Originally Posted by
BuzzBox
With both FMCs operating normally, one of them is active and the other is inactive. The active FMC synchronises the inactive FMC any time there is a change to the flight plan or performance data. If the active FMC fails with the FMC selector in AUTO (ie the usual position), there will be an automatic failover to the previously inactive FMC, which already has all the data it needs to take over as the active FMC. There is no "software reset" and LNAV remains engaged.
There are software resets, as stated in the FCOM.
“A software reset may occur while in single FMC operation. The active route becomes inactive, the performance data is erased, and LNAV and VNAV (if engaged) modes fail.”
It is not possible for an active FMC to synchronise to the inactive FMC if the link is severed.
If the active FMC always changes automatically, then there wouldn’t be a need to install a FMC selector switch for Left, Auto, or Right.