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Boeing says faulty sensor contributed to 737 Max crashes

The 737 Max 8 had two deadly crashes in five months, and authorities are zeroing in on what went wrong. Plus: Everything you need to know about the plane and what it'll take for it to fly again.

Boeing

After two deadly crashes of its 737 Max 8 that killed 346 people, Boeing is facing massive scrutiny over one of its newest and most critical aircraft models. The airliner remains grounded around the world, and Congress, the FBI and the Trump administration have called for an inquiry into the FAA's certification process.

The developments are a huge blow to Boeing, which has thousands of 737 Max orders on its books. The official causes of the crashes, which appear to be similar, are still under study. But so far, investigation teams in Indonesia and Ethiopia are focusing on faulty sensors and a flight control system designed to push the nose down in the air. Until the official reports are published, here's everything else we know:

What happened in the most recent crash?

On March 10, Ethiopian Airlines flight 302 departed Addis Ababa Bole International Airport bound for Nairobi, Kenya. Just after takeoff, the pilot radioed a distress call and was given immediate clearance to return and land. But before the crew could make it back, the aircraft crashed 40 miles from the airport at 8:44 a.m., six minutes after it left the runway. Aboard were 149 passengers and eight crew members representing more than 30 nationalities.

The aircraft involved was only four months old. Africa's largest airline, Ethiopian serves cities worldwide (including the US) and is a member of Star Alliance, which includes United Airlines, Lufthansa and Air China. And despite the crash, and contrary to what some television presenters might think, Ethiopian has a strong safety record. The airline's last fatal crash happened in 2010.

What was the previous crash?

On Oct. 29, Lion Air flight 610 crashed in the Java Sea 13 minutes after takeoff from Jakarta, Indonesia, killing 189 people. As with the Ethiopian crash, the flight crew lost control early in its flight and made a distress call. That aircraft was almost brand-new as well, having arrived at Lion Air three months earlier. 

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The 737 Max 9, shown here at the 2016 Paris Air Show, is a larger version of the Max 8, but with the same piloting system that's under investigation.

Kent German/CNET

What is the Boeing 737 Max 8?

One of Boeing's newest airliners, the 737 Max 8 made its first flight on Jan. 29, 2016, and entered passenger service with Malaysia's Malindo Air on May 22, 2017. Seating between 162 and 210 passengers, depending on the configuration, it's popular on shorter routes, but also has the range (3,550 nautical miles or about 4,085 miles) to fly transatlantic and between the mainland US and Hawaii.

The design of the Max 8 is based on the Boeing 737, an aircraft series that has been in service since 1968. As a whole, the 737 family is the best-selling airliner in history. At any given time, thousands of some version of it are airborne around the world and some airlines, like Southwest and Ryanair, have all-737 fleets. If you've flown before, you've most likely flown on a 737.

What's different about the Max 8?

Compared with previous 737 versions, the Max 8 has bigger, more powerful and more efficient CFM LEAP engines (more on those in a minute), improved aerodynamics and a redesigned cabin interior. It also can fly farther and carry more people than the previous generation of 737s, like the 737-800 and 737-900.

The 737 Max series consists of four models, of which the Max 8 is the most popular. The larger Max 9 has been flying only for a few months and the 737-10 is still in development and has yet to fly. A few airlines have ordered the smaller 737 Max 7, but Boeing has yet to complete any deliveries. (It flew for the first time last May.)

paris-airshow-onboard-boeing-787-10-737-max-36

Compared with previous versions of the 737, the Max's engines sit farther forward and higher up on the underwing pylons.

Andrew Hoyle/CNET

What caused the crashes?

The complete reports haven't been published yet. Crash investigations are tremendously complex -- it takes months to evaluate the evidence and determine a probable cause. Investigators must examine the debris, study the flight recorders and, if possible, check the victims' bodies to determine the cause of death. They also involve multiple parties including the airline, the airplane and engine manufacturers and aviation regulatory agencies.

