RNAV question

[newtie 533]

I'm trying out some RNAV approaches.

Last one was: KBFL > RNAV 34 gps LPV > KRDD

As I understand it, LPV provides vertical guidance. At the iaf at 3000ft. Iget the "vertical track" audio from my GTN750, buy no descent takes place? I tried the APPR button but that resulted in no descent either. Any help appreciated. Mark

[G550flyer 1,014]

Greetings, think of LPV as a precise vertical path. You can fly an RNAV in LNAV/VNAV mode and get vertical guidance. Now, in my early days, I flew a couple of jets that could not couple to VNAV. We could shoot RNAV approaches, but had to use LNAV minimums and fly it with vert speed. You just figure a descent rate based on the approach and ground speed flown. Now, the GV could do LNAV/VNAV all day, but could not use LPV minimums unless you had enhanced navigation. The good old G550 could do LPV minimums and we used the approach button to enable precision RNAV. I guess it depends on the aircraft you are flying. Utilizing the GTN750, the type aircraft and it's specifics will dictate how you would couple the vertical track if possible at all.

 

Rick

[newtie 533]

So, with LPV the glide slope indicator will not appear? That’s the problem I’m having.

[DavidP 1,413]

You should get a GS indication - what aircraft are you flying? Are you seeing "LPV" displayed on your GTN at or before the FAF?

[newtie 533]

1 hour ago, DavidP said:

You should get a GS indication - what aircraft are you flying? Are you seeing "LPV" displayed on your GTN at or before the FAF?

Hey David-

XtremePrototypes Lear 28 with RXP GTN750.

No LPV displyed on the GTN except for the Approach itself.

CDI is set to GPS. Only a red strip on the LH side of the heading indicator with GS printed on it.

[G550flyer 1,014]

14 hours ago, newtie said:

CDI is set to GPS. Only a red strip on the LH side of the heading indicator with GS printed on it.

I looked through their manual and as I suspect, this setup is like the DC10s and GIIIs I flew. As a rule of thumb, most aircraft that can couple to a GPS VPATH will have an FMS in it. It's not a hard and fast rule because those two jets I flew did have FMSs, but there were not any VNAV buttons to push to ARM/capture  the GPS VPATH. We had EFIS control knobs that allowed you to select VOR/ILS or LNAV for navigation sources for the EADI/EHSI. Even though we had the APP button in those jets, it was only for the G/S of an ILS. In this Lear setup, it's just like installing a GTN 750 inside of a Cessna. The GTN will display the proper indications for an RNAV approach, but, does the aircraft it's installed in has the avionics capability to couple to it. The XP Lear 28 gives you the ability to set the CDI source to GPS, but the G/S is strictly for the ILS. This is how my beloved DC10 and GIII worked. Now, in those jets, we would get the VPATH and the pregnant plus VPATH indicator, but we still could not couple the autopilot to it. We could couple to the lateral mode and then use vertical speed(V/S) to track the VPATH. Because of this, we had to use LNAV minimums.

Now, in the GV and G550, you actually have a VNAV button that allows the autopilot to be armed to capture and track the VPATH. I recommend that you fly your RNAV approaches with in GPS mode and use vertical speed to stay on the path. On a side note, there is probably a way to install a gauge or AP that can track the GTN path, but that's another can of worms lol.

[C2615 319]

I use RXP GTN750 and I can get almost any aircraft with "default enough" VHFNAV and AP to get a virtual G/S for APPR mode to capture, even without LPV.

[newtie 533]

After an afternoon of reading, I think I know enough to ask the right question.

I set up a flight in my Lear28 from KSTS to KRDD with and RNAV approach into Rwy34 (not at my computer right now). LPV was displayed in green on my GTN750 at the FAF.

My question is, how is vertical guidance displayed versus and ILS approach? I'm expecting to see a white triangle slowly descend to the middle dot and GPS switch to VLOC. I know they are different and just using this as an example. So, where do I look for vertical guidance and what does it look like?

