VNAV into Approach

[PT20J]

The GTN Xi Pilot's Guide has a clearer description of how VNAV works on an approach with a GFC 500. As Don noted above, the GTN Xi STC requires that Transition to Approach to be disabled with a GFC 500 installation. As near as I can discern, the GTN Xi has always had this VNAV capability, and the capability was added to the GTN at software v6.62 (enhanced VNAV). The most recent G3X Pilot's Guide is rev D from January 2022 and states that the GFC 500 does not have VNAV vertical guidance to the FAF. I think the change to enable VNAV to the FAF was in G3X v9.00 which also allowed simultaneous arming of VNAV and GP or GS. EDIT: Hopefully, Garmin will get all it’s documentation in synch with its software someday.

[Rmag]

5 hours ago, Marc_B said:

@Rmag I don't think I noticed that to use VNAV you must be in GPS mode but this makes sense...

This is incorrect.  I indeed used VNAV for the descent portion of the RNAV 19 approach into KWWD.  The autopilot descended to the 2,500 altitude constraint at KAGYS and then proceeded to the 1,700 altitude constraint at KIMBA before then capturing the GP at the FAF (KIMBA).

At 23:28 in my video I dialed the altitude preselect to the 1,700 FAF altitude at KIMBA, and at 23:30 you see me press the VNAV button on the AP.  Maybe what is confusing you is that the AFCS annunciation on the GFC 600 is “VPATH” and not “VNAV” like the GFC 500, it it is the same thing, just different terminology between the two AP’s.  The rest of the video beyond 23:30 shows the aircraft descend to the altitude constraints via VNAV (VPATH) before then capturing the GP at KIMBA.

I chose the prior video because it showed VNAV to the FAF.  I have another video where I shot the same approach in a Mooney using a GFC 500, GI 275, and GTN 750 Xi, but I only did VNAV to KAGYS (the IF prior to the FAF).  KAGYS inbound was then done via the GP.

if you rather see it in a Mooney with a GFC 500 start at 16:10 in the below video.  Just realize I do not use VNAV all the way to the FAF in the below video.

 

[ArtVandelay]

My most common mistake is forgetting to set the altitude preselect.

I’ve created a laminated checklist for autopilot functions.

[jlunseth]

I think the original question was about power setting. I don’t have the GFC500 so can’t address how it works in real life (versus the manual) during a descent. Don’t know which K you have, but in my 231 I slow to approach speed during the procedure turn or during vectors. Approach speed is 120 kts for me. In my aircraft that power setting is generally an MP of 24 point something, e.g. 24.5. That varies somewhat with OAT, in the summer it might be in the 25s. The important point is that I am managing to airspeed, not so much to a specific power setting. 120 kts is under gear extension speed. Then at the glideslope intercept I drop the gear and drop the MP initially to 14 something. 14+ will allow the plane to slow (with gear extended). When the airspeed is below Vfe (100 kts) I put in half flaps. When the airspeed is approaching 90 kts down the slope I adjust the power to about 19. That maintains 90 down the slope, which I use for all approaches. The power setting once on the slope is somewhat variable. The plane is right on the cusp of the power curve, so some days it wants a little less power, some days more. Again, I am managing to a stable airspeed. RPMs and fuel flow are not very important at this point in the power curve, in other words, the plane does not go significantly faster or slower if you push the prop full forward or enrich the mixture, the MP is the governing factor. That’s how it works for me. I use these settings for both AP and hand flying approaches.

Somewhere near the bottom of the slope you are going to have to change power settings again, obviously, because landing speed is 75, not 90, but once at 90 down the slope the plane will slow fairly quickly to 75 with a drop in power.

Edited April 19, 2023 by jlunseth

[kortopates]

The more complicated aspects of VNAV come in with different kinds of approaches which have similar concerns for VNAV that we originally had for when do we enable the approach mode and/or switch to LOC from GPS mode. As a result LOC approaches are the most complicated to master while any form of pure GPS approach guidance is pretty trivial once you understand the constraints.

