BOAC,
From Interim #2, para. 1.6.11.2:
The FPV is elaborated in the IR part of the ADIRU which, for this purpose,
uses inertial parameters and also an anemometric parameter: the barometric
vertical speed. It is thus necessary for the IR to have at least one valid ADR
at its disposal. From the perspective of the IR, an ADR is valid if the three
parameters, altitude, barometric vertical speed and true airspeed are valid
(SSM status is NO) If the three ADRs are considered invalid by the IR it is no
longer possible to calculate the FPV and the red FPV flag appears on the PFD.