This is a Boeing 737 in normal flight, pointed up at a so-called angle of attack of a few degrees. Planes are meant to fly within a certain range, effectively a safe zone for flight.

Boeing’s new 737 Max has larger engines placed higher on the plane, creating aerodynamics that can push the nose up in some conditions. If a plane’s angle of attack becomes too steep, it could stall and crash.

Boeing developed an automated system called MCAS that uses stabilizers on the tail to push the nose back down and help the plane to avoid a stall.

MCAS relied on only one of two sensors that measured the plane’s angle of attack. “That’s not a good engineering system,” said Bjorn Fehrm, an aeronautical engineer. “That’s where they screwed up royally.”

An angle of attack sensor is believed to have provided faulty data in the deadly Ethiopian Airlines and Lion Air crashes. On the Lion Air flight, a sensor showed that the plane was pointed up at least 20 degrees higher than the other.

The automated system responded by pushing the plane’s nose down to what it thought was a safer angle, investigators believe. But it actually pushed the plane to a potentially dangerous angle.

The Lion Air pilots were left to desperately read through a technical manual, struggling to troubleshoot malfunctioning software mid-flight.

Pilots could use stabilizer controls at their thumbs to temporarily counteract MCAS. But if the system still detected a dangerous angle, it would re-engage, pushing the nose down again. On the Lion Air flight, this created a tug-of-war.