Originally Posted by
FlyingStone
What happens when GPS or FMGC goes haywire? Or is that not the possibility you are taking into account?
If the FMS/GPS or whatever RNAV system in use can no longer meet the Required Nav Performance (RNP) for the particular phase of flight, then there will be an annunciation to that effect at which time the pilots will switch to alternate means of navigation as appropriate to their aircraft. There's no harm in having enroute navaids tuned up, but there is no requirement to do so either.