Boeing's 737 Max 9 jet is flying again. Here's what experts say about its safety.

• The Boeing 737 Max 9 aircraft has returned to service after a three-week grounding.
• The plane must undergo enhanced inspections of up to 12 hours before flying passengers again.
• Two ex-Boeing employees warned against flying on the jet, but some experts say it's 100% safe.

On January 5, an Alaska Airlines 737 Max 9 door plug broke off shortly after takeoff from Portland International Airport, leaving a gaping hole in the jet's fuselage. No one was seriously injured.

The Federal Aviation Administration quickly grounded 171 other Max 9 planes with the same door plug, mostly flown by United Airlines and Alaska.

Four critical bolts used to secure the door plug were missing from the jet when it left Boeing's assembly line, The Wall Street Journal reported, representing a massive quality control lapse.

This oversight has cast renewed scrutiny on Boeing's family of Max airplanes, which already saw 346 fatalities after two Max 8 jets crashed in 2018 and 2019.

Three weeks after the blowout, however, the Max 9 is carrying people once again. Alaska's COO Constance von Muehlen was on board the carrier's first passenger flight since the grounding and took the seat by the door plug, CNN reported.

The green light comes as regulators approve new inspection and maintenance processes in relation to the door plug. But questions surrounding Boeing's safety and quality control still remain on the minds of travelers.

Here's what to know about the Max 9 inspections, and how aviation experts view the return to service.

The Max 9's enhanced inspection process will take up to 12 hours

Last Wednesday, the FAA announced an "enhanced maintenance process" to be completed on the 171 grounded Max 9s before they could return to service.

This includes visual inspections of the left and right mid-cabin door plugs, as well as inspection of specific bolts, guide tracks, and fittings to ensure all critical components are correctly installed.

The agency also said it would cap production of the 737 Max, have "increased floor presence at all Boeing facilities," and ramp up oversight of the planemaker and its suppliers — particularly Spirit AeroSystems (not related to Spirit Airlines), which installed the door plug.

"The exhaustive, enhanced review our team completed after several weeks of information gathering gives me and the FAA confidence to proceed to the inspection and maintenance phase," FAA Administrator Mike Whitaker said in a January 24 statement. "However, let me be clear: This won't be back to business as usual for Boeing."

So far, carriers including United, Alaska, and Panama's Copa Airlines have re-launched their Max 9s after the required inspections, with Copa being the first to do so on Thursday, Reuters reported.

According to aviation analyst and president of Atmosphere Research Group, Henry Harteveldt, these inspections are "extremely thorough."

"There were at least four revisions to the inspection process before the FAA, Boeing, and the airlines agreed on the final procedure to be done to inspect these planes," he told Business Insider on Tuesday. "The initial inspection procedures were taking two to four hours; the agreed process takes between 10 and 12 hours."

Alaska noted the same 12-hour timeframe in a January 26 statement.

Not all experts agree on the Max 9's safety

It wasn't long after the Alaska Max 9 blowout that airline customers started voicing concerns about the safety of the plane after its return to service — with some travelers even opting to pay more to avoid flying on the jet.

According to the Washington Post, the travel booking website Kayak said its filter for the 737 Max significantly increased in the days after the incident.

Former Boeing senior manager-turned-whistleblower Ed Pierson told the LA Times in an interview published Tuesday that he would "absolutely not fly on a Max airplane."

"I've worked in the factory where they were built, and I saw the pressure employees were under to rush the planes out the door," he said. "I tried to get them to shut down before the first crash."

Joe Jacobsen, a former engineer at Boeing and the FAA, gave the paper a similar take: "I would tell my family to avoid the Max. "I would tell everyone, really."

In a statement to Business Insider, Harteveldt expressed confidence in the Max 9.

"I understand why people may be concerned about flying on the 737 Max 9 right now, but those planes would not be back in the air if the FAA did not think they were safe," he said. "No airline is going to dispatch a plane that it does not know to be 100% safe."

Harteveldt further explained that the Max 9 problem is not as complex as the fatal Max 8 crashes, which involved flawed flight-control software and a faulty sensor.

"The [Max 9] problem is isolated; the mechanics and engineers know exactly where to focus — and they did that — so it's a very different kind of problem for the plane," he told BI. "All of this certainly undermines the trust we place in Boeing, but, personally speaking, I would not hesitate to fly on a 737 Max 9 if the schedule and fare met my requirements."

However, Harteveldt emphasized that those who are concerned about flying on a Max 9 do have options. For example, Alaska and United currently allow passengers to switch their flight from a Max flight to a non-Max flight for no extra charge — similar to what carriers like Southwest Airlines did after the Max 8 crashes.

Passengers have more control over their personal safety

Embry-Riddle Aeronautical University associate professor and air crash investigator, Anthony Brickhouse, told BI that passengers should focus less on the aircraft they are flying on and more on their personal safety.

He listed things like wearing a seatbelt at all times, listening to the safety briefings, and leaving personal items behind in case of an emergency evacuation — the latter two helped save the lives of the 379 people on board a Japan Airlines Airbus A350 that caught fire in December.

"The federal regulators and the NTSB are going to do their job and make sure the aircraft we fly on are as safe as possible," Brickhouse told BI. "But passengers need to do more to impact their own safety at the end of the day."