SEATTLE, WA - JANUARY 29: The tail and a next generation winglet of a A Boeing 737 MAX 8 are pictured at Boeing Field after its its first flight on January 29, 2016 in Seattle, Washington. The 737 MAX is the newest version of Boeing’s most popular airliner featuring more fuel efficient engines and redesigned wings. (Photo by Stephen Brashear/Getty Images) (Photographer: Stephen Brashear/Getty Images)

First flown in 1967, Boeing Co.’s 737 has become the world’s best-selling commercial aircraft. The fourth generation of jets, known as the Max, made its debut in 2017 when Indonesian low-cost 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 led to a global grounding of the aircraft and put the 737’s previously sound safety record under the microscope. The crashes have also raised questions about how U.S. flight regulators came to certify the Max.

1. Are the two crashes linked?

It’s too early to say. A preliminary study of the Ethiopian Air flight data recorders shows “clear similarities,” according to Ethiopia’s transport minister. There were enough parallels for the U.S. Federal Aviation Authority -- the agency that originally certified the 737 Max -- to reverse its initial recommendation not to ground the aircraft. The Lion Air Max 8 that crashed was 2 1/2 months old, the Ethiopian Air jet just four months old. Both accidents took place not long after takeoff as the planes flew erratically and pilots asked to return to the airport. The investigation into the Oct. 29 Lion Air crash pointed to a malfunction of a safety feature, the Maneuvering Characteristics Augmentation System, that repeatedly forced the plane into a nosedive.

2. What is MCAS?

It’s a set of sensors and software that Boeing installed in the 737 Max to help pilots respond to a situation in which the wings are losing lift because the plane is climbing too steeply. Insufficient lift can produce an aerodynamic stall, which can cause a plane to plummet. If a so-called angle-of-attack sensor on the outside of the aircraft reports that its nose is aimed too high, MCAS is programmed to automatically lower it, allowing the plane to regain speed and lift. In the Lion Air crash, the system was activated by a reading from a single faulty sensor that had been improperly repaired after incidents on earlier flights. The pilots were baffled when the tool pushed the nose down multiple times, exerting more and more force until they lost control. All 189 people aboard died when it plunged into the Java Sea.

3. What does Boeing say?

It says there is a simple procedure for shutting off MCAS in such situations, but the Lion Air pilots apparently didn’t follow it. The day before the crash, an off-duty pilot on the same aircraft correctly diagnosed the problem and told the crew how to disable the system, saving the plane, two people familiar with the situation said.

4. What do others say about the system?

The Dallas Morning News reported that pilots in the U.S. repeatedly voiced safety concerns about the Max 8 autopilot system to authorities months before the March 10 Ethiopian Air crash. Many veteran 737 pilots first learned of the flight-control changes on the Max in the aftermath of the Lion Air crash, and some were furious with Boeing for omitting any description of them from most flight manuals. Subsequently, the FAA and other regulators took steps to notify pilots of the MCAS and to remind them how to overcome it in the event of a malfunction. Boeing issued further guidelines on how to override the plane’s automated systems.

4. Why is the FAA under scrutiny?

FAA employees warned as early as seven years ago that Boeing had too much sway over safety approvals of new aircraft, prompting an investigation by Department of Transportation auditors who confirmed the agency hadn’t done enough to “hold Boeing accountable,” Bloomberg reported. The transport department inspector general’s office was investigating the FAA’s approval of the Max even before the second crash took place. It’s seeking to determine whether the agency used appropriate design standards and engineering analysis to certify the anti-stall system, the Wall Street Journal reported. The Seattle Times reported that FAA managers pushed engineers to delegate key parts of safety assessment to Boeing and to speed up approval of the resulting analysis, as development of the model was nine months behind that of rival Airbus SE’s A320neo. The safety report Boeing delivered to the FAA for the MCAS had several flaws, the article said. Boeing said in response there were “some significant mischaracterizations” in the engineers’ comments. The Seattle Times says the FBI has joined the transport department’s investigation.

