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.

After seeing video of the frightening flight of a plane with a gaping hole in its side, passengers might reasonably be asking questions about safety.

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.

WILL PEOPLE AVOID THE 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.

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.

HOW DO I CHECK WHAT TYPE OF PLANE I’M ON?

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.

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.

IS BOEING IN TROUBLE?

The FAA is investigating whether Boeing and its suppliers followed proper safety procedures in manufacturing the part that blew off the Alaska jet. That could lead to sanctions.

In addition, the FAA says it won’t let Boeing expand production of Max jets until it’s satisfied that quality-control concerns about the company have been resolved.

Rival Airbus has pulled far ahead of Boeing, beating the U.S. company last year in both orders and deliveries of new passenger planes. Boeing’s latest crisis could make things worse. United CEO Scott Kirby says his airline will consider alternatives to the upcoming Max 10 because of uncertainty about when and whether the FAA will certify the plane, which is already years behind schedule.