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 the Federal Aviation Administration investigating its own certification process.

The developments are a huge blow to Boeing, which has thousands of 737 Max orders on its books. Indonesia’s aviation safety agency has published a report on the first crash, which blames a faulty sensor, problems with a flight control system and errors by Lion Air staff. The investigation into the second crash in Ethiopia is still underway, but teams are focusing on similar causes.

Boeing says it has completed the necessary changes to repair the plane and has submitted them for review by the FAA. But as of now, the agency has not said when that review will happen or when the plane could carry passengers again. And after many calls for his removal, Boeing CEO Dennis Muilenburg was fired by the company’s board of directors on Dec. 23. Until the Max is back in the air, here’s everything else we know.

What happened in the two crashes?

In the first crash, on Oct. 29, 2018, Lion Air flight 610 dove into the Java Sea 13 minutes after takeoff from Jakarta, Indonesia, killing 189 people. The flight crew made a distress call shortly before losing control. That aircraft was almost brand-new, having arrived at Lion Air three months earlier.

The second crash occurred on March 10, when 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, six minutes after it left the runway. Aboard were 149 passengers and eight crew members. The aircraft involved was only four months old.

What is the Boeing 737 Max?

The 737 Max is a family of commercial aircraft that consists of four models. The Max 8, which is the most popular version, made its first flight on Jan. 29, 2016, and entered passenger service with Malaysia’s (now defunct) Malindo Air on May 22, 2017. Seating between 162 and 210 passengers, depending on the configuration, it’s designed for short- and medium-haul 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 larger Max 9 first flew in 2017, and the 737-10 has yet to fly (it made its formal debut Nov. 22). A few airlines have ordered the smaller 737 Max 7, but Boeing has yet to complete any deliveries. (It flew for the first time in May 2018.)

The design of the 737 Max series 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 even occasionally, you’ve most likely flown on a 737.

What’s different about the 737 Max series compared with earlier 737s?

The 737 Max can fly farther and carry more people than the previous generation of 737s, like the 737-800 and 737-900. It’s also has improved aerodynamics and a redesigned cabin interior and flies on bigger, more powerful and more efficient CFM LEAP engines.

Those engines, though, required Boeing to make critical design changes. 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 of the engines 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. (For background on MCAS, read these in-depth stories from The Air Current and The Seattle Times.)

What caused the crashes?

On Oct. 25, the Indonesian National Transportation Safety Committee (KNKT) published its final report on the Lion Air crash. The report identifies nine factors that contributed to the crash, but largely blames MCAS. Before crashing, the Lion Air pilots were unable to determine their true airspeed and altitude and they struggled to take control of the plane as it oscillated for about 10 minutes. Each time they pulled up from a dive, MCAS pushed the nose down again.

“The MCAS function was not a fail-safe design and did not include redundancy,” the report said. Investigators also found that MCAS relied on only one sensor, which had a fault, and flight crews hadn’t been adequately trained to use it. Improper maintenance procedures, confusion in the cockpit and the lack of a cockpit warning light (more on that in a minute) contributed to the crash, as well.

Ethiopian investigators haven’t completed their report, but so far they’re also focusing on MCAS. 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).

And that’s not all. As 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, then-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 a software update will fix.

The actions of the Ethiopian flight crew are under investigation, as well. According to a 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 didn’t “completely” follow the procedures that Boeing had outlined to prevent a crash in the case of an MCAS malfunction.

While we wait for the final report, remember that 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.

What kind of MCAS training did 737 Max pilots receive?

Not much, which was a factor in the Indonesian crash. As the report said, “The absence of guidance on MCAS or more detailed use of trim in the flight manuals and in flight crew training, made it more difficult for flight crews to properly respond.”

Though MCAS was new to the Max, existing 737 pilots didn’t have to train on a simulator before they could start flying the plane. Instead, they learned about the differences the Max brought through an hour’s worth of iPad-based training. MCAS received scant mention. The reason? It was because Boeing, backed by the FAA, wanted to minimize the cost and time of certifying pilots who’d already been trained on other 737 versions.

