Originally Posted by
737 Driver
Do you have a reference? Looking at our FCOM and Maintenance Manual, I see no reference to a speed comparison. The STS seems to work primarily through the active FCC which should only be using the airspeed on the selected side. As a side note, in one Boeing-produced document that I have seen, MCAS is described as a sub-function of the Speed Trim System which leads me to think that the Stall ID function (flaps down) could have the same issue, except of course that it is inhibited by the control column cutout switches.
Not a pilot, but a programmer, and I have no details on the above systems. However, the key difference is that Speed Trim takes a large airspeed input, and turns out a small control movement, while MCAS takes a small AOA input and turns out a large control movement. The ratio between input error and output error is many times larger, between the two systems. From a control system theory viewpoint, that is a terrible design.
Edit: The jargon "error" is the difference between the measured value of a parameter, and the "desired" value (according to various rules). In this discussion it also has another meaning: Boeing's proposed fix allows a difference of 5.5 degrees between left/right AOA as acceptable for MCAS activation. AOA is a value that can easily be measured with the same degree of accuracy as something like airspeed (even if reliability is very high).
Edit: IMO such a disconnection tells you more than you need to know about the usefulness (or lack) of such a system.