Garmin VNAV on an LNAV approach

[iamtheari]

I was out flying practice approaches today and decided to try an LNAV to circling minimums. I have a G3X Touch experimental and a GTN 650 Xi. It should behave the same as a GFC 500 autopilot in case anyone knows how it is supposed to work on those.

Starting before the IAF, I set the altitude preselect to the circling minimums and activated VNAV mode. The autopilot and GTN did the step downs one at a time until the FAF, but after that the approach has a step down fix followed by the minimums and the VNAV didn’t sequence to either altitude. The glide path diamond came down through our altitude at the FAF but of course I didn’t activate APR mode because I didn’t want to fly the LPV.

Is there a step I probably missed to sequence down to the step down fix and/or LNAV minimums after the FAF when I want to fly a nonprecision version of a GPS approach, or do I have to use other autopilot vertical modes to get there?

 

[midlifeflyer]

Enroute VNAV does not go further than the FAF.

 

[Brad Z]

I assume there wasn’t a LNAV+V option for this approach?

 

[iamtheari]

I assume there wasn’t a LNAV+V option for this approach?

It’s an LPV so the avionics wanted me to fly the glidepath. I don’t know how to force an LNAV+V depiction when LPV is working.

 

[iamtheari]

Enroute VNAV does not go further than the FAF.

That makes sense, and it’s something I’ll have to practice with to get a feel for the best way to fly a non precision approach with all this fancy glass.

 

[wayneda40]

Enroute VNAV does not go further than the FAF.

As Mark says... ^^ This ^^... you'll need to engage APR to capture the Glideslope/Glidepath as you come into the FAF.
Graphically...

 

[eman1200]

Enroute VNAV does not go further than the FAF.

 

[midlifeflyer]

That makes sense, and it’s something I’ll have to practice with to get a feel for the best way to fly a non precision approach with all this fancy glass.

If you really want to emulate a WAAS failure, your best bet is to turn WAAS off in the GTN (so long as your transponder has its own WAAS for ADS-B Out reporting). Menu > System > GPS Status > SBAS >

 

[iamtheari]

As Mark says... ^^ This ^^... you'll need to engage APR to capture the Glideslope/Glidepath as you come into the FAF.
Graphically...

That's how I run it when I am flying a coupled approach with vertical guidance. But the scenario here was deliberately flying to nonprecision (circling) minimums.

If you really want to emulate a WAAS failure, your best bet is to turn WAAS off in the GTN (so long as your transponder has its own WAAS for ADS-B Out reporting). Menu > System > GPS Status > SBAS >
View attachment 133880

That works to simulate a WAAS failure. But my transponder gets its position from the GTN and I don't particularly want to turn off WAAS to fly a circling approach in normal conditions.

Skyvector seems to be down at the moment so I don't have ready access to grab and share the chart. I was flying the RNAV 12 at S25. The relevant fixes and altitudes are:

4500-6000 holding altitude at GERHE (IAF)
4100 for 5.3 nm to WILSN (FAF)
2960 for 3.6 nm to WATLI
2700 for 2.7 nm to RW12, category A circling minimums
2364 LPV DA

My VNAV sequenced down to 4100 at WILSN, consistent with prior posts above. Beyond that, I can think of three ways to fly this approach to circling minimums:

1. Use APR mode and let the autopilot follow the LPV glidepath down to almost 2700 and either engage altitude hold or just go missed if the runway isn't in sight. (Or continue to LPV DA and land downwind, a/k/a "alternate minimums." No, that's not for me.)

2. Use PIT or VS mode after WILSN and preselect altitudes for WATLI and the circling minimums. VS at 130 knots GS would need to be 500 ft/min to WATLI and 300 ft/min to the circling minimums.

3. Just dive and drive by hand the way I would have behind steam gauges without an autopilot.

Is there a better way?

 

[midlifeflyer]

That works to simulate a WAAS failure. But my transponder gets its position from the GTN and I don't particularly want to turn off WAAS to fly a circling approach in normal conditions.

Yes. That's why I added "(so long as your transponder has its own WAAS for ADS-B Out reporting)." Neither you nor the FAA want you to turn off your ADS-B Out.

I'm not sure what you meant by "normal conditions." Unless there was something wrong with it, I can even imagine wanting to turn it off except to practice a WAAS failure in "simulated conditions."

My VNAV sequenced down to 4100 at WILSN, consistent with prior posts above. Beyond that, I can think of three ways to fly this approach to circling minimums:

1. Use APR mode and let the autopilot follow the LPV glidepath down to almost 2700 and either engage altitude hold or just go missed if the runway isn't in sight. (Or continue to LPV DA and land downwind, a/k/a "alternate minimums." No, that's not for me.)

2. Use PIT or VS mode after WILSN and preselect altitudes for WATLI and the circling minimums. VS at 130 knots GS would need to be 500 ft/min to WATLI and 300 ft/min to the circling minimums.

3. Just dive and drive by hand the way I would have behind steam gauges without an autopilot.

Is there a better way?

I would not use anything other than #1 to get down to circling minimums. I would use #2 if there were a WAAS failure. Although I might increase my descent rate a bit beyond that of a 3 degree glidepath, I've pretty much given up dive and drive.

 

[iamtheari]

I'm not sure what you meant by "normal conditions." Unless there was something wrong with it, I can even imagine wanting to turn it off except to practice a WAAS failure in "simulated conditions."

By normal conditions I mean anything other than a simulated or actual equipment failure. In this instance, the normal operation would be flying an approach to circling minimums so I can circle to land when all of the navigational systems are working correctly. For example, if the big runway 9-27 at KRKS is closed for construction or the wind is blowing 210 @ 30G50, it might be necessary to fly the RNAV GPS RWY27 or RWY9 and circle to land on 21.

And you are exactly right: I have no desire to turn off navigational features of my airplane just to accomplish a circling approach. Or for any other reason except to simulate abnormal conditions like WAAS failure.

I would not use anything other than #1 to get down to circling minimums. I would use #2 if there were a WAAS failure. Although I might increase my descent rate a bit beyond that of a 3 degree glidepath, I've pretty much given up dive and drive.

Thank you. Presumably not the parenthetical add-on to #1. That's reserved for the specialest of YouTube pilots.