What you need to know about the Alaska Airlines 737 Max 9 plane that suffered a tire blowout

NEW YORK — An emergency landing by an Alaska Airlines jet has prompted US federal authorities to ground a number of Boeing 737 Max 9 aircraft, adding another episode to the troubled history of Boeing's Max line.

Here's what you need to know about the Max 9 plane involved and what comes next.

WHAT HAPPENED?

An Alaska Airlines jet blew out part of its fuselage Friday evening seven minutes after takeoff 3 miles above Oregon, forcing the pilots to make an emergency landing. None of the 171 passengers or six crew members were seriously injured, but the rapid loss of cabin pressure caused oxygen masks to fall from the ceiling. Jennifer Homendy, chairwoman of the National Transportation Safety Board, said the two seats next to the torn section were unoccupied.

HOW DO THE FEDERAL AUTHORITIES RESPOND?

The Federal Aviation Administration has ordered the grounding of a number of Boeing Max 9s owned by U.S. airlines or flown into the country by foreign airlines until they are inspected. The emergency order affects approximately 171 aircraft worldwide.

The NTSB has launched an investigation that will likely last months and is focusing on the paneled exit door that was blown off. On some jets with fewer seats, the so-called door plug is installed instead of an emergency exit panel. The aircraft ordered to be grounded by the FAA all have these panels installed.

Authorities are still searching for the door plug, which is believed to have landed near Oregon Route 217 and Barnes Road in the Cedar Hills area west of Portland.

HOW DO AIRLINES RESPOND?

Alaska Airlines has grounded its entire fleet of 65 Max 9s for inspections and maintenance. The airline initially kept 18 Max 9s in service on Saturday as they had undergone in-depth inspections as part of recent maintenance checks. But the airline took those planes out of service Saturday evening to comply with an FAA directive for all operators of Max9s to conduct specific inspections.

United Airlines, the world's largest operator of Max 9s, has grounded its entire fleet of 79 Max 9s.

Alaska and United are the only two U.S. passenger airlines that operate Max 9 aircraft. The companies operate nearly two-thirds of the 215 Max 9 aircraft in service worldwide, according to aviation analytics firm Cirium. According to Cirium, six other airlines use the Max 9: Panama's Copa Airlines, Aeromexico, Turkish Airlines, Icelandair, flydubai and Kazakhstan's SCAT Airlines.

Copa said it had temporarily suspended 21 Boeing 737 Max 9s to comply with the FAA order.

Alaska Airlines had canceled 163 flights on Sunday, or 21% of its schedule, according to Flightaware. United Airlines had canceled 247 flights, about 9% of scheduled flights. Not all cancellations were necessarily due to the Max 9 crisis.

Alaska Airlines said the groundings had led to at least 160 flight cancellations on Saturday evening and that disruptions will continue at least until midweek. United had said Saturday evening that the inspections would lead to about 60 cancellations.

Alaska Airlines has said affected passengers should go online to review rebooking options and request a refund. The airline has said that passengers whose flights are canceled will be moved to the next available flight, but they can also request a change or refund without being charged under a flexible travel policy. United Airlines has not implemented a specific policy for the Max 9 inspections, but the airline is waiving change fees for significant delays.

HOW SAFE IS IT TO FLY ONE OF THESE AIRCRAFT?

Federal officials and airline executives regularly tout the safety of air travel. There has not been a fatal crash of a US airliner since 2009, when a Colgan Air plane operating for Continental crashed near Buffalo, New York, killing all 49 people on board and one on the ground.

After arriving in Portland to investigate the Alaska Airlines incident, the NTSB's Homendy said the U.S. has “the safest aviation system in the world.” However, an increase in the number of close calls between planes at U.S. airports over the past year prompted the FAA to convene a “safety summit” last year, during which officials encouraged airlines and pilots to redouble their focus on flying carefully.

The incident has also raised new questions about the safety of Boeing's Max plane, the latest version of the company's legendary 737. There are two versions of the aircraft in use: the Max 8 and the Max 9, the larger of the two.

Regulators around the world have grounded Max 8 planes for nearly two years after a Lion Air flight crashed in Indonesia in 2018 and an Ethiopia Airlines Max 8 in 2019. Boeing changed an automated flight control system used in the crashes were involved.

Last year, the FAA told pilots to limit the use of an anti-icing system on the Max in dry conditions, fearing that the intakes around the engines could overheat and rupture, potentially affecting the plane. And in December, Boeing told airlines to inspect planes for a possible loose bolt in the rudder control system.

However, these past problems have nothing to do with Friday's eruption, which is extremely rare in air travel, according to Anthony Brickhouse, professor of aerospace safety at Embry-Riddle Aeronautical University. Brickhouse said fuselage panels sometimes tear off planes, but he could not recall a previous incident where a gaping hole was blown like the one that forced Friday's emergency landing.

In 2018, a passenger on a Southwest Airlines plane was killed when a piece of engine housing blew off and shattered the window she was sitting next to. However, that incident involved an earlier version of the Boeing 737, and not a Max.

WHAT IS BOEING'S RESPONSE?

The company, based in Arlington, Virginia, issued a brief statement saying “we deeply regret the impact this event has had on our customers and their passengers.” Boeing said it supported the FAA's decision to require immediate inspections and said it was providing technical assistance to investigators. The company has declined to make an executive available for interviews.