Originally Posted by
bsieker
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.
And if someone didn't catch it the first time, it repeast a few lines later:
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
Hi Bernd.
Thanks for your input. I do not pretend to be anywhere close to fully understanding these systems although contributors in this thread have certainly inceased my knowledge tremendously.
It's funny that your quoted passages are the exact sames ones I have underlined in my manual! I'm still not convinced though, that your last sentence is 100% accurate :
"That it is also active close to a stall is mostly a side-effect of the speed stability augmentation". The headline where my quoted paragraph is taken from, follows the section that gives the general explanation of the STS where your quotes are from, and it reads
Stall Identification. The first sentence here says: "Stall identification and control is enhanced by the yaw damper, the Elevator Feel Shift (EFS) module and the speed trim system. These three systems work together to help the pilot identify and prevent further movement into a stall condition." There are no caveats in terms of CG position, weight or thrust condition for this to work so it is atleast possible to interpret this as something more than just a side effect of the STS.
But this is not really my main point. The thing is, Boeing
has mentioned in several places in their manual that there is another system, in addition to the pilot and autopilot, that has the authority to run the THS in flight - the STS. If you consider this alongside the "Runaway Stabilzer" NNC and Boeing's opening statement that "....checklists cannot be created for all conceivable situations and are not intended to replace good judgement", it would not be too far fetched to see how Boeing could argue their case that any qualified crew should be able to handle the Lionair situation (and possibly the Ethiopean if it turns out the culprit is the same).
I'm not saying I agreee with this, in fact after having read on these pages how MCAS works, I'm in a state of shock and utter disbelief that a system like this could pass quality and safety checks. But no matter where in the chain the fault originates - from the AOA vane to the FCC - I think it will boil down to whether Boeing will find acceptance for their claim that the pilot should be the ultimate safety mechanism if something should go wrong in these scenarios. I do not think an AOA indicator or AOA DISAGREE lights would have made much difference - and the Boeings I flew had both. The major thing to me that sets these three flights apart (the two Lionair and the one Ethiopean) is the presence of a third crew member. This added mental capacity seems to have saved the day and the lives of many people. And I think this is what Boeing will argue. Not the introduction of a third pilot but the fact that experience, training, skills and knowledge of any two pilots in the cockpit of one of their airplanes should be able to free up enough capacity to deal with situations similar to those we are discussing.