VOR 13 LNAV +V

[donkaye]

I've attached a photo taken from the GTN 750 Xi today showing this approach into KSNS. I flew this approach in VFR conditions today and got Vertical navigation.  There was no caution like on ILS approaches about flying this completely in GPS mode.  If one were to switch to VOR at the FAF the VNAV would disappear.  This approach does not appear in the legacy GTN 750.

The question:  Can this approach be flown legally totally with GPS with no monitor of the VOR?

Many of us with the G500 TXi and 750/650 don't have a second VOR head in the panel, so could only monitor the VOR with a bearing pointer.

I have never seen an approach like this before.

[toto]

I may be missing something in the new interface, but I don’t see a photo of the approach?

[midlifeflyer]

Got me curious too. And confused. The only VOR approach I am aware of into SNS is the VOR 13,  a pretty vanilla VOR approach. And if that's the one, it's been in the database for a long time. Yes, the rules still require VOR raw data to be monitored while using GPS to fly the approach. Or are you asking about the addition of +V? I haven't seen that before but if it's new to the city, it wouldn't surprise me. That's just advisory to assist in establishing a stabilized CDFA.

Edited July 3, 2021 by midlifeflyer

[PT20J]

The approach plate says VOR RWY 13. (I know it well; used to practice it a lot when I lived in the Bay Area). Since it does not say "or GPS", my understanding is that there is no overlay and that you must use the SNS VOR for course guidance from the FAF inbound.

I tried it on the Garmin Xi sim. V20.12. The approach selections for KSNS included VOR 13 LNAV+V. When I loaded it I got the message NOT APPROVED FOR GPS. GPS guidance is for monitoring only. Load approach? When I flew it starting at SEWOF, I got a Message annunciation approaching SAMES (FAF) and the message says SELECT FREQUENCY. Select appropriate NAV frequency for approach. Seems that it was pretty clearly telling me that I had to use the VOR for navigation rather than the GPS. So the vertical guidance would only be good to the FAF, I think. Did you not get the messages?

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[midlifeflyer]

7 minutes ago, PT20J said:

The approach plate says VOR RWY 13. (I know it well; used to practice it a lot when I lived in the Bay Area). Since it does not say "or GPS", my understanding is that there is no overlay and that you must use the SNS VOR for course guidance from the FAF inbound.

I tried it on the Garmin Xi sim. V20.12. The approach selections for KSNS included VOR 13 LNAV+V. When I loaded it I got the message NOT APPROVED FOR GPS. GPS guidance is for monitoring only. Load approach? When I flew it starting at SEWOF, I got a Message annunciation approaching SAMES (FAF) and the message says SELECT FREQUENCY. Select appropriate NAV frequency for approach. Seems that it was pretty clearly telling me that I had to use the VOR for navigation rather than the GPS. So the vertical guidance would only be good to the FAF, I think. Did you not get the messages?

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When I tried it on the trainer, I got exactly the same as you. It's what I would expect in the real world too.

As an aside, these days, you  can use GPS for lateral guidance in the FAC, so long as you monitor the VOR raw data. That change took place several years ago.

[PT20J]

9 minutes ago, midlifeflyer said:

As an aside, these days, you  can use GPS for lateral guidance in the FAC, so long as you monitor the VOR raw data. That change took place several years ago.

Good point. I'm thinking about how I would set this up. You could tune SNS on the GTN Xi NAV or on NAV 2 if you have a way to display either. I think the GTN GPS will continue to provide LNAV+V guidance as I believe the SELECT FREQUENCY message is only advisory. The problem is that with many glass panel configurations you would only have one active CDI. You could put the VOR on a bearing pointer and perhaps that would be enough for "monitoring." It's such an unusual situation that it's probably not worth a lot of thought, but it does point out that a lot of things were pretty simple to figure out in the bad old days of dual VOR and a DME.

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[midlifeflyer]

Just now, PT20J said:

Good point. I'm thinking about how I would set this up. You could tune SNS on the GTN Xi NAV or on NAV 2 if you have a way to display either. I think the GTN GPS will continue to provide LNAV+V guidance as I believe the SELECT FREQUENCY message is only advisory. The problem is that with many glass panel configurations you would only have one active CDI. You could put the VOR on a bearing pointer and perhaps that would be enough for "monitoring." It's such an unusual situation that it's probably not worth a lot of thought, but it does point out that a lot of things were pretty simple to figure out in the bad old days of dual VOR and a DME.

Skip

You'd use the bearing pointer as the "green needle." It's pretty much SOP with a G1000. In the case of the GTN and xi, the AFMS limitations language reads,

When using the VOR or ADF receiver to fly the final approach segment of a VOR or NDB approach, GPS may be the selected navigation source so long as the VOR or NDB station is operational and the signal is monitored for final approach segment alignment.

ADF and RMI have used bearing pointers for decades. Good enough then for primary, good enough now for monitoring. That's basically what bearing pointers are. An RMI pointing to the VOR. 

As I think about it, it's probably the addition of the "LNAV+V" to the entry Don is referring to. That naming, as well as the addition of advisory vertical guidance to VOR approaches is new in the GTNxi.

[PT20J]

10 minutes ago, midlifeflyer said:

As I think about it, it's probably the addition of the "LNAV+V" to the entry Don is referring to. That naming, as well as the addition of advisory vertical guidance to VOR approaches is new in the GTNxi.

I think you are right. I never saw an LNAV+V on a non-overlay approach before, but, hey why not? 

[donkaye]

So the answer to my question was provided to me by Bruce Williams from the same posting I made on BeechTalk.  It was substantial and can be found here:

https://bruceair.wordpress.com/2021/05/14/vor-approaches-with-lnavv/

Basically this is an undocumented addition to the GTN Xi series.  Apparently, unlike the App version of the XI, there were no cautions associated with this approach in the actual GTN 750 Xi.  It can be fully flown with GPS provided a CDI OR Bearing Pointer is used to monitor the approach, which I did today.

Actually, that's pretty exciting that you can run a VOR approach and get vertical guidance on some approaches with the GTN Xi series of navigators.  It's getting closer and closer to the end of "dive and drive" on an approach.

Thanks to all who contributed their input above.

[midlifeflyer]

13 hours ago, PT20J said:

I think you are right. I never saw an LNAV+V on a non-overlay approach before, but, hey why not? 

Why not indeed.

If you check, even with an older GNS, your GPS is annunciating LNAV when you load  and fly a VOR approach, no matter whether you use it to feed your CDI or just track your position while the CDI is fed by VLOC mode.  So that's always been there. I an FAA goal is to encourage CDFA, for stability (it is), and we are allowed to use GPS for the primary CDI (we are), there's really no good reason not to use it with a VOR or NDB approach*. 

(*But not a LOC approach. The FAA still frowns on that GPS as primary on a LOC-based course,  probably because LNAV lateral guidance doesn't meet or exceed that spec).