Airbus used calculation for FPA = arc tan(Vzbi/GS)
Vzbi = baro-inertial Vertical Speed
(inertial for accuracy in dynamic maneuvers & baro for stability in time)
GS = Groundspeed
Vzbi is computed from Vacc (Vertical acceleration from IR), Hi (Inertial altitude from IR) and Hb (Baro altitude from ADR).
After upgrade to L4.2 standard the FPA is available with BUSS activated as it uses IR only for calculation.
FPAair = θ - (AoAc * cos(μ)) - (SSAc * sin(μ))
θ = pitch angle
μ = roll angle
AoAc = corrected Angle of Attack
SSAc = corrected Side Slip Angle