Recently there was a fascinating piece by Bjorn Fehrm on LEEHAM News and Comment explaining why Boeing had installed an automatic trim system (MCAS) on the 737 MAX to intervene aggressively should the aircraft be in danger of stalling even with the autopilot disconnected.
The reason for this was that though the 737 as originally designed was naturally stable it sat very low on the ground to facilitate loading at airports with then limited facilities. The bottom of the engine nacelles had to flattened to allow this. However, in order to install the much more efficient larger diameter engines for the 737 MAX, they had to be moved forward. As explained by Fehrm, this was fine at normal angles of attack, but should the angle of attack become too steep the nacelles themselves would produce lift far forward of the centre of gravity, which might result in a disastrous stall with the nose being pushed up further and further. To preclude this, Boeing installed MCAS, the Maneuvering Characteristics Automation System to force the nose down.
Unfortunately, in the Lion Air crash, the pilots did not know that unlike in the previous version of the 737 the forced down trim could not be removed by pulling back on the control yoke–the STAB TRIM CUTOUT switches have to be set to CUTOUT, which is what the pilots did when the same thing happened on an earlier flight of that aircraft.
Interestingly, in another post we cited the case of the XL Airways/Air New Zealand acceptance test flight, that crashed because the pilot could not overcome the upward maximum pitch trim and upward leverage of the low-slung engines causing the aircraft to stall will insufficient height to recover. In that case, two-out-of-three of the angle of attack sensors had frozen at the same angle and the computer dismissed the odd man out. In the Lion Air crash it seems the MCAS was triggered by just one angle of attack sensor showing too steep an angle, which was quite reasonable because the high pitch indicated by one might have been genuine and it was a precautionary measure, and better safe than sorry. If the pilot had been aware of what he should do, or thought of it like the pilot on a previous flight on that aircraft, there was no reason for it to crash.
It is a pity Air France Flight AF447 that crashed into the South Atlantic did not have something like MCAS both stopping the pilot stalling the Airbus A330, and preventing him from impeding recovery by pulling back continually on his sidestick.