I read this in the DallasNews:
U.S. regulators are mandating that Boeing upgrade the plane’s software by April but have so far declined to ground the planes. China, Australia and the European Union have grounded the 737 Max 8, leaving the U.S. and Canada as the only two countries flying a substantial number of the aircraft.
The Maneuvering Characteristics Augmentation System (MCAS) was included on the Max 8 model aircraft as a safety mechanism that would automatically correct a plane entering a stall pattern. If the plane loses lift under its wings during takeoff and the nose begins to point far upward, the system kicks in and automatically pushes the nose of the plane down.
After the Lion Air crash, the FAA issued an airworthiness directive that said: “This condition, if not addressed, could cause the flight crew to have difficulty controlling the airplane, and lead to excessive nose-down attitude, significant altitude loss, and possible impact with terrain.”
I read a report from Boeing that was similar, but...considering the high angle of attack of the aircraft at rotation and the subsequent lowering of the nose to gain speed for a better rate of climb, it seems the MCAS system has inerrant issues from the start.
I also read the system inputs elevator trim in gradual increments based on attitude and speed and can be overridden by the pilots with either input of manual or electric trim inputs.
Unless the controls are being pushed forward by the crew, it doesn’t sound like the MCAS system can force the aircraft into a nose-over attitude by itself. IIRC, it takes almost 10 seconds for the MCAS system to go to it’s full deflection.