5. How many airlines have purchased the 737 Max?

Plenty, but most orders have yet to be fulfilled. As of February, Boeing reported 376 deliveries of the single-aisle jets to 47 airlines or leasing companies. In total, orders from more than 80 operators exceed 5,000. 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 through February), American Airlines (24) and Air Canada (23). Norwegian Air, FlyDubai and several Chinese carriers also operate them. Chinese airlines account for about 20 percent of 737 Max deliveries globally. Click here for the full list.

6. How disruptive have the groundings been?

With fewer than 400 Max jets in service, not very, at least for most airlines. In the U.S., for example, the Max makes up about 3 percent of the mainline fleets for three carriers: American, Southwest and United. Airlines have reacted by using other planes in their fleet or leasing aircraft. Travelers may experience a bump in ticket prices, at least during peak season in some markets.

7. How long will planes be grounded?

The FAA says a more formal fix to redesign MCAS won’t be mandated until April. U.S. air-safety regulators are leaning toward approving Boeing’s changes to software and pilot training for the Max, the Wall Street Journal reported, citing people familiar with the matter. Boeing is meeting with customers and regulators to explain its plans for getting the plane back into service. Extensive changes to the plane’s software will make its automated stall-prevention feature less aggressive and more controllable, according to the Journal’s report. Training will highlight information about when the system engages and how to shut it off, the report said.

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

It has bigger engines, incorporates more automation, has a higher range (up to about 3,550 nautical miles, or 6,570 kilometers) and uses less fuel. Visually, the 737 Max differs from older models through its distinctive double winglets and serrated engine housing. Some reviews complained about the new design squeezing in smaller bathrooms and more seats.

9. Were the airlines involved in the crashes safe?

While Africa has a generally poor aviation safety record compared with global norms, Ethiopian Air is known for operating a modern fleet that features Boeing 787 Dreamliners and the latest Airbus SE A350, as well as the 737 Max. The state-owned airline, Africa’s only consistently profitable carrier, has built Addis Ababa into a major hub feeding travelers from around the world into dozens of African cities in competition with rivals such as Dubai-based Emirates. Captain Yared Getachew had amassed more than 8,000 flight hours, according to the airline, while first officer Ahmed Nur Mohammod had spent about 200 hours aloft. The plane that crashed had flown about 1,200 hours. Indonesia’s aviation safety record has been criticized. Its airlines, including Lion Air, were banned from flying to the European Union and the U.S. for almost a decade until 2016 because of safety concerns.

10. What does this mean for Boeing?

The 737 is the company’s largest seller and accounts for almost one-third of operating profit. There’ll be claims from airlines; Norwegian Air said it will seek compensation for the grounding of about 1 percent of its seats. And it may impact sales; Lion Air and several other carriers have suspended deliveries or put orders on hold. There’s also the prospect of substantial payouts to the families of passengers if Boeing is found responsible for the crashes. In the week after the second crash, Boeing lost more than $22 billion, or 12 percent, of its market value. Boeing issued a statement after the U.S. regulator grounded the aircraft saying it still has “full confidence” in the plane. According to an analysis of 47 commercial aircraft by Quartz, no model has been implicated in as many fatalities as quickly as the Max 8 since 1966. Boeing has suspended customer deliveries, but continues to produce them at a rate of 52 a month. Meanwhile, investigators in Addis Ababa are studying the Ethiopian Air black box, aided by experts from around the world, while Indonesian authorities say their final crash report may be ready in June.

Some of these questions came from our readers via WhatsApp. You can sign up to get news and analysis from Bloomberg daily on WhatsApp here.

--With assistance from Paul Geitner.

To contact the reporters on this story: Kyunghee Park in Singapore at;Alan Levin in Washington at

To contact the editors responsible for this story: Jon Morgan at, Grant Clark, Christopher Jasper

©2019 Bloomberg L.P.