Originally Posted by
bsieker
STS doesn't even have anything to do with stall,
Careful, the
function of STS is nothing to do with avoiding stall (ditto MCAS), but the implementation
does have an AOA input and
does behave differently at high AOA.
Copy/pasting from the AMM (my emphasis):
JPL 701-799, 803-899, 901-999 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.
If the roll angle from the ADIRU is more than 40 degrees, it opens an electronic switch and stops the speed trim signals.
JPL 801, 802 The stall detection circuit monitors the flap position and the angle of airflow. If it calculates that the airplane is near a stall condition, it opens a switch and stops the speed trim signals.
If the roll angle from the ADIRU is more than 40 degrees, it opens the same switch and stops the speed trim signals.
As I understand this, STS normally trims nose up when you add power (speed), on early revisions of the 737-800 this is prevented at AOA near stall (for obvious reasons). On later revisions this was modified so that near stall AOA, STS will trim down to allow it to then trim up if/when you add speed (and thus maintain speed stability). I believe I've seen rumours, maybe here, that this revision was needed to achieve EASA certification.
I think it is likely that the idea (and maybe some of the implementation) for MCAS came from this part of STS.