The crash of Ethiopian Airlines Flight 302 on Sunday — five months after the Lion Air crash of the same type of plane in Indonesia — has raised many questions about the Boeing 737 Max 8 aircraft. There are currently two types of Boeing 737 planes in use around the world that sound similar but are in fact very different aircraft with distinctive technology. The Boeing 737 Max 8, the model involved in the two deadly crashes, is the newer successor to the Boeing 737 Next Generation line of aircraft.

Boeing 737 Next Generation (737NG) is the overarching name given to a fleet of aircraft that began production in 1991. It includes the Boeing 737-600, Boeing 737-700, Boeing 737-800 and Boeing 737-900 — variations with different seating capacity. The 737NG featured improved fuel capacity, a 25 percent larger wing capacity, and an ability to fly 900 more nautical miles than earlier 737 models. Southwest Airlines was the world's first airline to take delivery in 1997, and by April 2012, 4,000 Boeing 737NGs had been delivered around the world.

Two decades after the debut of the 737NG, Boeing announced plans for a new model: the Boeing 737 Max 8.

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There are over 350 Boeing 737 Max 8 airplanes in fleets worldwide, although a growing number of airlines and nations decided to ground them in the wake of the second crash. President Trump said Wednesday that the FAA is grounding them in the U.S. as well.

Among U.S. carriers, least 69 Boeing 737 Max 8 and similar but slightly larger Max 9 aircraft were in use by Southwest Airlines, American Airlines and United Airlines.

A shot of Boeing's 737 Max 8 airplanes from the company's website. Eric Greer/Boeing

What's different about the Boeing 737 Max 8?

When it announced plans for the 737 Max 8 in 2011, Boeing said the new aircraft would be "the most fuel efficient, most capable airplane with the lowest operating costs in the single-aisle market." The Boeing 737 Max 8 flew its first flight in 2017.

One important distinction from previous 737s is the Boeing 737 Max 8 has a different software system. That software is now a focus of investigators.

The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after takeoff. To counteract this risk, Boeing developed software known as the Maneuvering Characteristics Augmentation System, or MCAS.

Max 8s come equipped with a sensor that reads the plane's angle relative to the wind flow, prompting MCAS to automatically trigger the plane's nose to angle downward if it gets a specific reading.

However, problems could arise if the MCAS system gets erroneous sensor readings. The system automatically pushes the plane's nose down, potentially surprising pilots who are unfamiliar with the system and overriding their commands. This is what investigators believe happened to Lion Air Flight 610 before it crashed in October, and investigators are are also looking into whether it played a role with Ethiopian Airlines Flight 302. (The FAA stresses the investigation into the Ethiopian Airlines crash has just begun and it is too early to draw any conclusions about the cause.)

According to The New York Times, when Boeing first rolled out the 737 Max 8, the Federal Aviation Administration determined there were not enough differences between the new model and the prior iteration of 737 to require pilots to go through simulator training — a decision that saved the airlines time and money, and made Boeing's new plane more competitive. That means, however, pilots remained unfamiliar with the MCAS system, even though it plays a key role in controlling the plane under certain circumstances. And even though the system could be influenced by a faulty sensor reading, The New York Times reports "there is no evidence that Boeing did flight-testing of MCAS with erroneous sensor data, and it is not clear whether the FAA did so."

Furthermore, older 737s are equipped with technology that allows pilots to manually control the plane by simply pulling back on the control column. Yet, per The Times' report from February, that feature was disabled on the Max 8 when MCAS is activated — another change pilots were unlikely to have been aware of.

Both Boeing and the FAA have released statements vouching for the safety of the 737 Max 8 planes. Boeing said Tuesday, "We have full confidence in the safety of the MAX." The FAA said Monday it "continuously assesses and oversees the safety performance of U.S. commercial aircraft. If we identify an issue that affects safety, the FAA will take immediate and appropriate action."

It took that action Wednesday, grounding the planes effective immediately in the U.S. after more than a dozen other countries had done the same.

Kris Van Cleave contributed reporting.