Pilot complaints about the lack of training emerged quickly after the Lion Air crash. On Nov 12, 2018, The Seattle Times reported that Max pilots from Southwest Airlines were “kept in the dark” about MCAS. The Dallas Morning News found similar complaints from American Airlines pilots four months later.

What about the warning light?

The Air Current reported March 12 that the Lion Air plane lacked a warning light designed to alert pilots to the faulty sensor 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.”

But on April 29, The Wall Street Journal reported that even for airlines that had ordered it, the warning light wasn’t operating on some Max planes that had been delivered (a fact the Indonesian accident report confirmed.). Then on June 7, Reps. Peter DeFazio, a Democrat from Oregon, and Rick Larsen, a Democrat from Washington, said they’d obtained information suggesting that even though the plane maker knew the safety alert wasn’t working, it decided to wait until 2020 to implement a fix.

Boeing responded to DeFazio and Larsen in a statement sent to CNET the same day.

“The absence of the AOA Disagree alert did not adversely impact airplane safety or operation,” the statement read. “Based on the safety review, the update was scheduled for the MAX 10 rollout in 2020. We fell short in the implementation of the AoA Disagree alert and are taking steps to address these issues so they do not occur again.”

Have any other reports been issued?

On Oct. 11, an international flight safety panel issued a Joint Authorities Technical Review that faulted both the FAA and Boeing on several fronts. For the FAA, it said the agency needs to modernize its aircraft certification process to account for increasingly complex automated systems by ensuring that aircraft incorporate fail-safe design principles that don’t rely too heavily on pilot input.

For Boeing’s part, the report cited the company’s “inadequate communications” to the FAA about the MCAS, inadequate pilot training and shortage of technical staff. The review was conducted by representatives from NASA, the FAA and civil aviation authorities from Australia, Canada, China, Europe, Singapore, Japan, Brazil, Indonesia and the United Arab Emirates.

What’s the current status of the Max?

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, Aerolineas Argentinas, GOL Linhas Aereas (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 issue a grounding order, saying in a statement tweeted the previous day that there was “no basis to order grounding the aircraft.” That was despite a public outcry from a group of senators and two flight attendant unions. But following President Trump’s decision to ground the Max that day, the agency cited new evidence it had collected and analyzed. Southwest and American quickly grounded their planes. Trump also 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.

How has Boeing responded?

Boeing has been fully involved with both investigations since early on. On Nov. 6, 2018, just eight days after the first crash, the company issued a safety warning advising 737 Max operators to deactivate MCAS if a flight crew encountered conditions like the Lion Air pilots experienced. It has expressed sympathy for victims’ families and pledged $100 million in support, and it quickly backed the US grounding order.

“There is no greater priority for our company and our industry,” Boeing said in a March 13 statement. “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 didn’t comment on preliminary findings of either 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.

Muilenburg 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.”

Following the Lion Air report, Muilenburg said the company is “addressing” its safety recommendations. “We commend Indonesia’s KNKT for its extensive efforts to determine the facts of this accident, the contributing factors to its cause and recommendations aimed toward our common goal that this never happens again,” he said.

Did Boeing know about Max problems before the crashes?

There is some evidence that it did. On Oct. 17, Boeing revealed it had turned over text messages between two of the company’s top pilots sent in 2016, which indicated the company knew about problems with the MCAS system early on. In one of the messages, a former chief technical pilot for the Boeing 737 described the MCAS’ habit of engaging itself as “egregious.”

Then during the week of Oct. 28 as he appeared before two Congressional committees Muilenburg admitted Boeing knew of the test pilot concerns in early 2019. “I was involved in the document collection process, but I relied on my team to get the documents to the appropriate authorities,” he said. “I didn’t get the details of the conversation until recently.”

Has Boeing made changes to its leadership?

