Originally Posted by
yoko1
It is reasonable to debate if MCAS was the appropriate solution to this problem, but the problem had nothing at all to do with aerodynamic stability. Aerodynamic stability has a very specific meaning in aviation. There are many online sites that discuss the topic including
here.
It's not as simple as that. The classical stability analysis you quote assumes that the control surfaces are fixed. Having movable surfaces complicates matters, for example as discussed
here. TL,DR: the last line of that paper states that the stick force gradient is proportional to the stick free static margin. In other words, the stick force gradient specified in the regulations is a measure of the static stability of the aircraft.
The MAX may not be unstable in that corner of the envelope where MCAS is needed - we don't really know since the data is not available in the public domain -, but we do know that it is not stable
enough.