[mgsouth]

Some sort of corrective system is required to adjust the raw aerodynamics of the 737 MAX. It some conditions it has characteristics which are not allowed for commercial planes in the US. [0] This is not, in itself, a problem. Many (most? all?) modern airliners have handling corner cases. A common way to fix them is to have sensors, computers, and actuators which adjust the aerodynamics or "push the controls". Boeing chose to use MCAS to adjust the handling in the undesired conditions. Unfortunately, they tried to extend existing sensors and software which were not sufficient for the job. They also, critically, tried to make it transparent to the pilots; this was to avoid extra training, and to avoid creating making the MAX a separate type with all the certification and logistics issues that involves.

The proposed fix is to change the software to admit to itself that it can fail, to limit the amount of control that MCAS can exert, and to train pilots to recognize failure modes and how, in the event of MCAS failure, manually handle the undesireable aerodynamics. Boeing can't simply disable MCAS and say that pilots have to always manually handle it; the rules specifically say the plane cannot behave in that manner.

[0] In some uncommon-but-not-impossible situations, pulling back on the control yoke and putting the plane closer to a stall is easier than pushing on the yoke and getting further from stalling. That isn't allowed for commercial aircraft.

https://www.nytimes.com/2019/06/01/business/boeing-737-max-c...

https://www.ecfr.gov/cgi-bin/text-idx?node=14:1.0.1.3.11#se1...

MCAS fixes this by detecting the condition and, in effect, pushing forward on a secondary control.