No references just by trial and error.
If you can reprogramme the FMC to redraw the vertical profile to show you low on profile with a lower alt set in the MCP while in VNAV ALT it will go to VNAV PTH as you join the profile from below.
Eg. Descending with HOLD & VNAV PTH say to reach AAA at FL290 when ideally the crossing alt would be say FL330.
As soon as SPD appears press the MCP alt knob still with 29000 in the window this will delete the 290 resriction in the FMC and will draw a new vertical profile and you'll get VNAV ALT, then reset the MCP window for your lower alt when the new vertical path comes down to meet you the A/C will transition to VNAV PTH and descend.
I've been told pushing the alt knob within 2nm of the waypoint will clear the Alt restriction in the FMC with the same alt set in the window (SPD activates usually within 2nm) I think I tried it at 1.9 still in HOLD and PTH and can't remember if it worked.
It also works by entering a new restriction in the FMC that once again redraws the profile above the A/C. This new alt may have to be in the MCP before entering it into the FMC.
I too would like to know what parameters allow it to go from VNAV ALT to PTH automatically.
Never works in the sim either?? Well not our ones anyway.
Last edited by SMOC; 30th Jul 2010 at 16:07.