The 737 Max is the newest version of the 737, the best-selling airliner ever. Since debuting in 2017, Boeing has delivered more than 350 of them in several versions that vary by size.
Dozens of airlines around the world have embraced the plane for its fuel efficiency and utility for short and medium-haul flights.
Boeing has taken more than 5,000 orders for the various Max versions, and they constitute the largest share of the company's backlog of nearly 5,900 planes. They carry list prices from $100 million to $135 million, although airlines routinely get deep discounts.
The plane suffered its first fatal accident in October, when a 737 Max 8 operated by Lion Air plunged into the Java Sea, killing 189 people. Boeing bounced back, however, with little apparent effect on new orders.
However, the second deadly crash for a Max 8 on Sunday in Ethiopia, which killed all 157 people on board, could prove far more damaging if investigators find fault in Boeing's design or airlines and their passengers lose confidence in the jet.
Already airlines in Ethiopia, Mexico, China, Brazil, Argentina and Indonesia have temporarily grounded their Max 8s, in addition to Caribbean carrier Cayman Airways, Comair in South Africa, and Royal Air Maroc in Morocco.
Crucially, however, there was no outward sign that the influential U.S. regulator, the Federal Aviation Administration, would do the same.
Transportation Secretary Elaine Chao said her department, which includes the FAA, was "very concerned" and monitoring developments around Sunday's crash. She said she met with acting FAA Administrator Dan Elwell to discuss the situation "and what are our possible paths forward." She didn't say whether the agency was considering grounding any planes.
It's unusual for authorities to take the step of grounding planes, and it's up to each country to set standards on which planes can fly and how those planes are maintained, said Todd Curtis, an aviation safety analyst who directs the Airsafe.com Foundation.
In the last major FAA recall, the agency grounded Boeing 787 passenger jets in 2013 after several instances in which lithium-ion batteries overheated. The plane was relatively new at the time. United was the only U.S. carrier affected, with six planes grounded.
Late on Monday, the FAA issued a statement saying that while others have drawn similarities between the Indonesia and Ethiopia crashes, the agency was not.
"This investigation has just begun and to date we have not been provided data to draw any conclusions or take any actions," the FAA said.
The FAA also said that no later than April it expects Boeing will complete changes, including new training for pilots in automated anti-stall technology that is suspected of playing a role in the Lion Air crash. Data released by Indonesian investigators indicates that pilots struggled unsuccessfully to counter the system, which repeatedly pointed the plane's nose down and may have sent it into a death spiral.
A consumer group, FlyersRights.org, urged the FAA to ground the Max 8.
"The FAA's wait-and-see attitude risks lives," said the group's president, Paul Hudson.
U.S. airlines repeated their belief that the plane is safe.
American Airlines, which operates 24 Max 8s, said it bases its judgment on collecting extensive data on its entire fleet, including the Max 8.
"We have full confidence in the aircraft and our crew members," the carrier's vice president of flight service, Jill Surdeck, said in a memo to employees.
Southwest Airlines operates the largest U.S. fleet of Max planes, with 34 Max 8s. Spokesman Brian Parrish said the airline remains confident in their safety. United has about a dozen Max 9s, which are slightly bigger than the Max 8.
Dozens of Max aircraft crisscrossed the skies over the United States on Monday, and passengers continued to board them and fly without incident. Passengers interviewed at Houston's Hobby Airport were keenly aware of the crash in Ethiopia. They expressed concern but no panic.
"I drive in Houston every day," said Brian Browder, who was waiting for a flight to Washington, D.C. "That's the place to be worried."
Still, Browder, who works for a construction-industry trade group and flies several times a month, said he would be "a bit apprehensive" about getting on a 737 MAX.
"They need to explain exactly what happened and why," he said.
Kelly Wells, a health care worker from Austin, believes a catastrophic accident is less likely to happen in the United States.
"Those pilots are very well trained to handle an emergency. I'm in capable hands," she said before a flight on Southwest, which operates the Max, although her flight was not among them.
Peggy Chang Barber, a lawyer for a Houston nonprofit who was on her way to New York, said U.S. airlines are "pretty vigilant" about maintenance and pilot training, and she won't be concerned "unless they find a fault with the plane."
It isn't clear yet whether the plane's technology played a role in the Ethiopian Airlines crash, or whether that accident is related to the deadly Lion Air accident on Oct. 29 in Indonesia.
Even if Boeing must make software or hardware changes to the plane, "it's nothing they can't get past, but it would be an expensive process," said Richard Aboulafia, an aviation consultant with Teal Group.
"Historically, (airlines) cancel orders because of (fuel) performance shortfalls or because somebody else gave them a discount — not because of safety," Aboulafia said.
The FAA stripped the McDonnell Douglas DC-10 of its airworthiness certificate after numerous safety problems including a 1979 crash in Chicago, yet it still outsold a rival plane made by Lockheed, he said.