There are important clues so far. Remember those larger CFM LEAP engines? Well, because they're bigger, and because the 737 sits so low to the ground (a deliberate design choice to let it serve small airports with limited ground equipment), Boeing moved the engines slightly forward and raised them higher under the wing. (If you place an engine too close to the ground, it can suck in debris while the plane is taxiing.) That change allowed Boeing to accommodate the engines without completely redesigning the 737 fuselage -- a fuselage that hasn't changed much in 50 years.

But the new position changed how the aircraft handled in the air, creating the potential for the nose to pitch up during flight. A pitched nose is a problem in flight -- raise it too high and an aircraft can stall. To keep the nose in trim, Boeing designed software called the Maneuvering Characteristics Augmentation System, or MCAS. When a sensor on the fuselage detects that the nose is too high, MCAS automatically pushes the nose down.

boeing-737-max-all-versions

Of the four 747 Max versions, only the Max 10 has yet to fly.

Boeing

Investigators in the Lion Air crash have said that a fault in the sensor may have been feeding incorrect data to MCAS, pitching the nose down into a dive. According to the preliminary report (PDF) from the Indonesian National Transportation Safety Committee, the Lion Air pilots were unable to determine their true airspeed and altitude and they struggled to take control of the plane before the crash, as it oscillated for 10 minutes. Each time they pulled up from a dive, the system pushed the nose down again. (For a thorough explanation of MCAS, see this story from The Air Current.) 

The report also noted that maintenance crews had replaced the faulty sensor two days before the flight and that pilots on the four flights preceding the crash reported incorrect airspeed and altitude information (a passenger likened one of those flights to a "roller coaster ride"). And like with the Lion Air crash, the sensor on the Ethiopian plane may have been damaged causing it to feed erroneous data to the MCAS system.

On April 29, during Boeing's annual shareholders meeting in Chicago, CEO Dennis Muilenburg said the incorrect data was a common link in a chain of events that led to both crashes. It's a link Boeing owns, he said, and one that the software update will fix.

Boeing 737-100

The original version of the 737 first flew in 1967.

Boeing

Would the pilots have known about the faults?

The Air Current and The New York Times reported that the Lion Air plane also lacked a warning light designed to alert pilots to the fault and that Boeing sold the light as part of an optional package of equipment. When asked about the warning light, a Boeing spokesman gave CNET the following statement on March 22:

"All Boeing airplanes are certified and delivered to the highest levels of safety consistent with industry standards. Airplanes are delivered with a baseline configuration, which includes a standard set of flight deck displays and alerts, crew procedures and training materials that meet industry safety norms and most customer requirements. Customers may choose additional options, such as alerts and indications, to customize their airplanes to support their individual operations or requirements."

On April 29, Boeing responded to a report from The Wall Street Journal published the same day that said a safety feature designed to alert flight crews to a faulty sensor wasn't operating on some Max planes that had been delivered to airlines. 

"The alert was intended to be a standard, stand-alone feature on MAX airplanes," the company's statement said. "However, the disagree alert was not operable on all airplanes because the feature was not activated as intended."

Then on May 5, Boeing issued another statement, which said the disagree alert is "not necessary for the safe operation of the airplane."

Etihad 777 flight

The previous model, the 737-900ER, doesn't have the MCAS flight control system.

Boeing/Ed Turner

Are the two crashes related?

It appears they are. Investigators in France are still examining the flight recorder (the "black box") from the second crash, but Ethiopia's Transport Minister said on March 18 that two crashes have "clear similarities." Though he didn't elaborate, satellite data released March 14 showed that the final flight track of the Ethiopian Airlines jet was similar to that of the Lion Air plane. The FAA cited that data in its grounding order.

Investigators at both crash sites have also recovered the jack screws, which manipulate the control surfaces on the horizontal stabilizer that pitch the nose up and down. Both jack screws were set to send the planes into a dive.

Do we know anything else about the Ethiopian crash?

According to the preliminary report released on April 4, the flight crew initially followed Boeing's emergency procedures to disable MCAS by cutting electrical power. For unknown reasons, though, they later turned the system back on as many as four times after they were unable to regain control under manual power. During his remarks at the April 29 shareholder meeting, Muilenburg said that in some cases pilots did not "completely" follow the procedures that Boeing had outlined to prevent a crash in the case of an MCAS malfunction.

What's the current status of the Max 8?

