Since speed is usually lower at low altitudes - re takeoff- in the simplified world-- then a logical question is how is altitude measured ? AGL or the standard baro altitude set at airport So what happens when airport alt is say 7000 above sea level - how is this compensated for re how much trim movement per second/ tweak?
While one can assume the whiz kids who programmed this kluge did take such issues into account- the Ethopian accident makes one wonder.- especially when one knows that mach number ( actual speed ) varies with actual temperature NOT altitude per standard tables. Which suggests that somehow a local ( to airplane ) temperature or at least at takeoff must somehow be fed into MCAS to prevent errors in stab movement.
Which then drives into the REAL effects of airspeed, altitude, temperature, airport altitude, inputs to MCAS and how derived/measured.
My recollection from hydrodynamics courses taken as part of a maths degree almost 60 years ago is that mach number is airspeed relative to the local speed of sound. The speed of sound depends on temperature and pressure (assuming constant air composition), so MCAS activation and operation should depend on air pressure, temperature and airspeed. It should be independent of altimeter setting and height above the surface (unless it is considered useful to involve the terrain clearance system in the picture). Perhaps if MCAS was overridden by TCAS we would not be having this discussion.