First flown in 1967, Boeing Co.’s 737 has become the world’s best-selling commercial aircraft. The fourth generation of 737s -- known as the 737 Max -- made its debut in 2017 when Indonesian discount carrier Lion Air became the first commercial operator. Yet two fatal crashes within five months -- Lion Air flight 610 in October off the coast of Indonesia and Ethiopian Airlines flight 302 in March outside Addis Ababa -- have put the 737’s previously sound safety record under the microscope.

1. Which airlines have purchased the 737 Max?

Plenty, but most orders have yet to be fulfilled. As of January, Boeing reported that it had delivered 350 of the single-aisle jets to 46 airlines. In total, orders from more than 80 operators exceed 5,000 planes. Most sales are the Max 8, the model involved in both crashes. (There’s also, from smallest to biggest, a Max 7, 9 and 10.) The main operators include Southwest Airlines (31 in the fleet), American Airlines (22) and Air Canada (20). Norwegian Air, FlyDubai and several Chinese carriers also operate them. (Chinese airlines account for about 20 percent of 737 Max deliveries globally, and further purchases of the Chicago-based planemaker’s aircraft were said to have been touted as a possible component of a trade deal with the U.S.) Click here for the full list.

2. Will the 737 Max be grounded?

Yes, in some places. A day after the Ethiopia crash killed all 157 people on board, China ordered the grounding of the 96 Max jets operating in the country. Ethiopian Airlines also grounded its planes, as did Indonesia.

3. What about in the U.S.?

The U.S. Federal Aviation Administration, which originally certified the 737 Max, elected not to ground it, at least for now. The Washington-based regulator issued a global notice of “continued airworthiness” and pledged to take “immediate and appropriate action” if it finds “an issue that affects safety.” Among U.S. airlines, Southwest said it’s confident in the safety of its fleet, including the Max, and American said it will keep a close watch on the Ethiopian probe. A U.S.-ordered grounding of an entire model of aircraft is extremely rare, especially so soon during an investigation when few details are known. The last time the agency did so was in January 2013 as a result of overheating lithium-ion batteries on Boeing’s 787 model. The agency acted only after the second such incident occurred.

4. How can I check whether my flight is on a 737 Max?

If you already have a ticket, you should be able to tell from the booking details. If you are making a booking online, many sites indicate the model. If not, websites such as http://flightstats.com allow you to dig into details of flights at least a few days in advance, including the make and type.

5. How is the Max different from earlier 737s?

It has bigger engines, incorporates more automation, has a higher range (up to about 3,550 miles, or 6,570 kilometers) and uses less fuel. The Max was produced partly as a response to a new model -- the A320neo -- from European rival Airbus SE. Following the Lion Air crash, it came to light that the 737 Max has software that forces the plane’s nose down in certain circumstances to prevent stalling. It also emerged that some pilots weren’t aware of the system. Boeing said that shouldn’t have been the case and issued further guidelines on how to override the plane’s automated systems.

6. Are the two crashes linked?

It’s too early to say. In both cases, the incidents took place not long after takeoff as the planes flew erratically and pilots asked to return to the airport. Yet veteran crash investigators say there’s too little data to draw a direct tie at this stage of the investigation. Others note that it would be surprising if the Ethiopian Airlines pilots had been unaware of the procedures highlighted after the Lion Air crash. A preliminary report into that disaster, which killed 189 passengers and crew, indicated that pilots struggled to maintain control following an equipment malfunction.

To contact the reporters on this story: Kyunghee Park in Singapore at kpark3@bloomberg.net;Alan Levin in Washington at alevin24@bloomberg.net

To contact the editors responsible for this story: Jon Morgan at jmorgan97@bloomberg.net, Grant Clark, Christopher Jasper

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