Why would Boeing's MCAS be preferable to a modified Feel and Centering Unit? According to some sources, the purpose of MCAS on Boeing's 737 MAX variants is to increase the back-force needed to further raise the nose when flying manually at high angles of attack, in order to give the airplane acceptable handling characteristics when approaching a stall.

Both the MAX variants and their predecessors, however, already have an elevator Feel and Centering Unit (page 8), the purpose of which appears to be the generation of appropriate stick force feedback in all stages of flight. If so, then, at first sight, this would be the appropriate unit in which to implement the function of MCAS, raising the question of why MCAS would be the preferred solution.

A few possibilities have occurred to me, but they are just guesses:


MCAS functionality needs angle-of-attack input, which may not be available where the Feel and Centering Unit is located (in the tail), and it would be complicated to get that information to it.
This answer states the the Feel and Centering Unit is a mechanical computer; given that, it therefore might not be easily modified.
Modifying the Feel and Centering Unit would require re-certifying it in toto, not just its new feature.
In addition to modifying the handling characteristics, MCAS is also seen as contributing to stall prevention directly, by reducing the angle of attack.


NB: Recently, Dominic Gates wrote an informative article in the Seattle Times about the origins of MCAS.
On the 737 NG, at high angles of attack, the nose of the plane would naturally pitch down, helping to recover from a stall and to increase airspeed.

The larger engine nacelles on the 737 MAX are located forward of the center of gravity, which means that at high angles of attack they are pushing the nose up. MCAS helps to push the nose down in this situation, similar to how the 737 NG would behave.

The added nose down trim has the side effect of requiring more elevator input at high angles of attack, but that was not the primary purpose.

There are several reasons that the Feel and Centering Unit would have a difficult time providing similar functionality:


The airplane should pitch down even the absence of control input, so it could not be done by simply changing the way the elevators respond to pilot input.
MCAS takes input from angle of attack, altitude, flap position, and airspeed. The Feel and Centering Unit currently just senses airspeed, so it would somehow need to get the other inputs.
The Feel and Centering Unit is a mechanical computer, so adding new inputs and changing the behavior could be very complex to design and certify.


Another possibility would be to use the Mach Trim system to adjust the Feel and Centering Unit. The Mach Trim system uses the flight computer to adjust the elevator neutral position to provide stability at higher speeds. While the flight computer should have all of the information needed, stabilizer trim provides much more control authority.