PJ2,
Thanks for posting that excellent graphic of the Airbus PFD in FPA mode. This originated with the Orca (A310) in 1983/4.
I think it may be helpful to some other readers, unfamiliar with Airbus glass cockpits, to clarify what we are looking at.
It is clear in the graphic that Alpha is +15 (but see my penultimate paragraph below). But if the drift increases, and particularly when varying amounts of bank are present, it's not so easy for the eye to interpret quickly.
The second problem is that, unfortunately, we are not looking at a primary indication of AoA, even though that could be made available from the appropriate AoA sensor. The "bird" is dependent entirely on IRU data (from the appropriate ADIRU), as Machinbird pointed out. So, of course, is the pitch indication on the same PFD.
HN39,
As far as I know, the FPA ("bird") derives its data from the IRU, not the AD. I would be astonished if it used VS from the AD, because the latter (presumably coupled with TAS) could only give a trajectory relative to the air – not the ground. When you are using the FPA to fly a steady glide-slope on a non-precision approach (standard procedure in British Airways), the FPA has to be relative to the ground.
Everyone,
My response to HN39 has belatedly reminded me that AoA CANNOT accurately be derived from a comparison of Pitch and FPA (flight-path angle): it is only strictly true in a stationary air mass.
Coming from a purely civil (airline) background, I've nothing useful to contribute on the concept of flying AoA as an alternative (or complimentary) to airspeed. We were just never taught it. To illustrate the point, both the 1960s VC10 and BAC 1-11 (and, presumably, Trident) had AoA sensors for their sophisticated stall-protection systems. But the only time we saw an AoA gauge in the cockpit was when the engineers fitted it temporarily for a flight test, after which it was removed.