Originally Posted by
lobo411
Speaking as someone with zero knowledge myself, I'd say that it's because the MACS system is both useful and necessary to safely operate the aircraft. There are lots of systems in any piece of technology that can be dangerous if used improperly. For example: in the Olden Days, you used to have to pump the brakes in an emergency situation to keep them from locking up. ABS became standard 30-40 years ago, so nobody pumps their brakes now because doing so actually makes things worse. But for the first 20 years after ABS, old people would still pump based on experience.
In theory, the MCAS system is a valuable safety aid to prevent the aircraft from stalling. It is designed to trim the nose down before the aircraft stalls if an excessive AoA is reached. However, the problem is that it is vulnerable to being mistriggered if an AoA sensor itself fails in a mode that causes it to read high (rather than, for example, reading zero or just sticking at the last reading). In that scenario, the auto-pilot will disengage and the nose will be trimmed pitch down. On large aircraft, where trim is achieved by moving the whole horizontal stabilizer, the trim authority is greater than the elevator authority. Therefore, it is likely that no amount of pulling back of the yoke will stop the ensuing dive - rather, the trim itself has to be adjusted. However, doing this only stops the dive for a short period (ten seconds I believe) before the MCAS dives again. The only sure fix is to disable the trim system completely (though the original FAA service bulletin also mentioned manually grabbing the trim wheel as a last, last resort if even this step failed).
So, in theory, a false MCAS activation is recoverable. However, put yourself in the position of this happening when you are 1,000 ft above the ground with only a few seconds to work out what is wrong and take the correct action. It might be easier if you had a nice message pop up to say that the AoA sensors weren't operating correctly. Unfortunately, this message is apparently only enabled if the airline has paid Boeing extra to get the AoA display option, even though it's only a software function.