For many fliers, “Am I booked on a 737 Max?” has become a crucial question after Boeing’s new plane was involved in a second crash that killed all on board.

Passengers are turning to social media to express their fears about the plane’s safety and to seek assurance from the airlines that fly the jet.

“We are fielding some questions from customers asking if their flight will be operated by the Boeing 737 MAX 8,” said Brian Parrish, a spokesman for Southwest, which doesn’t charge a fee to change reservations. “Our customer relations team is responding to these customers individually.”

Airlines declined to say how many passengers were seeking to change their flights. Carriers also dealt with questions like how travelers could tell what type of plane they’re booked on, what routes are flown with the Max and whether a Boeing 737-800 is the same as a 737 Max 8.

For most travelers, the information about their plane type is available at the time of booking, either during the seat-selection process or elsewhere online.

Experienced travelers — and especially those who frequently book longer flights — often head to FlightStats.com or SeatGuru.com, to determine their planes.

Even if passengers determine which type of plane they are booked on, airlines might change planes at the last minute, as required by logistics or a change of weather.

Southwest is the largest Max operator, flying 34 of the planes. Air Canada and American Airlines each have 24 and WestJet has 13. United Airlines does not operate any Boeing 737 Max 8 aircraft, but it does fly the 737 Max 9. Delta Air Lines does not fly any 737 Max planes.

United, which is also keeping its fees in place for rebookings, is parsing the difference between Boeing’s Max 8 and Max 9 on its social media accounts, noting that it operates the larger 9 and not the model that crashed in Asia and Africa. United has brought in 14 new Max aircraft since April, and is awaiting delivery of 122 more

Other U.S. and Canadian airlines are enforcing their ticket-change rules and charging fees for nervous customers who seek to swap from the Max. It’s a tricky dilemma for carriers: If they allow customers to rebook from a 737 Max flight without a penalty, they’re implicitly lending support to the notion that the new plane isn’t as safe as other aircraft they fly.

“Our standard policies for changes still apply,” said American Airlines. The carrier had 88 Max 8 flights scheduled Monday.

Fees to change tickets can cost up to $200 on a domestic itinerary, depending on when the switch is made, and $750 for international flights, according to airline websites.

Julie Loffredi, the manager of media relations for InsureMyTrip.com, a travel insurance comparison site, said that “fear” is usually not a good enough reason for insurers to pay compensation to travelers who cancel their flights.

“With traditional, standard travel insurance you wouldn’t be able to cancel and get your money back out of fear,” she said.

However, since the 9/11 terrorist attacks, more fliers have purchased coverage that allows for any cancellation, whatever the reason.

Passengers weren’t alone in being nervous about the aircraft. The Association of Professional Flight Attendants, which represents workers at American, told its members they would not be forced to fly the Max 8 if they felt unsafe. The Association of Flight Attendants-CWA called on the FAA to investigate the Max.

The Federal Aviation Administration tweeted on Sunday that it was “closely monitoring” developments in the Ethiopian Airlines crash, and on Monday, stressed its assurance that the 737 Max suite of planes were airworthy.