Most operators quickly grounded their planes in the days following the second crash. That list includes both Ethiopian Airlines and Lion Air, but also AeroMexico, Aerolíneas Argentinas, GOL Linhas Aéreas (Brazil), Turkish Airlines, S7 Airlines (Russia), FlyDubai, Air Italy, Cayman Airways, Norwegian, China Eastern Airlines, Fiji Airways and Royal Air Maroc.

More than 40 countries have also banned the 737 Max from flying in their airspace. China (a huge Boeing customer and a fast-growing commercial aviation market) led the way and was joined by Indonesia, Thailand, Malaysia, Australia, India, Oman, the European Union and Singapore, Canada initially hesitated, but soon reversed course.

Up until March 13, the FAA also declined to join the grounding order, saying in a statement tweeted the previous day that there is "no basis to order grounding the aircraft." That was despite a public outcry from a group of senators and two flight attendant unions. But along with Trump's order, the agency said it issued the order due to new evidence it collected and analyzed (see below). The grounding will remain in effect pending further investigation.

Southwest and American have pulled their 737 Max 8s from service and have adjusted their schedules to make up for the lost aircraft. The only other holdout, Panama's Copa Airlines, has grounded its planes, as well. The order also has grounded the 737 Max 9, currently in service with United Airlines.

Older 737 models, like the 737-700, 737-800 and 737-900 don't use the flight control system under investigation and aren't affected. 

Now playing: Watch this: Boeing CEO: 737 Max soon to be one of the safest planes
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How has Boeing responded?

On Nov. 6, Boeing issued a safety warning advising 737 Max operators of the potential for a sensor failure and instructing them how to deactivate MCAS by flipping a switch. But two days later, The Seattle Times reported that Max 8 pilots were not specifically trained on using MCAS. The reason? According to The New York Times, it was because Boeing, backed by the FAA, wanted to minimize the cost and time of certifying pilots who had already been trained on other 737 versions. An investigation by The Dallas Morning News found that several Max 8 pilots had complained about the inadequate training. 

Following the second crash, the company expressed sympathy for the victims' families and said it was sending an investigation team. Then on March 12, Boeing said it was continuing to work with Max 8 customers that have grounded their aircraft.

"Safety is Boeing's No. 1 priority and we have full confidence in the safety of the 737 Max," the statement said. "The United States Federal Aviation Administration is not mandating any further action at this time, and based on the information currently available, we do not have any basis to issue new guidance to operators."

On March 13, however, in response to the US grounding order, the company said it supports the action

"There is no greater priority for our company and our industry," the statement said. "We are doing everything we can to understand the cause of the accidents in partnership with the investigators, deploy safety enhancements and help ensure this does not happen again."

As is common after a crash, Boeing has not commented on specific aspects of the investigation, but on March 11, the company said it would issue a software update that would include changes to MCAS, pilot displays, operation manuals and crew training. (For more details on those changes, see link above.)

But even that timeline is in question. The Wall Street Journal reported March 12 that an MCAS update could have come in January before the second crash. The newspaper says it was delayed, however, by the 35-day US government shutdown earlier this year.

Dennis Muilenburg, Boeing's chairman, president and CEO also published a letter April 4 expressing confidence in the fundamental safety of the 737 MAX. "All who fly on it -- the passengers, flight attendants and pilots, including our own families and friends -- deserve our best," he wrote. "When the MAX returns to the skies with the software changes to the MCAS function, it will be among the safest airplanes ever to fly."

What does this mean for the FAA?

The agency is under fire on multiple fronts over the crashes. Congress, the FBI and Secretary of Transportation Elaine Chao have called for investigations of the FAA's certification process. Under scrutiny is whether Boeing employees acted on behalf of the FAA during the certification process and whether pilots flying the 737 Max should have received additional training. The Justice Department's criminal division also is investigating the airplane, The Washington Post reported.

What has to happen before the Max can fly again?

First off, investigators need to agree on a cause for both crashes. Secondly, once Boeing deploys the relevant fixes, the FAA needs to certify them as safe and airlines need to implement them. There's no telling how long that'll take. On April 18, Muilenburg said the company was making "steady progress to certification" after 135 test flights. Then on May 16, Boeing said the update is complete and ready for evaluation by the FAA.

