This discussion is starting to beg the question of whether the concept of AoA is so meaningless as to have no relevance to flight whatsoever!
Quote (my emphasis):
"Genghis is right, it depends on the choice of reference for the aoa gauge.
PDR1 is right, the choice is biased towards something easy to set up and check. Fuselage centreline datum is often chosen"
Thanks, Owain. Presumably the same practicality may apply to the pitch attitude, as shown on the display above and the PFD/ADI? That assumption would at least be consistent with the fact that when most jets I've flown were parked or taxiing on a horizontal pavement the pitch attitude displayed on the PFD/ADI was roughly zero.
Unfortunately, however, that raises a dilemma that has been bugging my subconscious for years. Quote:
"the wing/body setting which is usually chosen to get a more or less level cabin floor in cruise."
So why is the indicated pitch typically about +2.5 in the cruise? That suggests a 4% gradient on the fuselage axis, which itself seems unlikely to be much different from the cabin floor(s).
Secondly, in the example pictured above by the OP, pitch is +2.7 (see also the PFD) and AoA -0.8. If they are both referenced to the fuselage axis, the incoming airflow at the relevant AoA sensor must be angled at -3.5 relative to the horizon. (In the conventional model, where the aircraft is stationary, as in a wind tunnel, that would represent a
downwash of 3.5 degrees relative to the horizontal.) Is that feasible?
Both of the above points are predicated on the datum for pitch attitude being the fuselage axis. If it isn't, then what can it be?