I was told by one very knowledgeable Gentleman, that the main parameter used to derive the FPA is the AoA and was shown some drawings explaining how it worked. Seems plausible to me and actually quite clever - but I am looking for some official reference.
The only problem I have is that in this case the FPA would be referenced to still air - ie. headwind/tailwind component wouldn't affect it, only TAS and VS. Does anyone know if this is the case?