But that's just in the United States. Aviation regulatory agencies around the world, like the European Aviation Safety Agency, also need to approve the fix before they'll let the Max fly to the countries they oversee. Traditionally, they've followed the FAA's lead on such matters, but there's no guarantee they will. For example, Canadian Transport Minister Marc Garneau has said that he disagrees with an FAA report that Max pilots wouldn't need additional simulator training to learn the updated Max software

Even with the grounding order, Boeing is permitted to conduct test flights. Also, airlines are allowed to ferry empty Max aircraft back to their home airports and maintenance bases. 

boeing-737-max-test-flight

A Boeing 737 Max 7 lands at Boeing Field in Seattle after a test flight to evaluate the MCAS software fix.

Paul Christian Gordon/Boeing

Are airplanes now too complicated?

On March 12, Trump tweeted that airplanes are "becoming far too complex to fly." The reality isn't quite that simple. Commercial airliners have used automated systems for decades (that's what an automatic pilot is). The Lockheed L-1011, introduced in 1972, could land itself. Most airliners flying today also are "fly-by-wire," meaning that a pilot's commands are carried as electronic signals (rather than over hydraulic lines) to an aircraft's control services. Flight computers also continually stabilize an aircraft during flight without input from the flight crew. Boeing and Airbus have different philosophies for this interaction, but explaining those could take a book.

So, the basic concept of MCAS is nothing new. But crews need to be properly trained to use automated systems, recognize when they may be at fault, and override them if necessary. As the initial reports have indicated, a lack of training about MCAS may have contributed to the Max 8 crashes. Airline pilots are thoroughly trained to fly an aircraft under extraordinary circumstances, but they need accurate information about factors like airspeed and altitude to be able to make quick decisions in an emergency.

Has a commercial aircraft been grounded before?

Yes. In the most recent example, the FAA grounded the Boeing 787 for three months in 2013 after a series of nonfatal battery fires. Before that, the FAA grounded the Douglas DC-10 for a month in 1979 after a crash near Chicago O'Hare Airport killed 271 people on board, plus two on the ground. (Outside of the Sept. 11, 2001, terrorist attacks, that remains the deadliest airplane crash on US soil.) The Chicago crash was ultimately attributed to improper maintenance. The crash of a DC-10 in 1974 in France, killing 346 people, was caused by a design flaw on a cargo hold door latch.

Outside the US, both Qantas and Singapore Airlines voluntarily grounded their Airbus A380s for a couple of days after a Qantas flight from Singapore to Sydney in 2010 had an uncontained engine failure

How important is the Max series to Boeing?

Hugely important. The market for 150- to 200-seat aircraft is fiercely competitive. Airbus, Boeing's perennial archrival, sells the similarly sized A320neo and China is seeking customers for its new Comac C919. As of Feb. 28, the company has more than 5,000 firm 737 Max orders. Boeing says the 737 Max is the fastest-selling airplane in its history. But two crashes in five months is a troubling record for a plane that entered service barely two years ago and airlines will have to reassure passengers the planes are safe. 

As of Feb. 28, Boeing has delivered 376 Max aircraft to more than 50 airlines. Currently the three largest customers (in order) are Southwest Airlines, American Airlines and Air Canada. 

The news has touched Boeing's other aircraft, as well. As a result of the latest crash, Boeing has postponed the rollout of its 777X, which was scheduled for later this month.

Originally published March 13.
Updates, March 13: Adds Trump's order and Boeing's statement about it. Also deleted a question about how you know if you're booked on a 737 Max 8; March 14: Includes questions about what it'll take for the Max to fly again and on Trump's comments about automated aircraft; March 19: Adds questions about whether the crashes are similar and about the FAA facing new scrutiny; March 22: Includes information about Garuda Indonesia canceling its order for the 737 Max, adds info on the warning light, includes information on earlier release date for software fix; April 4: Adds a question about other findings from the Ethiopian crash. May 7: Adds information from Boeing's annual shareholders meeting and a question about whether pilots would have known about the faulty sensors. May 17: Adds Boeing statement on completion of software fix.