Originally Posted by
KelvinD
Declaration: I am not a pilot! (Have flown gliders only)
Having got that out of the way, I have to ask a question that has been bothering me since this issue first arose: From what I read here, the MCAS problems arise from erroneous AoA indications (plus the obvious follow ons such as the correct action to take etc). Why does the only indication of AoA have to be from a fallible instrument stuck outside on the airflow? Outside of aviation, there are many devices that can perform the same function, such as inclinometers etc, either working from a bubble indication or an object floating in a liquid. Why can a pilot not have an instrument, mounted inside the cockpit, that will give him an indication of AoA, regardless of what the airflow over an outside instrument tells him., At least, he would then have the opportunity to think about whether or not something is wrong.
Angle of attack is independent of pitch and acceleration. Inclinometers and bubbles indicate the direction of local acceleration. This is only reliable if the vehicle they are in cannot accelerate and add or subtract from the acceleration due to gravity. The closest one can get is to differentiate (mathematical process) to determine the path and compare that to the apparent pitch alignment to that path. Even then it's not a certainty as that depends on the air being stationary and uniform, which it usually isn't.
Essentially the only way to account for the local relative wind's likely effect on the aerodynamics of the plane is to use an instrument that is directly affected by the local relative wind. Some less-direct aerodynamic sensors do exist, such as laser Doppler meters that measure the effect of the local wind on dust particles in the air, and I think there are hot-wire probes that detect the thermal convective characteristics, but those are more prone to getting broken and are much more expensive their fragility.