So for example, if we're doing any GPS approach (LPV, LNAV, LP with or without +V), we can follow the flight plan constraints before the FAF using VNAV. Its as simple as starting with a using VNAV to get down to the IAF and follow the constraints to FAF by setting the altitude to the FAF constraint altitude. Then once we've started the VNAV descent we can next Arm Approach mode on the GFC AP and the now the the AP will seamlessly  transition from VNAV down to FAF and then GP down to the missed (if there is any form of vertical guidance).

But on the LOC approaches with lots of step downs and no GP is where VNAV really shines but is also the most complicated use case since it involves switching to LOC mode from GPS and when we do, we'll loose VNAV guidance since its GPS. Before VNAV and doing a full LOC approach (not VTF) I would switch to LOC mode as soon as I was turning onto the final approach course to navigate past several step down before the FAF - exactly as called out in AC 90-108. The Garmin GTNs and earlier GNS's will also automatically switch to LOC mode from GPS when you have turned on automatic switching in Setup, once turned inbound onto the final course completing a procedure turn/course reversal - also per AC 90-108. So to really take advantage of VNAV capabilities on such approaches you have to disable automatic switching so you can keep it in GPS mode as you descend through the multiple step downs prior to the FAF and then manually switch to LOC mode on your way to the FAF. Although counter to AC 90-108 its legal to use GPS prior to the FAF as along as we're using LOC crossing the FAF; just don't wait till the last second so you can verify all is as expected. As an example, consider the KSEE LOC-D approach using the GPS VNAV to descend to and past BARET IAF (with or without PT) all the way to just prior to FAF SAMOS and then go to LOC mode and now switch to a largely manual VS mode to descend past the FAF to minimums. 

 

  

[BornToFly]

To the Original Poster:

One workaround technique would be to create an Along Track Offset (ATO) waypoint one or two miles BEFORE the FAF.   Set the altitude constraint for this new Along Track Offset waypoint to be the same altitude as the FAF.  What will happen is that VNAV will fly you down to the ATO waypoint and level you off in ALTS hold mode.  You will cruise along in level flight until you intercept the GP and head the rest of the way down.

This will allow you a mile or two of level flight before the FAF to slow down, get the gear and/or flaps down and otherwise become configured for the approach before you start down the GP at the FAF.   

As I mentioned, it is a workaround of sorts, but I have used it several times for different reasons and it works very well.

[donkaye, MCFI]

3 hours ago, BornToFly said:

To the Original Poster:

One workaround technique would be to create an Along Track Offset (ATO) waypoint one or two miles BEFORE the FAF.   Set the altitude constraint for this new Along Track Offset waypoint to be the same altitude as the FAF.  What will happen is that VNAV will fly you down to the ATO waypoint and level you off in ALTS hold mode.  You will cruise along in level flight until you intercept the GP and head the rest of the way down.

This will allow you a mile or two of level flight before the FAF to slow down, get the gear and/or flaps down and otherwise become configured for the approach before you start down the GP at the FAF.   

As I mentioned, it is a workaround of sorts, but I have used it several times for different reasons and it works very well.

Several GTN System Updates ago Garmin resolved the issue and VNAV works through GP intercept where [V] changes to [GP].

[Fly Boomer]

8 hours ago, BornToFly said:

As I mentioned, it is a workaround of sorts, but I have used it several times for different reasons and it works very well.

Welcome to MooneySpace!

[Marc_B]

I've looked into the "Transition to approach" a little more and now I'm even more confused.  I think that the VNAV transition to approach is default enabled.  But at some point my shop corrected this to "disabled" per previous install instructions.  This is the only thing I can come up with that explains why the altitude constraints at the FAF are now white and unable to be changed/confirmed.  So I'm currently back to VNAV only functioning down to FAF-1, leveling off, and capturing the GP/GS at higher altitude along the segment prior to the FAF unless I manually descend using VS down to the FAF constraint.

