"" Can anyone explain in what situation you would use the "UPDATE ARM" selection on FMC POS REF Page 2? I'm guessing that it would be used in a situation where you were in an area with no GPS or Radio reception and wanted to update the position, for instance just prior to coast-in? Wouldn't the FMC automatically do this, though, as soon as radio or GPS reception was again available? Is it a defense against GPS spoofing? ""
Yes when the aircraft tells you too !
B787 = EICAS FMC VERIFY POSITION , run the check list its all there..
or
B777 = FMC Alert Message VERIFY POSITION (primary /secondary FMC position differs by more than the greater of RNP or 0.4 NM for 5 seconds or FMC position differs with navaid used by greater than 12nm for 5 seconds.)