Does anyone know the logic that the new FMC soft installed on B737 uses to revert from VNAV PATH to VNAV SPD automatically?
If your aircraft is operating on update 10.8 like ours is then this from our FCOM should apply:
VNAV reverts to VNAV SPD if a limit speed will be exceeded.
also this;
A path descent must be initiated while within the allowable cross–track error for LNAV , however LNAV may be disengaged during descent while remaining in the path mode. VNAV will remain in path regardless of cross-track.
Hope that helps.