The only way I could find that would allow the FAF altitude constraint to be active was to reenable "Transition to Approach".  Not sure if any of you have seen an all clear for enabling this for the GFC500.  (best I can ascertain is that initially GFC500 could not follow both the VNAV and GP/GS concurrently so sometimes the GP/GS would be missing if the transition to approach was enabled...but Garmin has made several updates since 2019 when this behavior was described I think)

Can anyone confirm that with GFC500/G5/GTN Xi that the VNAV will descend to the FAF and smoothly pick up the GP/GS? 

 

[hubcap]

4 hours ago, Marc_B said:

Can anyone confirm that with GFC500/G5/GTN Xi that the VNAV will descend to the FAF and smoothly pick up the GP/GS? 

 

Yes. I recently flew an approach where I was given the following clearance - "Cleared direct WOMPO....Cross 20miles east of WOMPO at 6,000...cleared for the RNAV 36 Approach into KLXT"

The GFC 500 flew to the TOD then VNAV kicked in and it crossed 20 miles east of WOMPO at 6000' crossed WOMPO at 3000' while turning onto approach heading, then crossed the FAF at 2600' and switched to glidepath at the FAF.

It was very impressive to me, and it was in solid IMC until about 400'

[midlifeflyer]

7 hours ago, Marc_B said:

Can anyone confirm that with GFC500/G5/GTN Xi that the VNAV will descend to the FAF and smoothly pick up the GP/GS? 

 

Yes. It's one of my standard tasks when doing transition training into that avionics combination. Love it.

I have a video doing it in a G1000/GFC700 setup. It's even better with the GFC500/G5/GTN Xi combo than the flavor of G1000 I used.

But VNAV doesn't "descend to the FAF". VNAV descends to the fix before the FAF. When the FAF becomes the active waypoint, the V caret morphs to the G caret and you pick up the glidepath, which brings you down to the FAF and beyond (assuming, of course both were armed). That's why the FAF altitude is white. It's not part of the VNAV profile.

[LANCECASPER]

12 hours ago, midlifeflyer said:

I have a video doing it in a G1000/GFC700 setup.

I would like to see that. Do you have that posted anywhere?

[donkaye, MCFI]

Marc's persistence paid off, but it's not what I wanted to hear.  Here is what Garmin said today about "Transition to Approach" for the GFC 500.

From: Aviation Training Webinar <AviationTraining.Webinar@garmin.com<mailto:AviationTraining.Webinar@garmin.com>>
Sent: Wednesday, September 20, 2023 8:15 AM
To: Marc Breen 
Subject: RE: Transition to Approach with GFC500

The GTN by itself and when coupled to a GFC 600 is capable of using VNAV all they way to the final approach fix.

I may have misspoken about transition to approach when paired with a GFC 500. (my highlight)  As it stands right now the GTN install manual has the installer turn off transition to approach when paired with a GFC 500.

I have been talking to both the GTN and GFC teams to see if this can get updated but so far this feature has to be turned off.  If and when they update this, then it will simply be a setting on the GTN that will allow this feature.

I apologize for any confusion this may have caused.

If you have any other questions please let me know and If you want, time permitting I can still give you a call.

Best Regards,

Matthew Clark
Senior Aviation Pilot Instructor
1200 E. 151st Street | Olathe, KS 66062
Email: Aviation.Training@garmin.com<mailto:Aviation.Training@garmin.com>


 

[midlifeflyer]

4 hours ago, LANCECASPER said:

I would like to see that. Do you have that posted anywhere?

Sure...  The most significant difference (in what's shown here) between the GFC700 in this G1000 and the GFC500 is the 5-minute gotcha (about 6:10 into the video). The GFC700 here prompts you to reconfirm that you really want VNAV if you arm in more than 5 minutes from top of descent. If you are outside that parameter, the white VPTH-armed indication will flash a minute out from TOD. If you don't reconfirm my tapping VNV again, it will turn off. The GFC500 does the opposite. It doesn't matter how far out you arm it, the GFC assumes you want it. The white armed indication will flash green when it begins to engage and steady green once engaged.

