on Friday. United aims to follow suit on Sunday, but a spokeswoman said the airline might use them as spare planes Friday or Saturday.

Those are the only two U.S. airlines that operate this particular model of the Boeing 737, which gained widespread attention earlier this month when a door plug blew out minutes after takeoff, exposing passengers to a gaping hole. 

The Federal Aviation Administration has detailed the process that airlines must follow to inspect — and if necessary, repair — the panels called door plugs, one of which broke loose on Alaska Airlines flight 1282 on Jan. 5. The plugs are used to seal holes left for extra doors on the Max 9 when an unusually high number of seats requires more exits for safety reasons.

FAA Administrator Mike Whitaker says his agency's review of everything that has happened since the accident, including gathering information about inspections of door plugs on 40 other planes, gives him confidence that they will be safe so long as the new inspection process is followed.

Why were the planes grounded?

Alaska Airlines grounded all 65 of its Max 9 jets within hours after one of the two door plugs in the back half of the cabin of flight 1282 blew away while 16,000 feet above Oregon. The FAA grounded all Max 9s in the U.S. the day after the blowout.

Even though none of the passengers were seriously injured, regulators acted quickly because the accident could have been far worse.

By a stroke of luck, the two seats closest to the panel that blew off the plane were empty when flight 1282 took off from Portland, Oregon. And the plane had not yet reached cruising altitude of more than 30,000 feet, when passengers and flight attendants might have been walking around instead of being belted into their seats.

Airlines found problems on other planes. Alaska CEO Ben Minicucci told NBC this week that "many" of the planes they inspected had loose bolts that are supposed to help secure the door plug to the airframe of the jet. United Airlines made similar similar findings.

What is being done about it?

The FAA is requiring airlines to conduct "detailed visual inspections" of the door plugs and other components, adjust fasteners and fix any damage they find before putting Max 9s back into service. The agency says the process was developed by what they learned from inspections of 40 grounded planes.

United says the process involves removing an inner panel, two rows of seats and a sidewall liner from the cabin. Technicians open the door plug, inspect it and the surrounding hardware, and make any necessary repairs before resecuring the panel.

In a statement, Alaska Airlines said it will only return those 737 Max 9s that have undergone "rigorous inspections." Each plane must be deemed airworthy, according to FAA requirements, it added. 

"The individual inspections are expected to take up to 12 hours per aircraft," the airline said.

Are travelers canceling flights on 737 Max 9 planes?

Alaska Airlines officials said Thursday that they have lost a few sales among people purchasing flights into February — a phenomenon called "booking away" in the airline business. They didn't say how many people have booked away from the Max 9, but they predicted it would only last a few weeks.

Minicucci, the Alaska CEO, said "at first, people will have some questions, some anxiety," but that "over time" confidence in the plane's safety will be restored.

Travelers returned to the Boeing 737 Max 8 after two of them crashed in 2018 and 2019, killing 346 people. In that case, Boeing had to redesign an automated flight-control system before the FAA would let Max 8s and Max 9s resume flying after a 20-month grounding.

How can I tell the type of aircraft I'm booked on?

Most people don't bother to look up the type of plane they are booked to fly, although there was an uptick after flight 1282. Scott Keyes, founder of the travel site Going, said once FAA clears the planes to fly — and if there are no more incidents — the public's memory will quickly fade.

Airline websites typically now include the type of aircraft to be used on a particular flight, but finding the information varies.

On American Airlines' website, the type of plane shows up right on the search-results page. On the United and Alaska sites, however, you will need to take one more step: Click on "details." On Southwest Airlines, you'll have to click on the flight number — it's in blue — to see the aircraft type.

What if I don't want to fly on a Boeing 737 Max 9?

United told CBS News that it will allow passengers who don't want to fly on a Max 9 aircraft to change flights without additional cost, depending on seat availability. 

Meanwhile, Alaska Airlines said that it still has its flexible travel policy in place, which will allow passengers to rebook their flights for travel through February 9. 

Is flying safe? 

It's much safer than driving and also safer than rail travel on a per mile basis, according to U.S. Department of Transportation figures.

Airline officials and aviation regulators like to point out that there has not been a fatal crash of a U.S. airliner since 2009. However, in the past year there has been an sharp increase in close calls being investigated by federal officials.

In:
• Plane Crash
• Alaska Airlines
• United Airlines
• Alaska