747-4/8 "NG FMC" - VNAV ALT not changing to VNAV PATH automatically on approach
Aircraft in question is the B-747-400 and 747-8 NG FMC with blockpoint 4.0 software.
When when descending in VNAV SPD and below the VNAV path, the vertical flight mode annunciation changes to VNAV ALT once capturing the MCP altitude. When cleared for the approach, on a non-ILS or a published transition route to an ILS, typically the pilots will set either minimums or final approach fix altitude in the MCP altitude window depending on the type of approach. When on approach logic (flaps out of up, first approach waypoint sequenced, or close to the airoprt) and in VNAV ALT, most of the time, the FMA will automatically changet to VNAV PATH and begin a descent toward the next altitude constraint in the FMC or end of descent point, automatically as the VPI/VNAV path intersects the aircraft's altitude from above.
My question is: Why does VNAV ALT sometimes automatically change to VNAV PATH and resume descent when the the Vertical Path indicator crosses the airplanes altitude from above, but not always? Why does it sometimes remain in VNAV ALT and require a pilot to press the "altitude intervene" switch on the MCP to follow the path down? I realize you can press altitude intervene early and get VNAV path while level and below path (VPI), however I would just like to know why the FMC will automatically do this sometimes, and others not. I could not find this scenario mention in the FCOM Vol II; the scenario of VNAV ALT automatically changing to VNAV PATH on approach isn't specifically addressed that I could find.