 

 

[kortopates]

Marc's persistence paid off, but it's not what I wanted to hear.  Here is what Garmin said today about "Transition to Approach" for the GFC 500.

From: Aviation Training Webinar AviationTraining.Webinar@garmin.commailto:AviationTraining.Webinar@garmin.com>>
Sent: Wednesday, September 20, 2023 8:15 AM
To: Marc Breen mailto:>>
Subject: RE: Transition to Approach with GFC500

The GTN by itself and when coupled to a GFC 600 is capable of using VNAV all they way to the final approach fix.

I may have misspoken about transition to approach when paired with a GFC 500. (my highlight)  As it stands right now the GTN install manual has the installer turn off transition to approach when paired with a GFC 500.

I have been talking to both the GTN and GFC teams to see if this can get updated but so far this feature has to be turned off.  If and when they update this, then it will simply be a setting on the GTN that will allow this feature.

I apologize for any confusion this may have caused.

If you have any other questions please let me know and If you want, time permitting I can still give you a call.

Best Regards,

Matthew Clark
Senior Aviation Pilot Instructor
1200 E. 151st Street | Olathe, KS 66062
Email: Aviation.Training@garmin.commailto:Aviation.Training@garmin.com>


 

Thanks for sharing. Unfortunate indeed. Luckily my installer chose not to disable this feature and i have thoroughly vetted it and have grown to really appreciate. It’s especially useful to me on our Localizer approach with many step downs but no GS. Of course it enables getting me to FAF on VNAV staying in GPS mode, where i then have to switch to LOC mode and specify a descent rate to continue to the MAWP. In fact VNAV has changed how i fly a full localizer approach - i no longer go to localizer mode till i close to the FAF as long as i can leverage VNAV. Disabling the transition to the FAF would probably reduce the utility on approaches by 50% or more.
So now i ponder why i would want to disable is. It is after all merely implementing the FAA guidance to fly a stabilized approach with a continuous descent angle (CDFA) while meeting all step down fix altitude constraints - just now we’re extending it further out from the FAF.

On an RNAV approach it’s pure elegance in its simplicity, since once VNAV has started I merely have to touch the Aprch button to Arm the GS to capture it seamlessly from VNAV approaching the FAF.

Sent from my iPhone using Tapatalk

[donkaye, MCFI]

Paul, I totally agree with you, since it appears be a paperwork issue, but as of now it is not correct on the GFC 500 to have Transition to Approach Enabled.

[Marc_B]

In speaking to my avionics shop they said that the GTNs default to Transition to Approach enabled. But the install manual says to disable it for all but GFC600 (six) applications. 

In  trying to figure this out I read somewhere here or on BT that the GFC500 had cases where there may have been issues?? following both the VNAV and GP, and GP/GS wouldn’t show up, but  it was fixed by disabling Transition to Approach.  The odd thing is there have been some random times when I selected a Visual approach and the advisory GP didn’t show up. So no clue if these are related or just a quirk of what I was doing at the time. 
I’ll see if I can find the thread…. Garmin, nor my avionics shop had any idea if there would be an issue with turning it on or not for the GFC500.  

EDIT: From Garmin - You can expect this to be available in the near future as we are currently awaiting STC approval for this feature on G5 and GI-275 installations. I don't have an exact timeline, but my best-educated guess would be 2024. 

Edited September 21, 2023 by Marc_B

[Marc_B]

Not sure if it’s a faux pas to post a link to BT here… mods feel free to delete if so.

NO GP on LPV Approach. SOLVED

https://www.beechtalk.com/forums/viewtopic.php?f=7&t=199658

this is one of the only things that I could find pertaining to the Transition to Approach for the GFC500, but it was with GI275. 

[kortopates]

3 minutes ago, Marc_B said:

In speaking to my avionics shop they said that the GTNs default to Transition to Approach enabled. But the install manual says to disable it for all but GFC600 (six) applications. 

