One of the terrific features available in the latest digital autopilots is VNAV, or vertical navigation. If you have a Garmin GTN-series navigator and a GFC 500 or GFC 600 autopilot, you can use VNAV to descend to the charted altitudes along the intermediate legs of an instrument approach, until you reach the final approach fix.
VNAV is also available on most G1000 systems equipped with the GFC 700 autopilot, including the new G1000 NXi, and some other manufacturers, such as Avidyne, plan to add the feature to their systems.
To see another example of using VNAV, visit VNAV with a GFC 600 Autopilot.
The 10-minute video below demonstrates how VNAV can help you fly a smooth, precise descent prior to the FAF. The video explains the basic concepts, uses the Garmin PC Trainer Suite to highlight key details, and then shows VNAV at work as I fly the RNAV (GPS) RWY 12 approach at Kelso, WA (KKLS).
For more information about VNAV, see Garmin GTN VNAV here at BruceAir. Garmin has also posted a detailed explanation of VNAV in the following video.
BruceAir, LLC 
Just used VNAV on my last three approaches. The descent to to the crossing altitude at the FAF seemed steeper than normal. Not sure why the TOD was placed so close to the FAF. The approach was an LNAV + V approach. I put in a request yesterday to ForeFlight to add a TOD marker on the course. I always wish I could check for it on ForeFlight rather that toggling to the map page on the 750.
Especially in high-performance/complex aircraft, VNAV takes some getting used to. And it may require a change of old habits. The feature puts you exactly at the charted altitude at the charted position, which may suggest that you want to slow down, extend flaps, etc. earlier than you have in the past, when you planned descents along the intermediate legs, often with the intention of reaching the altitude a mile or so outside the fix so that you could level off and slow down before extending flaps or landing gear.
I’m still practicing with the the feature and taking care during preflight planning and approach briefings to decide how best to fly a particular approach.
The system (GTN) defaults to a 3 degrees descent slope which is too high resulting in a high rate of descent. If say cruising at 160 kts, descent rate would be around 800-900 FPM. TOD will be close to the next specified altitude including the FAF. Go to the VNAV page (Utilities) in the GTN and select (for example) 500 FPM descent rate. It will then calculate a lesser decent slope. TOD then is further out from the fix in question. However, you never want a TOD behind you.
That’s a good recommendation, which I make to my customers who fly with GTNs. I set up my GTN 750Xi for a 600 fpm descent rather than the default descent angle, which as you noted, typically results in an uncomfortable rate of descent in high-performance aircraft like Bonanzas.
I’m not sure how the FAF fix is defined in this context. The Maltese Cross is the FAF for a non-precision approach; but the FAF for a precision approach is GP/GS interception. (This has regulatory relevance to Part 121 operations but not Part 91.) Usually GP/GS interception occurs at the published attitude at or near the Maltese Cross.
If the Maltese Cross is chosen as the lowest altitude for VNAV, then it also becomes the BOD. What this implies, is that at the Maltese Cross (at least in the GFC 500) if the pilot delays switching from VNAV to APR, the airplane will overfly the GP/GS and will usually not couple.
Easy solution. Chose the pervious fix as the lowest altitude for VNAV (usually an IAF). At that fix, plane will level off, and at that time switch to APR from VNAV. Airplane will maintain altitude until intercepting the GS/GP which then results in descent all the way to the runway. The only catch is that at the previous fix, we need to be below the GP/GS.
Your suggestion will work in many cases, but not all. If that’s your plan, you should brief it for each procedure and ensure that it will work out. And it’s not necessary if, like me, you have a GFC 600 autopilot. With that AP, you can arm APR mode while using VNAV, and the AP changes modes when you intercept the GS or GP.
As for defining the FAF, if you’re flying a precision approach that does not have a FAF marked by the Maltese Cross, you can set the GS or GP intercept altitude as the bottom of the VNAV descent. See https://bruceair.wordpress.com/2021/02/06/the-final-approach-fix-on-an-ils/
Unfortunately the GFC 500 does not allow to fly VNAV as active mode and arm APR mode at the same time.
I understand that. Hence my emphasis on briefing procedures carefully. I have a presentation on the topic on my YouTube channel: Briefing IFR Procedures (https://youtu.be/vbB4s7-Fv1c)
I did the calculation and my recommendation would work at KKLS. Choose UXALE as the BOD for VNAV. At UXALE, the GP is at 2860 ft MSL, arm APR, fly level at 2700 FT MSL for a short distance, intercept the GP and follow it down. However, this really needs to be determined prior to the flight prior to flying and not during the briefing.
There is a new (for me) Garmin term: Transition to Approach. In the GFC600 it is enabled, in the GFC 500 it is not. This means that the GFC 500 VNAV will not fly up to the point where APR automatically switched over from VNAV. By switching, I mean capturing the GS/GP. Apparently to provide this function requires testing and updating the STC.
The GFC 600 supports automatic switching from VNAV to GS/GP capture. At present, that feature is not available in with the GFC 500. See p. 48 in the GFC 600 pilot guide.
Thank you , I’m fully aware of that the GFC 600/700 has that function, but not the GFC 500. That is why I was initially confused with the great demo in your Bonanza with the GFC 600. Naively, I assumed the feature would also have been available in the GFC 500. It took me awhile to figure it out reaching out to several people especially since I was assuming it was a pilot error (me) or a problem with the installation of the GFC 500. I did have a few minor bugs with my installation, natural to assume maybe an installation error or defective equipment. I do wonder, if the function is enabled in the GFC 500, would it allow seamless transition from VNAV to APR?
If the feature is made available in the GFC 500, and if it works as it does in the GFC 600, then it would provide a seamless transition from VNAV to APR mode. It’s a nice feature–but as always, you must take when you using it. Most IFR pilots recommend waiting to select APR mode until you are cleared for an approach. Engaging it too early could create a conflict with ATC. But if you are following a series of stepdowns and if you have been cleared for an approach, then the capability is nice.
To the best of my incomplete knowledge, the functionality is available, it is “disabled” in the GFC 500 and enabled in the GFC 600. Check the CONFIG in your GFC 600.
As always, with such questions, it’s best to consult with an avionics shop and review the documentation–especially the AFM supplement–for the avionics in your aircraft. As you know, Garmin and other avionics manufacturers often update the system software of the newest gizmos, which are, essentially, computers. What was true when the boxes were installed may not be accurate after one more updates.
I can’t tell you how happy I am I found your article! I have a GTN-650, Dual G5’s, and a GFC-500 (which replaced an S-Tec 50) in a 182. My home airport is KDLS and the RNAV-A approach is useful…but I’ve been struggling to fly coupled approaches with the GFC-500. Yesterday I used VNAV to the IF and then NAV (as the G5 manual for GFC500 says) to fly LP…and figured I could trim the plane myself on the step-downs – not so. I ended up flying part of it (in VMC) using the pitch wheel on the GFC-500 which was weird…I didn’t like it and just hand flew it instead.
This morning it occurred to me to try VNAV for the step downs and then I went searching for info. and found your page. Thank you! Will try it in the Sim and then go try it in VMC to confirm.