Originally Posted by
barit1
It would be pretty simple for an AOA display to be biased appropriately by flap position, so that the indicated value for Vso remains constant.
Actually, it wouldn't be (and indeed, isn't!)
The first question is: what stall AoA do you want to display - the one you would achieve if you were to conduct a 'standard' 1 kn/sec slowdown from your current speed, or the one you'd achieve if you were to pull a wind-up-turn at your current speed? Because they won't be the same, and the latter may be noticeably less than the former. If you display the 1'g' stall AoA then you may be giving the pilot a false sense of confidence in manoeuvring flight; if you display the manoevring stall AoA you will find that the displayed AoA moves around as you speed up and slow down (anyone who's watched their "low speed cue" or "green line" has probably seen it moving around on some aircraft, where the input data are Mach-dependent).
There are also issues with the accuracy of AoA sensing at low angles/high speeds, compared to high angles/low speed - as one gets faster the intrinsic accuracy of the airspeed system improves relative to the AoA sensors - for things like stall or shaker speeds the AoA system is generally more accurate than the airspeed system, when the two are compared, but the AoA resolver accuracy becomes more and more significant as the AoA drops. You'd have better luck trying to fly a precise cruise speed than a cruise AoA I expect.
John Farley's remark that automated systems expect AoA as an input is correct, but FBW systems also can have gains and such as a function of AoA; most autopilot systems don't, to my knowledge, use AoA as a control parameter.