This is a weather vane which needs to be exposed to the elements, its not something that can be inside the aircraft. It needs to be exposed to super harsh conditions to get its reading.
If the IMU and airspeed sensors are good enough to feed the artificial horizon, autopilot, etc, they're surely good enough to produce an AoA estimate that's good enough to serve as a cross-check on the physical sensor.
Some more reliable than others. You have no fucking clue what direction the wind is coming down on the wings based on horizon.
You have a skewed perception of what wind is. On the ground it is always horizontal because it can't go into the ground. Once you get up into the sky the dynamics are insane. Get out of your armchair.
It's basic flight physics. The lift developed by the wing is equal to CL.q.A. A is the wing area, the dynamic pressure q is basically indicated by the airspeed indicator, and CL is a known function of AoA. Lift is by definition equal to the load factor * mass.
For a good, in-depth explanation, see http://www.av8n.com/how/htm/aoa.html#sec-ias-aoa
CL(AoA) = Az.m/(q.A)
(Az = vertical acceleration.) The relationship between coefficient of lift and AoA is a known function that depends on the airfoil. As long as you're below the critical angle of attack, it's an invertible function. (If you're not below the critical AoA, you've already departed controlled flight so it's too late for MCAS.)
This is not true at or near stall conditions.
(If the flight conditions are that unstable, a mechanical AoA vane will struggle too, since it has mechanical inertia.)
While you are right in theory - if you knew the load factor and indicated airspeed (IAS) you could determine AoA - how would you measure IAS in practice? Normally a pitot tube would give you incorrect readings at non-zero angles of attack, and that's why airplanes use an input from an AoA sensor for correction. Which brings you back to where you started - an AoA sensor.
You are second-guessing tens (hundreds?) of thousands of engineers who thought about these problems for more than a hundred years.
It also can't withstand failure of the selected pitot-static system, because it depends directly on it. So now you need a mechanism to detect such failures, and use a different pitot-static system, and do all of that switching while maintaining the requirements in FAR 25.207. I'm unconvinced that's feasible.
To be clear: I didn't imply that you'd want to use this as the single source of AoA, only that you can use this in concert with the physical sensors to determine if the sensors are producing a valid value. You need 3 sources to be able to throw out a bad measurement, 2 AoA sensors and an airspeed-based measurement should be a perfectly valid, single-fault-tolerant, way to outvote a bad sensor.
Remember that these accidents happened at high airspeed. You'd rip the wings off the airplane before you'd stall it at those speeds. It's not a subtle failure.
In a turn, an airplane experiences a higher load factor as a function of the bank angle. The lift from the wings is what makes the airplane turn. Since some of the lift is being diverted to the horizontal, the total lift must be increased to keep the airplane from sinking. The result is a higher load due to the acceleration in the turn.
The beginning of a climb or descent also changes load, but once established, the load returns to 1.
CG doesn't enter into it except to second order due to horizontal stabilizer lift.
Horizontal stabilizer lift is a downward force and must be countered with lift from the wing to maintain level flight. It's effectively the same as adding weight. Angle of attack absolutely changes as CG changes. This is a central purpose of weight and balance computation. W&B also changes as fuel is consumed. So you'd have to take that into account.
Anyway, you can't infer angle of attack or the coefficient of lift from only airspeed, load factor, and mass. You need more information.
This is a common misconception, but is not true. The angle of attack does not care if you are in level flight, climbing, or descending. As long as you are in unaccelerated flight, the AoA is identical.
It's not exactly the same, because if the descent (or climb) angle is significant, the lift only has to supply part of the weight of the airplane (the rest being supplied by drag or thrust), so the AoA will be slightly less. But that's accounted for because the load factor will also be slightly <1G.
(The edge case is if the plane is flying straight up or down. In that situation, the wings provide no lift at all and the AoA will be zero.)
What you're saying is true, but the effect of AoA is not huge. Stabilizer lift is a small fraction of main wing lift, because it has a huge leverage. The corresponding effect on aoa (effectively stall speed) is noticeable but also not huge. CG matters mostly because of its effect on controllability, not because of its effect on stall speed (although the effect is there.)
I maintain that, for the purpose of judging whether an alpha vane has failed or you're actually about to stall, you can ignore this effect when estimating AoA.
It's exactly the same as making the statement "If I'm flying at 1.5 Vs0, I know I'm not stalling (unless I'm in a very steeply banked turn or is pulling up sharply out of a dive.)
I saw you're a CFII. If a student made the above statement, would you berate him for failing to consider CG position?