Does anyone know if MCAS was just a software addtion to the Max, or is it an actual mechanical system in addition to the STS and the Elevator Feel and Centering system, (which ARE two systems, with separate pitot-static sources).
Tx...PJ2
Well, someone obviously knows but it doesn't appear to be public domain. What we do seem to know is that there was a software mod to FCC in the NG, allegedly for EASA cert reqs. - see e.g.
There are 2 FCC software versions regarding STS with following fundamental differences:
1. (POST SB 737-27-1220 issued in Feb 1999 and installed by now on most NGs and all MAXes) The stall detection circuit monitors the flap position and the angle of airflow. Near stall, the speed trim function trims the stabilizer to a nose down condition to allow for trim above the stickshaker AOA and idle thrust. The trim continues until the stabilizer gets to its limits or the aft column cutout position is exceeded.
and from an NG FCOM:
As airspeed decreases towards stall speed, the speed trim system trims the stabilizer nose down and enables trim above stickshaker AOA.
Now,
that is for the NG, definitely a software change, possibly a change
to STS or possibly software
in addition to STS.
The MAX appears to be different - unless Boeing bulletin and the E-AD are wrong and a common issue has appeared on the MAX first when it has two (or maybe three) orders of magnitude fewer flight hours than the NG. There is much internet commentary stating that the larger engines set further forward mean the MAX has different pitch stability to the NG at high AOA, leading to the MCAS requirement.
We also know (see further up this thread) that there are hardware changes to the stab trim cut out switches on the MAX (labelling
and function), and hence probably to physical wiring. We don't know if this is related to MCAS or not, but it seems likely and I highly doubt the change was made for fun or because someone said "no one ever uses these switches separately so lets make them do the same thing".
I think it is very likely that MCAS is an FCC software function evolved from the previous STS mod on the NG. The FCC does have
both set of ADIRU data available (there is a cross link between the two FCCs) whether and how MCAS uses both AOA inputs, we don't know. This is based on info from various sites including:
https://www.satcom.guru/2018/11/737-...n-command.html
You mentioned the Elevator Feel Computer having separate pitot-static sources, which it does, however it also takes an input from EFSM (Elevator Feel Shift Module) which is triggered by
either SMYD (Stall Management Yaw Damper) which in turn take inputs from the on-side ADIRU, so a single AOA error would trip SMYD on that side (and hence stick shaker motor on that side,
and the EFSM...