What travelers need to know about the Boeing 737 Max 9 grounding

(CNN) —  A terrifying Alaska Airlines incident on Friday that left a hole in the side of a Boeing 737 Max 9 aircraft shortly after takeoff from Portland, Oregon, has raised safety questions and put many would-be air travelers on edge.

Investigators are looking closely at the failure of a mid-cabin door plug that detached during the flight, leading the Federal Aviation Administration to ground all US 737 Max 9 aircraft that have the door plug feature until the aircraft can be thoroughly inspected.

Here’s what we know so far about how the situation affects air travelers:

How long will the planes be grounded?

The timeline for a return to service is unclear. The FAA’s Emergency Airworthiness Directive prohibits flight by US airlines or in US territory of all Boeing 737 Max 9 aircraft that have a mid-cabin door plug installed until they pass inspection.

Alaska Airlines acknowledged on Wednesday the Max 9 will not fly for at least several more days and canceled all flights scheduled on that plane through Saturday. The cancellations amount to between 110 and 150 flights per day.

“We hope this action provides guests with a little more certainty, and we are working around the clock to reaccommodate impacted guests on other flights,” the airline said in a statement.

The FAA said on Tuesday that Boeing is revising its instructions to operators for inspections and maintenance of the affected 737 Max 9 aircraft.

“Upon receiving the revised version of instructions from Boeing, the FAA will conduct a thorough review,” the FAA said. “The safety of the flying public, not speed, will determine the timeline for returning the Boeing 737-9 Max to service.”

Jennifer Homendy, chair of the National Transportation Safety Board, said Wednesday she thought it would be difficult to verify the planes were fixed and safe to fly without a better understanding of the issue.

The FAA order applies to the US. What about operations of this model elsewhere?

The FAA order grounded 171 of the world’s 737 Max 9 aircraft. There are 215 in service.

Some international carriers operate Max 9s that are not affected because they don’t have a mid-cabin door plug. Some other airlines, flying aircraft with the door plugs, have followed the FAA’s lead and grounded their planes.

Mexico’s Aeromexico and Copa Airlines in Panama are among the international carriers with the most 737 Max 9 aircraft in their fleets. Aeromexico said in a statement that it grounded its Max-9s over the weekend in accordance with the FAA’s directive. Copa Airlines said in an updated statement on Wednesday that operations of its 21 Max 9 aircraft remain suspended as the airline awaits further details on inspections.

Turkish Airlines, which has five Max 9 aircraft in its 400-plus plane fleet, said that it will withdraw the aircraft from its fleet “until the technical investigation process is completed and the measures requested by the authorities are implemented.”

While the FAA does not have authority over the operation of aircraft operated by some international carriers, those airlines often follow the agency’s lead.

“The world still looks to the FAA … as the gold standard,” according to Kathleen Bangs, an aviation expert and former commercial airline pilot.

How have flight operations been affected?

Hundreds of flights have been canceled since Friday on both United Airlines and Alaska Airlines, the two US carriers using 737 Max 9 aircraft.

The airlines have been able to shuffle equipment and accommodate some of those passengers on other aircraft, but the grounding was still affecting operations midweek.

Copa and Aeromexico have also logged numerous cancellations in recent days, according to flight tracking site FlightAware.

Airlines with larger fleets will generally have an easier time shuffling aircraft around to meet customer demand. Location can also play a role. Airports capable of supporting a larger number of aircraft are more likely to have spare planes to replace the grounded ones.

What do I do if my flight is canceled because of this grounding?

First off, you can get a refund.

“Under federal law, if an airline cancels or significantly changes your flight – no matter the cause – you’re entitled to a full cash refund if you choose not to travel,” said Scott Keyes, founder of travel site Going, in an email interview with CNN Travel on Tuesday.

“This is true across the board, even if you booked a nonrefundable fare (as most tickets are) and even if you’re in basic economy,” he said.

If you still want to make the trip, “the airline will reaccommodate you on a different flight,” Keyes said. “The simplest way to do this is self-service through the airline’s mobile app (which have gotten quite good in the past year or two), or you can call up the airline and an agent will rebook you free of charge.”

The two US carriers operating affected planes are offering limited-time refunds to some passengers.

“Alaska is currently allowing anyone scheduled to fly on a 737 MAX-9 through January 20 to get a full refund if they choose not to travel. United is currently offering the same but only through January 10,” Keyes said.

Is it still safe to fly?

David Soucie, an aviation safety analyst for CNN and former FAA safety inspector, is taking a wait-and-see approach on the Boeing 737 Max 9 while the investigation continues to pinpoint the exact source of the problem.

“I haven’t made the decision to not fly in this aircraft if it’s returned to service,” he said Monday in an interview on CNN.

Soucie said he has called on Boeing to offer more details about when the aircraft interior on the Alaska Airlines plane involved in the incident was installed, which could indicate whether Boeing or the airline last worked on the door plug in question.

At an “all-hands” Boeing safety meeting on Tuesday in the wake of the Alaska Airlines incident, a company source suggested that “the mistake in question is clearly the assembly that was built up through the supply chain getting to the customer that contributed to the incident,” referencing the mechanism meant to hold a door plug in place that ultimately separated from the plane.

Bangs, the former airline pilot, said she understands “why people are frustrated – and nervous – about this,” noting that it’s just been a couple of years since another member of the Max family of aircraft, the Boeing 737 Max 8, was returned to service after being grounded for nearly two years. That followed two fatal crashes attributed to problems with the aircraft’s automated Maneuvering Characteristics Augmentation System, or MCAS.

Bangs said she was impressed with the FAA’s “right decision” to quickly ground the Max 9 on Saturday, a move it was slow in making for the Max 8. She said that industry members were hoping that the issue with the Alaska Airlines plane was a one-off involving a single occurrence that somehow weakened its structure.

But United and Alaska Airlines both reported additional jets with loose hardware upon inspection Monday.

“And so now we know we have a quality control problem because these are airplanes produced at different times, you know, different deliveries and at different airlines,” Bangs said.

But Bangs believes that authorities will ensure that all of the door plugs are secure, “but I certainly understand, when you add that to the problems of the Max 8, why people are getting nervous about flying on this particular version of the 737.”

Bangs said she’s not nervous because most of the pilots she’s talked to who fly this model like the planes. And she added that she’s confident that once an issue has been identified and addressed, it is safe.

NTSB Chair Homendy personally inspected the Boeing aircraft with the damage and told CNN on Monday that “our focus right now is on this aircraft to determine what happened, how it happened and to prevent it from happening again. And once we determine that, we can see if there’s a greater concern that we want to issue an urgent safety recommendation for.”

But is it safe for anyone to fly on these Boeing Max jets right now?

“Our aviation system Is the safest in the world. We are the gold standard for safety in our air space, but we need to maintain that,” Homendy said. “And when an event occurs like this, it is up to us to take a close look at what happened to make sure we maintain safety in the air.”

How can passengers figure out what type of plane they’ll be on?

Bangs pointed out potential passengers can see what kind of equipment the flight is operating on in the “details” section when they book flights.

Keyes explained that “when you purchase a flight, it will say on your receipt what aircraft you’re scheduled to fly (typically near the flight number).” People can also also use the website ismyplanea737max.com, he said.

But be prepared for last-minute changes, Keyes warned. “Do note that aircraft swaps happen occasionally; what you were originally scheduled to fly is usually but not always the same model you ultimately fly.”

In that case, Bangs said passengers can use the tail number of the plane at the gate to double-check the type of aircraft.