Vertical Nav on a Precision Approach


Apr 17, 2005
Oxnard, CA
Sorry if this has been answered before, but my search of the forums and reading the Pilots Guide about 5 times still didn't answer my question:

Assuming a certificated WAAS/GPS navigator (430W or 650) is installed and integrated through the ARINC-429 box, will the VNAV function allow the autopilot to to follow a vertical path from the IAF to the FAF as well as capturing the GP/GS from the FAF to the MAP?  As an example, selecting VNAV on a Garmin integrated navigator prior to the IAF and setting the ALT bug to the DA/MDA will allow the AP/FD to fly the "step-down" from the IAF to the FAF as a stabilized approach.  My assumption from what I've seen and tried with my installation is that the Skyview AP will only provide vertical guidance on the final segment of the approach (GS/GP).  If that's correct then I realize VS will have to be used for descent prior to the FAF. 

It would really be nice if someone could post a training video of a few precision and non-precision approaches with the "buttonology" needed to make them work.  Maybe they are out there on YouTube somewhere, but I haven't found any to answer this question.   :D


Active Member
Nov 10, 2008
Gods Country - west Texas
Once you have a glideslope available on the HSI, then the VNAV function is enabled and you can select it - prior to that the VNAV function is disabled (greyed out and unselectable) because it has nothing to follow. It will not follow a "step-down" without vertical guidance being received either by a certified ILS receiver or a certified GPS receiver, talking to the ARINC 429.

Approaching the IAF, use the altitude hold and climb/descend as you normally would to capture a target altitude, including any step-downs enroute. When you get close to, and stabilized on, the approach path then the glideslope will appear on the HSI and now you can toggle the VNAV on. The AP will remain on altitude/VS hold mode until it intercepts the glideslope from below, and then track it down.