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VNAV Transition to Approach

rongawer

Well Known Member
I really enjoy the new VNAV capability the Garmin has provided with the v6.5 update to the GTN, combined with the recent G3X update. However, the updated Pilot Guide states that for the G3X, VNAV capability is only for visual conditions. That actually makes sense, because in IFR flight, you’ll be on an assigned or prescribed altitude. In fact, my operational experience has found that if you pick a fix to start and approach, set the altitude constraint and hit “save” in the GTN, and then arm VNAV, the AP will not automatically transition to the approach; it will steer laterally to the inbound course, but vertically, altitude hold will take effect for the last constrained altitude - even if a GP or GS is prescribed in the approach. I wondered how I could get the VNAV feature to make IFR flight easier as it wasn’t obvious to me in either the GTN and G3X PG’s.

Well, if you have a GTN coupled to your G3X, I’m happy to report that there is a feature called VNAV Transition to Approach that works very well, but requires using the Direct to VNAV feature. A common occurrence for me is to have ATC say “proceed direct to XXXXX and cross at XXXX, cleared for blah, blah, blah” - direct to VNAV works great for this.

It wasn’t obvious to me how this all worked, and in the event that I’m not the only person who hasn’t got it all figured this out - after MUCH playing around with the system - I’ve come up with a way to use VNAV as part of my IFR flying and thought I’d share my process.

I have a GTN625, GMC307 AP controller and G3X dual displays - so this is applicable to that setup, or a similar one.
- Load my applicable flight plan in the GTN
- Be in NAV mode (on the GMC, select NAV); VNAV will not work in HDG mode.
- Load my approach and verify routing to the IAF is active.
- Verify the altitude constraint for the fix; the system automatically loads the published altitude, such as 4500’ for DOCAL on my local approach, but ATC often gives a different crossing altitude, such as 4700’, so I just update that. Then - and this is a key point - press the D-VNAV button on the GTN, not “SAVE”.
- Press the APR button on GMC (APPR on the G3X if you don’t have a GMC) to arm the approach. An option would be flying on vectors in heading mode and then activate this and then proceed on the approach once cleared.
- And then press the VNAV button. The APR button will stay lit - armed. You’ll also note that if the approach has a glide path (GPS) or glideslope (ILS) a GP or GS will also appear on the AP control panel showing armed in grey color.
- You’ll note a altitude constraint in magenta above the altitude tape appears and the VNAV magenta “<“ on the altitude tape starts descending.

The AP will guide descent to the waypoint at the constrained altitude and start the turn onto the approach leg. Once you are at the constraint altitude, (it seems to capture at about 50’ above), the VNAV will transition automatically to the GP or GS.

Enjoy watching the magic happen. It’s pretty slick.
 
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VNAV button on GMC507

Thank you for this tip, I am very new with Garmin G3X and A/P and learning all of these.
I am not sure if I am all clear about the VNAV button all on its own on the GMC507. Do I understand this correctly that it will not work in HDG or TRK mode but is its function in such way that when you set vertical navigation, lets say you want to arrive a 1000' @5NM from your destination, this will guide you to that point in space?
 
I have a GMC 307, which does not have a TRK mode, but I can confirm that VNAV does not work in HDG mode, but rather only works in NAV mode on my unit; this makes sense as you would want to descend to a waypoint that you are also laterally navigating to, and neither heading nor track mode take you to a specified waypoint.

You do have it right in that the purpose of VNAV is to provide the vertical portion of navigation to the altitude constraint defined by the waypoint(s) you have selected in your current flight path. Let's say your flight plan is KWLW GIFME HAIRE C83 and that you are cruising at 7500' after taking off from Willows, but want to be at 2500' by GIFME, and then 1000' at HAIRE for landing at C83. You would enter the altitudes in the GTN for the fixes and then select VNAV on your GMC unit to arm the descent. Once you reach the TOD, the AP will start the descent as planned. That's enroute VNAV, and used for VFR flight.

For IFR flight, instead of enroute VNAV, you want to use Direct VNAV; and this makes sense to, because it's unlikely that you'll get to decide when and where you descend on an IFR flight plan, as you would when VFR (an enroute descent), but rather ATC will control that and you'll descend on their clearance (a direct-to descent). Let's say you have the RNAV approach in for C83 and have MOVDD as the IAF. Then assume ATC provides vectors to MOVDD (as they usually do) and finally say to "proceed direct MOVDD, cross at 3000', cleared for RNAV 30" - you would select MOVDD on the GTN, enter 3000 (which is probably already populated in the altitude constraint as it's the published crossing altitude), then select "D-VNAV". Then select the VNAV button on the GMC (if you have not already done so, select APR as well). The APR button arms the approach and selecting VNAV starts your descent to MOVDD. You should note that on the G3X autopilot status box VNAV will turn green and GP will be white (armed), and the magenta '<' will be on the glideslope indicator tracking your descent. Your current VNAV altitude target will be displayed at the top of the altitude tape as well.

Once nearing MOVDD, the AP will start a GPSS turn inbound on the approach, crossing MOVDD at 3000', and then seamlessly capture the Glidepath (GP) and the '<' will turn into a diamond. The AP will then fly the approach both vertically and laterally.
 
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Thank you Ron for the information and guide. It should save me a few gallon of fuel learning it in the air :)
 
It was definitely a head-scratcher for me until I did a couple approaches and tried out the various scenarios. Glad to help. I highly recommend doing a few practice approaches before going “iron hand” with it.
 
I tried the VNAV (not an instrument approach) on my autopilot and had a airport in my destination and VNAV configured on G3X to arrive at the airport 1000' AGL @ 3 miles before the airport. I do see the highway in the sky showing up on the G3X but it never starts descending to reach that point (1000 and 3 miles)

What other step am I missing in this? I don't need to arm the approach correct, since there has not been any approach loaded.

One other observation today, on the 650 it gives me the visual approach and when I select it, it gives me the runways available. In this case I selected run 23, my heading was 060 as I was going towards the airport. The A/P turned the plane to the 23 direction, even though I was still about 7 miles to the airport. But when I selected the 05, it turned me around and tried to land.
So I guess the visual approach will work only if on a long final.
 
For enroute VNAV, you need to be in NAV mode to a waypoint. You don't say what your starting altitude was, but for enroute VNAV, you would need to be outside of the TOD distance, however you have that configured, or the VNAV should have given you a message saying it was unable to capture the VNAV. If you did all that right, then you might want to email G3Xpert for further help, but I would check that you have the right software levels and configurations.

If you're doing the visual approach, it is handled like any other approach for the GTN, so you need to use direct to VNAV and then arm the AP for APR. You can't use the enroute VNAV to start the approach, but need to use direct-VNAV.

You should be in NAV mode direct to the 3NM Visual fix and then select D-VNAV for the waypoint on the GTN. And then press APR to arm the approach. It should then descend to the FA fix in VNAV mode and then capture the glide path in approach mode.

One other thing, and I think this is a glitch for the G3X - or at least something that should be changed, but the HITS stays level with my altitude selection when I'm on an enroute VNAV and does not show a descent like an approach with a glide path would. My fix is that once I've started down the VNAV, I adjust my altitude preselect to the next descent point and then I capture the HITS when I get there.
 
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