The FPA is calculated using Vzbi, also called baro-inertial vertical speed, a parameter that uses both barometric and inertial vertical speed. The exact formula is FPA=arctan(Vzbi/groundspeed). So, if the barometric altitude is unreliable, Vzbi is unreliable, and the FPA will also be unreliable. If all ADRs are turned off, Vzbi is replaced by GPS parameters.