In  trying to figure this out I read somewhere here or on BT that the GFC500 had cases where there may have been issues?? following both the VNAV and GP, and GP/GS wouldn’t show up, but  it was fixed by disabling Transition to Approach.  The odd thing is there have been some random times when I selected a Visual approach and the advisory GP didn’t show up. So no clue if these are related or just a quirk of what I was doing at the time. 
I’ll see if I can find the thread…. Garmin, nor my avionics shop had any idea if there would be an issue with turning it on or not for the GFC500.  

I can only tell you mine works flawlessly in this regard. It take learning the constraints on VNAV before I got to work as expected, but to me I consider it very safe to use the transition since I can always see both the background hollow GS position as I am approaching the FAF will VNAV is still active thus if there was ever a concern I can also take immediate action. But what its doing is so basic anyway in meeting each step down fix altitude constraint including the FAF altitude.

No all visual approaches have vertical guidance; especially where I live in the southwest due to terrain. But there are constraints in activating the vertical guidance on a visual too,

[Marc_B]

@kortopates Next time you fly a visual approach can you see if the Transition to Approach enabled allows the 3NM waypoint altitude reference to be an altitude constraint for VNAV approach down to?  I'm curious.

[Marc_B]

So found out that with VNAV transition to approach that it allows the 3NM final waypoint on a Visual Approach to have an active altitude constraint.  Where as if this is disabled it shows you a reference constraint but doesn't allow VNAV to use this.  So with Transition to Approach enabled it allows you to use VNAV from your altitude, direct to 3NM waypoint, APR armed, and it will smoothly descend and perfectly capture the advisory glidepath.

Basically Transition to approach "turns on" the altitude constraint for the FAF and for the 3NM visual approach waypoints so that VNAV can use these.

[PT20J]

I reached out to Joey Ferreyra at Garmin and asked why the GFC 600 is approved for transition to approach but the GFC 500 is not. He didn't directly answer this, but he did say that the development team is looking at it and may approve it in a future release. He didn't say if a software change might accompany the release. 

Skip

[PeteMc]

5 hours ago, PT20J said:

why the GFC 600 is approved for transition to approach but the GFC 500 is not.

Always amazes me to see the total lack of coordination with some companies.  As much as I like Garmin, I also think they are one of the worst managed companies there is.  

The GTN Xi with no Bluetooth.  The belief that anyone buying a GTN would not by a GNC as a Comm 2 so there was no need for them to talk to each other - even though the Engineers brought it up and it was still nixed.  I'm sure others here can add multiple faux pas to the list.

 

[ArtVandelay]

Always amazes me to see the total lack of coordination with some companies.  As much as I like Garmin, I also think they are one of the worst managed companies there is.  
The GTN Xi with no Bluetooth.  The belief that anyone buying a GTN would not by a GNC as a Comm 2 so there was no need for them to talk to each other - even though the Engineers brought it up and it was still nixed.  I'm sure others here can add multiple faux pas to the list.
 

The experimental division (GFC500, G3X) and certified division (500 TXi, GFC 600) are effectively different companies under the corporation umbrella.

[PeteMc]

6 hours ago, ArtVandelay said:

different companies under the corporation umbrella.

Oh, I get being a different Co/Division.  That doesn't bother me at all for a company that size.  But they have NO coordination.  There should be a HQ department that keeps track of what they're all doing and what they're all looking to release in the next 12-24 months.  Other than Patches for BUGS, it's not like they develop the new features in a week.  (Not talking writing the code, but actually the planning and research.)  So when they see something for the 600 that would work for the 500, then they let them know for continuity.  Same with Experimental vs. Certified....  curious if there's anything the Exp. versions can do that the Cert. versions could benefit by?  That would be another perfect example of lack of coordination.  Right now we Users will be the only way the Cert. crowd will hear about a great Feature Request.  That's pretty bad IMHO.