For Boeing, the 737 MAX was supposed to be a promising new frontier in its impressive domination of the mid-size airliner market. Better efficiency, updated cockpit displays, and a fairly minor change to the way pilots needed to land the plane.
Pilots who knew how to fly the 737 wouldn't need a whole lot of new training.
No major changes.
Or so pilots thought.
It turns out there was another update on the 737 MAX: something called the maneuvering characteristics augmentation system, or MCAS.
It's invisible to pilots, but it's meant to automatically adjust the pilot's inputs for smoother, safer flying.
But just like any other system, it can malfunction.
The MCAS is supposed to help avoid a stall, which is what happens when the plane is going so slow or is pitched up so high that airflow over the wings all but stops. The plane loses lift and starts to fall.
Pilots know how to recover from a stall — they train for it all the time.
But the MCAS can control the plane's "horizontal stabilizer" — what looks like little wings on the tail of the plane — which controls pitch. The MCAS can make minor adjustments to the horizontal stabilizer, letting it move in the opposite direction the pilot is trying to move it in.
The 737 MAX has sensors that read how high the plane is pitched. If the MCAS sees one that says the pitch is too high, it forces the nose of the plane down.
But what if that sensor were wrong?
The MCAS would force the nose of the plane down anyway — leading to a nosedive.
Now, common sense might tell you that in that scenario, pilots should disable the MCAS and pull the nose back up. They can do that by using manual adjustments.
But until November 2018, they couldn't be expected to know the MCAS was to blame because they didn't know the MCAS existed.
After a Lion Air 737 MAX crashed in Indonesia, the FAA sent out an alert to airlines that fly the MAX and their pilots. That alert effectively informed them of the MCAS' existence.
In the wake of the Lion Air crash, pilots called for more transparency from Boeing about the systems on the plane and how pilots should be trained on them.
Pilots did receive training, and a spokesperson for American Airlines' pilots union told Newsy that once that training happened, the plane was safe.
But then an Ethiopian Airlines MAX 8 with 157 people on board crashed minutes after takeoff from Addis Ababa.
To be clear, there's nothing public that suggests the MCAS had anything to do with the crash in Ethiopia. But two brand new planes of the same type crashing in such a short period of time raises questions about the plane and its systems.
The FAA, Boeing and the U.S. airlines that fly the MAX have insisted the planes are safe.