Wiedehopf # 189, “Knowing you have an AoA problem means you might more easily disregard a stick shaker when it doesn't make sense. Also on takeoff crosschecking speeds AND AoA seems like a sensible idea”.
You appear to have overlooked the main points about comparison in a dual system, which can detect a difference, but not which one of the sensors is incorrect; hence the stick-shake could be real.
Whilst the converse could be true, it is fail safe; see discussion on 737 stick-shake in Tech Log.
An optimum design with a dual systems ‘disagree’ is to remove ambiguity particularly when there is no alternative display to cross check. e.g. AoA disagree, low speed awareness remove; IAS disagree, annunciate and check best 2 out of 3 displays for continued use.
zzuf, I would agree with the view in # 1902 that MCAS is more related to stability than stall.
For those who continue to focus on stall, consider that forward trim might be applied at any time over a wide speed range - at or below Vref down to Vss. MCAS appears to involve a continuous assessment involving speed, AoA, and thrust; pitch and pitching moment, not as a result of a specific AoA as in stall warning / identification.
Also, for elevator effectiveness, there is a significant difference between the low speed stall situations, and nose-down trim at high speed.
piratepete # 1907, and MickG0105 # 1909, re un airworthy.
As discussed previously, how might an operator determine ‘un airworthy’ in this instance.
What is the industry norm ?
Resetting maintenance logs might be a function of BITE, which was run (report page 17) and concluded ‘no faults’, i.e. a serviceable system.
Depending on functionality, BITE might only be able to check the integrity of the connection to each AoA vane but not identify an offset fault.
Underlying these operational problems is the complexity of the overall control, flight guidance, display and alerting systems. ‘Intractable and close coupled.’
The overall ‘system’ is difficult, impossible to describe at the sharp-end. The level of knowledge required is insufficient to relate all interconnections, implications, and consequences of sensor interactions over the complete operational envelope. Thus neither pilots nor maintenance can be expected to deal with difficult failures without accurate descriptive guidance and checklists.
Another thought, is that the manufacturer was similarly ‘unable’ to describe, or have awareness of the overall system; e.g. flight control /aerodynamics use AoA input (dual) to overcome a stability issue, but with a viewpoint of a redundant (triple) system - AFCS / trim. Such a situation can arise from ‘egg-box’ like organisations or extremely large, complex, and interacting subsystems; in these cases then there should always be some one in design, airworthiness, or certification who can ask ‘the question’, but what is the question, who asked, when. Similarly for the FAA.