Originally Posted by
TTail
Now that the MCAS is being classified as a subsystem of the STS wouldn't the below paragraph - possibly without the EFS module part - read like something you'd find in a Boeing MAX AFM if the intent were to give a brief non-technical description of MCAS?
"As airspeed decreases towards stall speed, the speed trim system trims the stabilzer nose down and enables trim above stickshaker AOA. With this trim schedule the pilot must pull more aft column to stall the airplane. With the column aft, the amount of column force increase with the onset of EFS module is more pronounced."
No, because STS is triggered by
change in airspeed (in either direction), and MCAS is triggered by
angle of attack, and only by high, not by low values. So although they sit in the same box, and one may even be considered a subsystem of the other, their functions and activation criteria are fundamentally different.
STS doesn't even have anything to do with stall, or stick forces, but is about speed stability. Only the first sentence of the paragraph you quoted is about the STS, the rest is about the Elevator Feel Shift module.
The speed trim system (STS) is a speed stability augmentation system designed to improve flight characteristics [...]. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change.
And if someone didn't catch it the first time, it repeast a few lines later:
As the airplane speed increases or decreases [...], the stabilizer is commanded in the direction to return the airplane to the trimmed speed.
Nothing about stall at all. That it is also active close to a stall is mostly a side-effect of the speed stability augmentation.
Bernd