>>1296914MCAS was created in response to the fact that the 737 is a 1960s designed regional jet which originally had 2 JT8D engines mounted under the engine.
They had to seriously modify the CFM-56 for the Classic and Next Generation models by placing the gearbox in the 3 o'clock position instead of the 6 o'clock position it was in on the A320 and for the MAX, the CFM LEAP was so large, they couldn't even fit the engines under the wing, even after raising the landing gear, so instead, they placed the engines in front of the wing while still using the 3 o'clock position for the gearbox while the A320neo keeps the gearbox in the 6 o'clock position for the exact same CFM LEAP engine.
This engine position, in front of the wings, means that the aircraft will naturally nose up without any inputs being made. Boeing then developed the Manouveuring Characteristics Augmentation System (or MCAS) to automatically trim nose down while the aircraft is in Flaps 0, even in level flight. This was supposed to prevent the aircraft from accidentally stalling thanks to the engine positions.
On the Lion Air flight, the MCAS proceeded to set the trim nose down for some reason. The pilots responded by using the trim switches on the yokes, but the MCAS would undo this. At some point, the pilots proceeded to carry out the Runaway Stabalizer proceedure, it it was too late by this stage, as the aircraft was already in a nose dive, and the artificial feel system on the Boeing 737 MAX made it physically harder to pitch up on the yokes.
On the Airbus, Autotrim is the default unless the aircraft enters Direct Law. There have been no incidents of any erroneous nose down inputs causing Airbus crashes, although there have been 2 recorded incidents of A330s doing nose dives in cruise flight suddenly, however reverting to Alternate Law following Overspeed warnings. Airbus Flight Augmentation Computers have more failsafe states.
