Can't comment on the 777, on the 737 the FPV is inertial but uses barometric altitude inputs from the ADIRUs. Boeing warns that it is unreliable with unreliable primary altimetry information. However, there is no information about speed information. There is no information about the FPV in the airspeed unreliable checklist, however there is a big note which could explain while it is not entirely wise to trust it completely.
Note: Erroneous or unreliable airspeed indications may be caused by blocked or frozen pitot-static system(s), or a severely damaged or missing radome.
Since the FPV takes static system inputs any malfunction that includes the static system may lead to wrong indications.
Last edited by Denti; 12th Jan 2013 at 17:36.