GNC 420 non WAAS Vertical Speed Required (VNAV) Feature

[MBDiagMan]

I am working on understanding the VNAV feature for non-WAAS units. It appears that it can be used to indicate the descent rate from the final approach fix to the the missed approach point.

Has anyone ever tried this?

Example: Refer to the RNAV 17 Approach at KOSA. If I set Target Altitude to 736 Before WPT and Target Position to 2.2 Before Target Reference (which would end up as KOSA, then hit the FAF at 2000 feet, it would show me the Vertical Speed Required to descend to the MAP at my current speed. I realize that I am seeking an altitude for determining if I miss, but this should give me a useful indication while I “Dive and Drive.”

 

[RussR]

I use the VNAV functionality all the time, but for enroute descents, not for the purpose you are attempting.

You would need to reference the runway threshold, not KOSA, which in this case would be about 0.5 nm further than the runway, but at other airports could be much more.

And what you're measuring to by your method is not the missed approach point, but the VDP.

Also, why not just set it to descend to 1100 MSL, not 736 above target?

However, this seems like a lot of unnecessary knob spinning to do what you want to do, especially when there's a simple rule of thumb to do the same thing. It's a 3 degree glideslope, like most are. Take your groundspeed, add a zero and divide by 2, and the result is your VSR, within normal limits of accuracy. 90 knots = 450 fpm. 100 knots = 500 fpm, and so on.

 

[MBDiagMan]

Yes, on the little Cessna I can set 1850 RPM, and pitch for 80 Knots and I get 400fpm descent rate.a little adjustment depending on ground speed and I’m there. I am just exploring my new toy and it seemed that it might be helpful on a dive and drive.

 

[Ravioli]

and it seemed that it might be helpful on a dive and drive.

I'm probably wrong, but "dive and drive" is for Non-precision approaches and should be 800fpm. Precision approaches are 500fpm.

or so I've been told.

 

[MBDiagMan]

Yes dive and drive is for non precision and yes you descend at a faster rate.

 

[chemgeek]

For GPS approaches with no vertical guidance (LNAV), I would simply smartly descend to each stepdown to maximize the potential of gaining ground contact earlier in the approach. No calculations required. For normal approaches with a 3 degree profile, 700+ fpm should be sufficient to make stepdowns before the next one or MAP is encountered. If the profile is steep, closer to 1000 fpm will work.

One way to get in to trouble with LNAV+V (advisory) vertical guidance is to assume it clears all the stepdown altitudes (it should, but...) and all obstacles to touchdown (it doesn't). The same caveats would apply to using advisory vertical guidance of any sort. A particularly graphic example is the GPS 7 at N23 (Sidney, NY). The advisory vertical guidance will happily fly you into the hill if you follow it below the MDA. Of course, you are not supposed to do that--but it is possible to make the false assumption that the vertical CDI is assuring terrain clearance all the way to the runway.