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A United Airlines Boeing 737 Max 9 aircraft lands at San Francisco International Airport on March 13. Photo: Justin Sullivan/Getty Images

With the global fleet of the brand new Boeing 737 MAX jet grounded indefinitely after two deadly crashes, we're learning more about what may have brought us to this point.

Why it matters: No plane crash is ever the result of one factor alone, but rather a series of events that conspire to cause a disaster.

  • In the case of the Lion Air crash in October and the Ethiopian Airlines crash last week, the information coming from the official crash investigations is focused squarely on a system built into the aircraft to prevent it from stalling, or running out of lift.
  • Another emerging thread from the investigation is the way that the plane — which has a total of more than 5,000 standing orders from carriers worldwide — was certified to fly in the first place.

How it works: Media outlets have been shining a spotlight on how the plane came to be approved as if it was simply another version of the best-selling 737 series, despite having so many new pieces of hardware. That includes new, more powerful engines, a redesigned tail, new wing design and more, along with complex, novel software.

  • The MAX’s larger, more powerful engines are strapped farther forward on its wings compared to other 737 derivatives, which changed the aerodynamic characteristics of the plane and could push the nose upward in certain situations.
  • Boeing installed software, known as the Maneuvering Characteristics Augmentation System, or MCAS, to push the nose down and avoid a high speed stall. But this system gets its input from just a single angle of attack sensor mounted near the nose of the aircraft.
  • It's this system that is suspected of causing the two crashes, potentially due to faulty data from the angle of attack sensor.
  • Had the MCAS compared data from both of the angle of attack sensors on the plane, it could have ignored erroneous data from one of them.

Context: Despite the aerodynamic differences and new software system, many 737 pilots were not required to undergo intensive training in flight simulators, and they could instead do so on an iPad, according to the New York Times. The simulators also weren't ready when the plane began rolling out of the factory and taking to the sky.

  • The lack of simulator training was a selling point for the jet, since it meant airlines that already flew other versions of the 737 could save money on training.
  • The NYT reported that pilots at United Airlines, which flies the Boeing 737 MAX 9, were not initially trained on the MCAS system.

Meanwhile: Another strand emerging from the accident investigations concerns how deferential the FAA was to Boeing in certifying the MCAS system and other aspects of the plane's certification.

  • The Seattle Times reports that deadline pressure prodded the FAA and Boeing to delegate more safety decisions to Boeing engineers working on behalf of the FAA. That's because across the Atlantic, Boeing's arch-rival company, Airbus, was developing a similar plane, known as the Airbus A320neo.
  • The Times story also shows that the original safety assessment for the MCAS system may have misstated its full capability in moving the tail of the plane, as well as how many times it could kick in before resetting.
  • A federal grand jury issued a subpoena on March 11 to at least one person involved in the development of Boeing's 737 MAX aircraft on behalf of the Justice Department, though it remains unclear if the action is related to government scrutiny of the FAA's approval of the plane's MCAS anti-stall safety system, the Wall Street Journal reports.

What's next: Boeing has said it plans to roll out a fix to the MCAS software in April that should prevent a similar crash in the future. But it's likely that pilots will be briefed on it via additional personal computer training, since there is just one 737 MAX simulator available for training use in the U.S.

CEO Dennis Muilenburg issued a new statement on Sunday:

"While investigators continue to work to establish definitive conclusions, Boeing is finalizing its development of a previously-announced software update and pilot training revision that will address the MCAS flight control law's behavior in response to erroneous sensor inputs."

Editor's note: This story has been updated to include the WSJ report of a federal criminal investigation.

Go deeper:

Go deeper

Updated 25 mins ago - Politics & Policy

Omicron dashboard

Illustration: Shoshana Gordon/Axios

  1. Health: CDC director says number of U.S. Omicron cases "likely to rise" — Two years of COVID-19 — Prior coronavirus infections may not protect well against Omicron.
  2. Vaccines: Data demonstrates most-vaccinated counties less vulnerable to worst of COVID — Omicron adds urgency to vaccinating world — Omicron fuels the case for COVID boosters.
  3. Politics: Nevada to impose insurance surcharge on unvaccinated state workers — New Jersey GOP lawmakers defy statehouse COVID policy — Oklahoma sues Biden administration over Pentagon vaccine mandate.
  4. World: Vaccine mandates lose steam in the U.S. while Europe doubles downWHO: Delta health measures help fight Omicron — COVID cases surge in South Africa in sign Omicron wave is coming.
  5. Variant tracker: Where different strains are spreading.

Vulnerable Democrats: Less Trump talk

Illustration: Aïda Amer/Axios

Vulnerable House Democrats are convinced they need to talk less about the man who helped them get elected: President Trump.

Why it matters: Democrats are privately concerned nationalizing the 2022 mid-terms with emotionally-charged issues — from Critical Race Theory to Donald Trump's role in the Jan. 6 insurrection — will hamstring their ability to sell the local benefits of President Biden's Build Back Better agenda.

Updated 2 hours ago - Politics & Policy

Bipartisan tributes flood in for "giant of the Senate" Bob Dole

Then-Vice President Joe Biden and former Sen. Bob Dole at an event put on by the World Food Program where he was awarded the first “McGovern-Dole Leadership Award” in December 2013. Photo: Douglas Graham/CQ Roll Call

Republican and Democratic politicians, including former Senate colleagues, are sharing condolences and memories commemorating the life of Bob Dole, who passed away at 98 on Sunday morning.

The big picture: Dole, the Republican presidential nominee in 1996, was the longest serving Republican leader in the Senate until 2018, when current Senate Minority Leader Mitch McConnell surpassed his record.