Yes, but they didn’t come quickly. Though Muilenburg apologized to the victims’ families in an interview with CBS News (CNET News is published by CBS Interactive, a unit of CBS) in May, he came under sharp criticism for his response to the crashes. On Oct. 11, Boeing announced it had taken away his role as chair so that as CEO, Muilenburg could “focus full time on running the company as it works to return the 737 Max safely to service.” Muilenburg spent the next two months resisting calls for his resignation from his other position, but on Dec. 23 the company announced that he had stepped down.

“The Board of Directors decided a change in leadership was necessary to restore confidence in the company moving forward as it works to repair relationships with regulators, customers, and all other stakeholders,” Boeing said in a statement. Muilenburg will be replaced by current Chairman David Calhoun, a change that will go into effect Jan. 13, 2020. In the meantime, Boeing Chief Financial Officer Greg Smith will serve as interim CEO.

Also, on Oct. 22, the company said it replaced Boeing Commercial Airplanes CEO Kevin McAllister, the official overseeing the 737 Max investigation, with Stan Deal, former president and CEO of Boeing Global Services.

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?

Aviation safety agencies around the world need to certify Boeing’s fixes as safe and airlines need to implement them. Even with the grounding order, Boeing is permitted to conduct test flights.

On April 18, Muilenburg said the company was making “steady progress to certification” after 135 test flights. On May 16, Boeing said the update is complete and ready for evaluation by the FAA. Five months later, on Oct. 22, the company said it had made “significant progress” toward that goal by adding flight control computer redundancy to MCAS and three additional layers of protection. It’s also completed a dry run of a certification flight test and conducted simulator tests for 445 participants from more than 140 customers and regulators. Boeing provided a further progress report Nov. 11.

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 but on Sept. 5, the EU said it will conduct independent tests of the plane before it can resume commercial flights to and within Europe. Previously the FAA and regulatory officials from Canada, Europe and Brazil agreed in principle to work together to review Boeing’s changes to the Max and restore the aircraft to service.

There’s no word, though, on when that might happen. Most airlines have canceled Max 737 Max flights until next April at the earliest.

Will pilots have to train in a simulator before they can fly the Max again?

After Boeing announced it was ready to deploy the MCAS software update, the FAA initially said that simulator training still wouldn’t be required. But some pilots and regulatory officials from other countries, like Canadian Transport Minister Marc Garneau, have disagreed with that decision. They won an influential supporter on June 19 when “Miracle on the Hudson” Capt. Chesley B. “Sully” Sullenberger argued before a Congressional committee that simulator training should be required before pilots take the Max back into the air. He also said the original design of MCAS was “fatally flawed and should never have been approved.”

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 accident 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 Nov. 30, Boeing had almost 4,932 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 Nov. 30, Boeing had delivered 387 Max aircraft to more than 50 airlines. Currently, the three largest customers (in order) are Southwest Airlines, American Airlines and Air Canada. Following the second crash, airlines stopped ordering the aircraft and some airlines have canceled Max orders. But on June 18 at the Paris Air Show, International Airlines Group said it would consider buying 200 737 Max 8s and 10s. And at the Dubai Air Show in October, Boeing reported 737 Max orders from Air Astana and SunExpress.

But on Dec. 16, Boeing said it was suspending production starting in January until the certification process takes place. The news has touched Boeing’s other aircraft as well. As a result of the latest crash, Boeing postponed the rollout of its newest aircraft, the 777X. Instead of the usual media event, Boeing instead held a low-key introduction for the 777X, attended only by its employees.

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; June 10: Expands question on whether pilots would have known about the fault; July 2: Adds question on simulator training; July 24: Adds second quarter earnings call; Oct. 13: Adds information about the Joint Authorities Technical Review; Oct. 19: Adds information about 2016 text messages between Boeing pilots; Oct. 23: Adds information about the latest fixes to MCAS and Kevin McAllister’s departure as Boeing Commercial Airplanes CEO. Nov. 1: Adds information about the final Lion Air accident report. Dec. 16: Adds information about Boeing suspending Max production. Dec. 23: Adds information about Muilenburg’s firing.