A six axis sensor could probably measure AoA in a pinch. It’s just like how a stability control system works in a car (which is measuring slip angle, which is the lateral “angle of attack” between tires and road). If you detect upwards rotation but no matching acceleration forces, you’re detaching from the air stream.

Obviously this system will drift over time, but it could more than adequate to detect a failed AoA sensor and revert to the second one.

Since angle of attack is by definition the angle between a reference line of a body and the fluid it is moving through, I fail to see how you could measure it without measuring the relative motion of the fluid.

Your suggestion would work under the assumption that there is never any vertical motion of air, but that's not the case in real life. E.g. in a downdraft your sensor would fail to detect a stall.

That’s true, but it would still be good enough to cross check the aerodynamic AoA sensors upon a disagree between the two and select the one that’s more reliable.

> it would still be good enough to cross check

What are you basing this assumption on?

Past professional experience with similar sensor systems.

I also assume the AoA sensor failure was evident over time, through several phases of the flight and not momentarily. A sensor fusion algorithm would notice the divergence over time between the IMU and the two AoA sensors, then select the one AoA sensor that was closer.

Even in the case of a momentary failure, it would take an extreme coincidence to have the readout pattern of the failed sensor seem closer to the IMU based AoA model than the working sensor.

Keep in mind I’m only talking about cross checking the AoA sensors here; the flight control algorithms are only considering the aerodynamic sensors for AoA calculation as before, but they have an independent external reference to gauge the data quality of each sensor.

It could at least provide a reference to infer if readings were within a sensible range. Outside of that range it might be best to scream bloody murder and require manual human piloting with automated assistance at producing the desired results. (E.G. If the pilot's pulling back on the stick, they PROBABLY want the airframe to go up as a result rather than down.)

Is it really necessary to measure the relative wind or is that just how it's always been done?

What would be wrong with using an accelerometer to measure/calculate velocity with respect to pitch give you a better reading of AoA? In conjunction with a second velocity source to provide redundancy?