Originally Posted by
KelvinD
Declaration: I am not a pilot! (Have flown gliders only)
... Why does the only indication of AoA have to be from a fallible instrument stuck outside on the airflow? ...
A recurrent question, not least because of the ready availability of both GPS and high-quality inertial-frame-of-reference devices.
But the AoA is defined as the angle between the wing and
the mass of air in which the plane is flying. Here are pictures of 4 planes
just about to stall (from
https://www.apstraining.com/resource/whats-the-big-deal-about-angle-of-attack/)
... and the velocity shown here is relative to the local air-mass, not something you could measure with GPS.
Smoke-tails at acrobatic displays confirm that AoA and attitude may have startlingly little relationship to each other, as in this eye-candy
AOA versus pitch
PS In clear air it might be possible to measure the the relative velocity of the air to the plane by some laser-based optical device. However
in cloud, fog, heavy rain, icing-conditions, ... no way. (After AF447 they considered this avenue for airspeed measurement.)