What we know so far about the mid-air blowout aboard Alaska Airlines flight

Investigators are trying to determine what led to the terrifying incident aboard a Boeing 737 Max 9 jet Friday, when a piece of the aircraft blew out mid-flight forcing pilots to make an emergency landing with a hole in the fuselage.

Alaska Airlines Flight 1282 had 171 passengers and six crew on board and was flying at an altitude of more than 4,800 metres when a door plug a panel in place of an optional exit door located near the rear of the aircraft ripped off about 20 minutes into the Jan. 5 evening flight from Portland, Ore., to Ontario, Calif.

Fortunately, the two seats next to the panel were vacant and there were no serious injuries, though several passengers did require medical attention after the pilots made an emergency landing at Portland International Airport.

By the next day, U.S. aviation authorities had grounded certain Max 9 planes in order for emergency safety inspections to be carried out.

It's still unclear what caused the door plug on the Alaska Airlines flight to blow out.But this is just the latest safety concern for Boeing's 737 Max series, following two deadly crashes involving Max 8 planes five years agoin Indonesia and Ethiopia, resulting in a worldwide grounding that lasted nearlytwo years.

Here's what you need to know about what's happening with the Boeing 737 Max 9 and the investigation into the frightening flight.

WATCH | Emergency landing after blowout:

FAA investigates after Boeing 737 cabin panel blows out at 16,000 feet

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U.S. airline regulators have temporarily grounded 171 Boeing 737 Max 9 planes after a terrifying non-fatal incident aboard an Alaska Airlines flight. A cabin window blew out and depressurized the passenger cabin in mid-air, forcing an emergency landing.

Which planes were grounded?

Saturday, the U.S. Federal Aviation Authority (FAA) issued an Emergency Airworthiness Directive to the owners and operators of Boeing 737 Max 9 planes with similar mid-cabindoor plugs.

The FAA said the directive affected 171 planes out of 218 Max 9sin operation worldwide.

In the U.S., only two airlines fly the 737 Max 9:Alaska Airlines, which operates 64 of the aircraft, and United Airlines with79 Max 9s in its fleet.

Both airlines have been forced to cancel hundreds of flights.

On Monday, three days after the mid-air incident, Alaska Airlines said it was forced to cancel 141 flights, 20 per cent of its overall flights, while United's 221 cancellations amounted to eight per cent of its total scheduled flights for the day.

Why did the plane have a door plug?

Some larger Boeing 737s have emergency exits on fuselages behind the wings to meet a federal requirement thatplanes be designed so passengers can evacuate within 90 seconds even if half the exits are blocked.

The more passenger seats there are on a plane, the more exits are required.

Some carriers, including Indonesia's Lion Air and Corendon Dutch Airlines, havemore than 200 seats ontheir Max 9s, so they must have extra emergency exits. However, Alaska Airlines and United Airlines configure their 737 Max 9s to have fewer than 180 seats, so the planes don't need the two mid-cabin exits to comply with U.S. evacuation rules.

On Alaska and United, those side exits near the back of the plane are replaced with a permanent plug the size of an exit door approximately 1.2 metres high andweighing28.5-kilograms which is fixed in place with four bolts.

As a result of the emergency inspections, United said Monday it found loose bolts on door plugs of Boeing 737 Max 9 jetsgrounded after the blowout on the Alaska Airlines flight.

"Since we began preliminary inspections on Saturday, we have found instances that appear to relate to installation issues in the door plug for example, bolts that needed additional tightening," said Chicago-based United, which is the second largest airline in the world.

LISTEN | Passenger describes the moment part of the plane tore off:

As It Happens5:07Nurse describes the moment oxygen masks came down on Alaska Airlines flight

Alaska Airlines Flight 1282 made an emergency landing on Friday when a door-sized section on the side of the plane blew out. Nurse Vicki Kreps was on that flight with her two grandchildren Brady, 7, and Brynlee, 5. Kreps spoke to As It Happens host Nil Kksal about the experience.

What do investigators know so far?

Investigators are now in possession of the door plug after an Oregon man found it in his backyard Sunday night. The National Transportation Safety Board (NTSB) has sent the plugfor examination at its Material Laboratory in Washington.

Alaska Airlines said theMax 9 involved in the incident was only delivered for use on Oct. 31, so it's not believed wear and tear is a factor.

Arthur Rosenberg, an aviation lawyer and former pilot, believes it may have been what he called a"manufacturing defect" that led to the door plug blowing out, as opposed to any sort of design defect.

"The bolts and the fasteners which were holding it in placefailed for some reason," he told CBC News Network, explaining the recovery of the plug and the parts remainingof the structurefuselageshould allow investigators to determine what happened.

The NTSBsaid Monday evening it is uncertain if therecovered cabin panel was properly attached or if bolts had been installed.

The NTSB said it will be able to determine if the bolts had been installed through testing.

NTSB chair Jennifer Homendyalso told reportersinvestigators have"no indication whatsoever" that a previous issue with a cabin pressurization control system on the plane was related to the door plug blowing out.

The aircraft's auto-pressurization warning light went offonthree occasions between Dec. 7 and Jan. 4, including one instance during a flight.

Alaska Airlines restricted the aircraft from long flights over water it had been used to fly to Hawaii but that was in line with the airline'sinternal maintenance policy not a federal aviation regulation, shesaid.

The flight data recorder has beenexamined, but theNTSB said Sundaythe plane's cockpit voice recorder was "completely overwritten" after the flight landed safely Friday evening.

Homendy explained the data was not retrieved within two hours when recording restarts, erasing previous data.

The U.S. requires cockpit voice recorders to log two hours of data, while in Europe, planes made after 2021 are required to log 25 hours of data.

The FAA proposedchanging its requirementslast spring,to mandate a 25-hour recording time for cockpit voice recorders in new aircraft, butnoted at the time that theprocess could take years.

Since 2016, the International Civil Aviation Organization hascalled for 25-hour recording on planes manufactured since 2021.

Do Canadian airlines fly Max 9 planes?

No. According to The Canadian Press, airlines including Air Canada, WestJet, Flair Airlines and Lynx Air all say they fly the Max 8 jetliner, not the Max 9.

Transport Canada grounded Boeing 737 Max 8 aircraft in the country in March 2019 following the two overseas crashes that left 346 people dead, but the order was lifted in January 2021.