There will always be plane crashes due to human error. Usually there will be a chain of irregular events or circumstances, sometimes the humans just malfunction and crash a perfectly good aircraft.
In the attempt to provide protection from one threat, another can be inadvertently created. A good example is Germanwings 9525. To prevent cockpit intrusions, the cockpit doors have been secured after 9/11, but that allowed the suicidal co-pilot to lock the captain out and crash the plane into the Alps. Unintended consequences.
The MCAS is another case in point. The more powerful engines on the 737 MAX, which are really too big for this airframe, had to be mounted farther forward on the wing, creating a notable tendency for the aircraft to pitch up when full thrust is applied. Since that gets the wings closer to the critical angle of attack, the MCAS was invented to automatically counter-trim nose-down. Great, so it won’t stall. But it creates the opposite danger if a single sensor is faulty, and even if the pilots turn it off, it reactivates itself if the false readings continue.
And that’s where most pilots’ criticism lies with Boeing. Automation is fine, and generally computers are much better and more reliable than people. Commercial aviation has an insanely good safety record compared to all other modes of transportation including walking down the stairs in your house. But this is the first time that Boeing put a system into a plane that could not be switched off and control taken over by the pilot, but which continued to fight until it overwhelmed pilots which were maybe less than top-shelf.
