Following two deadly crashes, Boeing is confronting momentous problems with one of its newest and most critical aircraft models, the 737 Max 8.

For now, the aircraft continues to operate in the United States, but most airlines operating the 737 Max 8 have grounded their fleets and a swath of countries have banned it from flying completely. Two crashes in five months is a troubling record for airliner that entered service barely two years ago and the developments are a huge blow to Boeing, which has thousands of 737 Max orders on its books.

The causes of both crashes, which at this point appear to be similar, are still under investigation and the official reports won't be published for months. Until then, here's what we know:

On March 10, Ethiopian Airlines flight 302 departed Addis Ababa Bole International Airport bound for Nairobi, Kenya. Immediately 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 4 months old. Africa's largest airline, Ethiopian serves cities worldwide (including the United States) and is a member of Star Alliance, which includes United Airlines, Lufthansa and Air China. And despite Sunday's 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.

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.

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

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 138 and 158 passengers, depending on the configuration, it's popular on shorter routes, but also has the range (3,850 nautical miles) to fly transatlantic and between the mainland United States 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.

Though its design represents the evolution of an existing aircraft, the Max 8 is essentially a new bird. It has bigger, more powerful and more efficient CFM LEAP engines (more on those in a minute), improved aerodynamics and a redesigned cabin that feels spacious for a narrow-body plane. 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 has yet to enter passenger service. (It flew for the first time last May.) A few airlines have ordered the smaller 737 Max 7, but Boeing has yet to complete any deliveries.

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

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

Hugely important. The market for 150- to 200-seat aircraft is fiercely competitive, with Airbus, Boeing's perennial archrival, selling the similarly sized A320neo. As of Feb. 28, the company has more than 5,000 firm 737 Max orders.

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.

We don't know the official reason, and we won't know for a long time. 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 recorder (the "black box") and, if possible, check the victims' bodies to determine the cause of death. Though an investigation team arrived in Jakarta hours after the Lion Air crash, a full report isn't due until August or September. We'll be on a similar schedule for Ethiopian 302.

Crash investigations also involve multiple groups, including the airline, the aircraft and engine manufacturers and government agencies like the US National Transportation Safety Board that regulate airline safety. Coordinating their efforts takes time.

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 airport with limited ground equipment), Boeing moved the engines slightly forward and raised them higher on their underwing pylons. (If you place an engine too close to ground, it can suck in debris while the plane is taxiing.)

That new position changed the aircraft's center of gravity, creating the potential for the nose to pitch up during flight. Boeing designed software called the Maneuvering Characteristics Augmentation System, or MCAS, to overcome this action. When a sensor on the fuselage detects that the nose is too high, which could eventually cause the plane to stall, MCAS automatically pushes the nose down.

Investigators in the Lion Air crash have said that a fault in the the sensor may have been feeding incorrect data to MCAS, pitching the nose down into a dive. According to the investigation, the Lion Air pilots struggled to take control of the plane before the crash as it pitched up and down. (For a thorough explanation of MCAS, see this story from The Air Current.)

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.

On March 10, the company expressed sympathy for the victims' families and said it was sending an investigation team. Then on March 12, Boeing said it's continuing to work with Max 8 customers that have grounded their aircraft.

"Safety is Boeing's number one 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."

As is common after a crash, Boeing has not commented on specific aspects of the investigation, but on March 11 it said it would issue a software update by the end of April that would include changes to MCAS, pilot displays, operation manuals and crew training. When Boeing issues the update, the company says the FAA will issue an airworthiness directive ordering 737 Max operators to make the change.

The Wall Street Journal reported on March 12 that the software might have come earlier, but it was delayed by the 35-day US government shutdown earlier this year.

Most 737 Max operators have temporarily grounded their planes. The list includes not just 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.

Additionally, several countries have banned from the 737 Max from flying in their airspace. That list includes Canada, China (a huge Boeing customer and a fast-growing commercial aviation market), Indonesia, Thailand, Malaysia. Australia, India, Oman, the European Union and Singapore.  

Yes. In the most recent example, the FAA grounded the Boeing 787 for three months 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.) Though the Chicago crash was ultimately attributed to improper maintenance, a 1974 DC-10 that crashed in France, killing 346 people, was caused by a design flaw on a cargo hold door latch.

Outside the United States, 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.

Southwest, American Airlines and Panama's Copa Airlines have not announced plans to ground their fleets. Likewise, United Airlines continues to fly its Max 9 aircraft, which also has the MCAS system. 

Reaction from flight crews at those airlines has been mixed. Though the Southwest Airlines Pilots Association has expressed confidence in the aircraft, the Association of Professional Flight Attendants and the Association of Flight Attendants-CWA want it grounded.   

Despite a public outcry -- including from a group of senators -- for the FAA to ground the aircraft, the agency has declined to do so. In a statement tweeted March 12, it said there is "no basis to order grounding the aircraft." 

Your aircraft type will be indicated on your itinerary. Typically it's listed under "flight details," which may require an extra click. Some airlines will spell out the full aircraft name, while other carriers may shorten it to "7M8." You also can find it by looking at your flight's seat map. The Points Guy has a deeper explanation by airline, including if you'll be charged a fee for changing your flight. Unfortunately, that's likely to happen. 

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