Originally Posted by
galaxy flyer
This is Collins-specific and maybe type specific. In any approach with a vertical guidance (ILS, LPV, Baro VNAV) once the vertical guidance is captured, the FD/AP goes into VGP on the FMA. Where the altitude is set is no longer a constraint, so it goes to missed approach altitude, except in one case. Collins FMS database has visual approaches for every, or nearly every, runway at airports in the database. In that one case, it's a LNAV/VNAV mode and the FD/AP will respect any altitude selection. Put 500' in and at 500', it levels off and you lose the vertical VMC path to 50' over the threshold. It's a visual, not instrument approach, just giving guidance. In a visual, you have to set RWY to have the path to the threshold.
I'm not sure if 737 is the same but the Honeywell FMC in other Boeings has approach logic even for the VFR approaches loaded from the database. They are flown in LNAV/VNAV. So it will fly and guide the approach even when the altitude selected in the MCP is higher than current altitude, as long as you are past the generated "FAF".
So, you put 500ft in, and once you're past the "FAF", you can set your missed approach altitude and it will still keep flying the approach to the 50ft TCH, unless you select TOGA.