Originally Posted by
safetypee
This thread requires a large placard reiterating the purpose of MCAS - a stability enhancement, it is not an aid to help pilots avoid, or be anti anything to do with stall.
Stability, in this case relates to the relationship between speed and trim, which translates to stick force - the feel of the aircraft. Changing trim would not normally change pitch attitude during manual flight, which MCAS is limited to.
The 737 Max is not unstable; without MCAS, in a specific area of the flight envelope, the aircraft exhibits less stability margin than required for certification. The aircraft is flyable without MACS with low risk; however, the aircraft appears very difficult to fly with large trim offsets resulting in high stick forces, more so with distracting stick shake and alerting (more associated with UAS). Unfortunately these aspects and associated risk appear to be confirmed with hindsight.
Neither should we resort to hindsight in judging the quality of the design. Given the problem and resources (timescale), the design appears to be adequate in normal operation - as judged by many normal flights. The critical point in these accidents is that there was an erroneous input - probably from the AoA vane, but not yet proven. The effect of this error appears to differ from what might have been expected in design and certification.
The mechanism for managing this error in flight appears hold many assumptions about the severity of the fault, and the ability of human intervention; compounded by the lack of system information.
Great post! A design isn’t complete when the system just performs its intended function. It is important, and not trivial, to understand how it might behave when it or related systems fail, to identify the failure modes, correctly classify them and design protections from their consequences. In this case, it seems that the failure conditions were not adequately understood and/or the ability of pilots to recognize and react correctly in a timely manner was optimistic.