What to know about the Boeing 737 MAX 9 panel that blew off in mid-air

Federal aviation regulators have grounded certain Boeing 737 MAX 9 jets for further inspection after a piece of fuselage tore off the left side of an Alaska Airlines-operated aircraft on Friday, causing the airliner to make an emergency landing after the cabin depressurized.

After Alaska Air Flight 1282 took off from Portland, Oregon, en route to Ontario, California, it was at about 16,000 feet of altitude and climbing when it experienced rapid depressurization after a panel that serves as a plug for an emergency exit door used on some variations of the Boeing 737 MAX 9 was torn off. The pilot immediately signaled the plane would have to land, it safely returned to Portland with all 171 passengers and six crew members aboard.

No serious injuries were reported in the incident, though Alaska reported that some passengers experienced minor injuries that required medical attention – all of whom were medically cleared after receiving treatment. Nobody was sitting in the seats adjacent to where the door plug blew out according to the airline. Parts of the seat next to the fuselage, including the headrest, were missing.

The Federal Aviation Administration (FAA) issued a directive Saturday requiring safety inspections of 171 Boeing 737 MAX 9 aircraft operating worldwide. The National Transportation Safety Board (NTSB) dispatched a "Go Team" with experts in structures, operations and systems to investigate the incident. 

WATCH: ALASKA AIRLINES PASSENGER CAPTURES HORRIFYING MOMENTS AFTER PLANE PART BLEW OFF MIDAIR

Why is there a door plug panel?

The 737 MAX 9 is Boeing’s largest single-aisle plane and can seat up to 220 people. It includes an optional extra door to allow for the approved number of evacuation paths in cases where carriers choose to install the maximum number of seats on the aircraft.

Planes that aren’t configured to maximize seating capacity can replace that emergency door with a panel, or plug. Door plugs have been used to adapt aircraft and offer flexible layouts across the industry for years.

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How complex is the production process for the panel?

The fuselage for Boeing 737s is made by Kansas-based Spirit AeroSystems, which separated from Boeing in 2005. Spirit is one of two suppliers that makes the plug doors on the MAX 9, but Boeing also has a key role in the plug installation process, Reuters reported.

The installation is done in a two-part process involving manufacturing at Spirit’s fuselage plant in Wichita, Kansas, and a Boeing factory outside Seattle, sources told Reuters. 

Spirit sends the 737 fuselages by train with the special door assembly "semi-rigged" to Renton, Washington, where Boeing typically removes the pop-out doors while loading interiors, then puts the part back to complete installation according to the Reuters report. The hull is then pressurized to 150% to ensure everything is working properly.

How many 737 MAX 9 planes are there?

Boeing has delivered 200-plus Boeing 737 MAX 9 planes, of which 171 have a door plug panel instead of an emergency door. Before U.S. airlines can resume flying the planes, the FAA must agree to detailed inspection criteria proposed by Boeing, and it’s unclear if it will do so on Sunday.

U.S. air carriers that use the panel on their 737 MAX 9 airliners are Alaska and United. Alaska halted the use of 18 of its MAX 9 planes on Saturday. On Sunday, it canceled 163 flights, or 21%, and said travel disruptions from the grounding are expected to last through at least mid-week. United canceled 230 flights on Sunday, or 8% of its scheduled departures.

Ticker	Security	Last	Change	Change %
ALK	ALASKA AIR GROUP INC.	53.16	+0.56	+1.06%
UAL	UNITED AIRLINES HOLDINGS INC.	97.44	+0.61	+0.63%
BA	THE BOEING CO.	156.54	+1.10	+0.71%

Other carriers that have grounded planes for inspection include Panama’s Copa Airlines, Aeromexico and Turkish Airlines.

What they’re saying

Boeing said in a statement shared with FOX Business: "Safety is our top priority and we deeply regret the impact this event has had on our customers and their passengers. We agree with and fully support the FAA’s decision to require immediate inspections of 737-9 airplanes with the same configuration as the affected airplane. In addition, a Boeing technical team is supporting the NTSB’s into last night’s event. We will remain in close contact with our regulator and customers."

Alaska Airlines said in a post on X, formerly, on Saturday: "As of this morning, inspections on more than a quarter of our 737-9 fleet are complete with no concerning findings. Aircraft will return to service as their inspections are completed with our full confidence."

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National Transportation Safety Bureau (NTSB) chair Jennifer Homendy spoke at a news conference Sunday and referred to what happened as "an accident, not an incident," 

"We are very, very fortunate here that this didn’t end up in something more tragic," Homendy said. "With that said, I imagine this was a pretty terrifying event. We don’t often talk about psychological injury, but I’m sure that occurred here."

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Homendy declined to speculate about the cause of the accident but noted that the outcome could’ve been much worse if it had occurred once the aircraft reached its cruising altitude.

"Think about what happens when you’re in cruise. Everybody’s up and walking, folks don’t have seatbelts on. They’re going to restrooms. The flight attendants are providing service to passengers. We could have ended up with something so much more tragic," she said.

Homendy also asked the public for help locating the missing door, saying it’s believed to be in the the west side of the Portland metro area near Oregon Route 217 and Barnes Road in the Cedar Mills suburb.

FOX Business’ Landon Mion and Reuters contributed to this report.