The problem with the Max is aerodynamics: Civil airplanes are designed with negative aerodynamic feedback, so the nose-down torque (“pitching moment”) increases as the AoA increases. But on the Max at very high AoA this feedback torque becomes somewhat smaller due to interaction of airflow around the engines and the wing. That by itself is not a problem because the nose-down torque still exists and airframe is still stable.
There is a regulatory requirement that the amount of pilot’s control force required to maintain a given AoA must be a non-decreasing function of AoA[1]. Due to the Max’s aerodynamics, this requirement is not met.
Boeing’s MCAS was a bandaid to make the plane meet regulations by applying nose-down trim while at high AoA. The trim results in a higher yoke force, so the plane meets the requirement. A better method would have been a “stick pusher” (which have been used as stall prevention devices for over 50 years, though not on the 737) or addition ventral fins (like on the Beech 1900). But either of those would have probably required recertification
[1] The purpose of this is to reduce pilot-induced oscillations: see what happened with AA flight 587, where the lack of a similar requirement for rudder pedals and yaw led to PIO which ultimately resulted in the tail falling off an Airbus.
The problem with the Max is aerodynamics: Civil airplanes are designed with negative aerodynamic feedback, so the nose-down torque (“pitching moment”) increases as the AoA increases. But on the Max at very high AoA this feedback torque becomes somewhat smaller due to interaction of airflow around the engines and the wing. That by itself is not a problem because the nose-down torque still exists and airframe is still stable.
There is a regulatory requirement that the amount of pilot’s control force required to maintain a given AoA must be a non-decreasing function of AoA[1]. Due to the Max’s aerodynamics, this requirement is not met.
Boeing’s MCAS was a bandaid to make the plane meet regulations by applying nose-down trim while at high AoA. The trim results in a higher yoke force, so the plane meets the requirement. A better method would have been a “stick pusher” (which have been used as stall prevention devices for over 50 years, though not on the 737) or addition ventral fins (like on the Beech 1900). But either of those would have probably required recertification
[1] The purpose of this is to reduce pilot-induced oscillations: see what happened with AA flight 587, where the lack of a similar requirement for rudder pedals and yaw led to PIO which ultimately resulted in the tail falling off an Airbus.