Thanks, Mark

[jcomm 5,939]

According to my friend "COPILOT":

"This calculation is not based on any published surveyed approach data. It comes purely from an algorithm in the GTN software. Therefore, the best procedure is to simply intercept the vector at five or six miles at an altitude below the glide path at the intercept. First, activate the approach, then select a heading to intercept it at a location of your choice. When you turn on course select APR on the autopilot to arm the glideslope.

Please note that the glide path guidance during an RNAV LPV approach can be affected by certain settings in the GTN 750. For example, if LinkVor = False with the GTN 750, there is no Glide Path guidance during an RNAV LPV approach. Setting LinkVor = true prevents using the RMI needles for situational awareness when the GTN750 is in GPS mode. This is because if the aircraft autopilot is custom made to directly command the aircraft controls and if the autopilot is only based on the VOR variables then the GTN needs to override them."

There are also many youtubes, but you can watch this one in particular:

Garmin GTN750 +G5 RNAV/LPV approach with VNAV (youtube.com)

🙂

Edited January 3 by jcomm

[newtie 533]

8 hours ago, jcomm said:

According to my friend "COPILOT":

"This calculation is not based on any published surveyed approach data. It comes purely from an algorithm in the GTN software. Therefore, the best procedure is to simply intercept the vector at five or six miles at an altitude below the glide path at the intercept. First, activate the approach, then select a heading to intercept it at a location of your choice. When you turn on course select APR on the autopilot to arm the glideslope.

Please note that the glide path guidance during an RNAV LPV approach can be affected by certain settings in the GTN 750. For example, if LinkVor = False with the GTN 750, there is no Glide Path guidance during an RNAV LPV approach. Setting LinkVor = true prevents using the RMI needles for situational awareness when the GTN750 is in GPS mode. This is because if the aircraft autopilot is custom made to directly command the aircraft controls and if the autopilot is only based on the VOR variables then the GTN needs to override them."

There are also many youtubes, but you can watch this one in particular:

Garmin GTN750 +G5 RNAV/LPV approach with VNAV (youtube.com)

🙂

Not sure what Copilot is saying? There isn't a LinkVor=false on my system. Is this to be added to an RXP .ini file or something?

[jcomm 5,939]

8 minutes ago, newtie said:

Not sure what Copilot is saying? There isn't a LinkVor=false on my system. Is this to be added to an RXP .ini file or something?

Supposedly, yes, a specific GTN option, but I never used the GTN sw, so...

LinkVOR and LinkOBS Settings - RXP GTN 750/650 Touch - The AVSIM Community

https://forums.x-plane.org/index.php?/forums/topic/217728-gtn-750-and-autopilot/&do=findComment&comment=1960658

Edited January 3 by jcomm

[newtie 533]

Solved. 'Connect GPS to VOR1 Indicator' must be checked in order for the GS to appear.

Thanks, Mark

[jkeye 152]

On 1/1/2024 at 6:20 PM, newtie said:

I'm trying out some RNAV approaches.

Last one was: KBFL > RNAV 34 gps LPV > KRDD

As I understand it, LPV provides vertical guidance. At the iaf at 3000ft. Iget the "vertical track" audio from my GTN750, buy no descent takes place? I tried the APPR button but that resulted in no descent either. Any help appreciated. Mark

I do not think LPV is simulated in P3D

[Bob Scott 7,721]

On 1/4/2024 at 3:29 PM, jkeye said:

I do not think LPV is simulated in P3D

LPV glidepaths are simulated, not by P3D itself, but in some advanced add-on RNAV units and/or aircraft panels.  The position data received from P3D via SimConnect serves as a proxy for the GPS satellite-derived position data.

[jkeye 152]

12 hours ago, Bob Scott said:

LPV glidepaths are simulated, not by P3D itself, but in some advanced add-on RNAV units and/or aircraft panels.  The position data received from P3D via SimConnect serves as a proxy for the GPS satellite-derived position data.

This is good to know.

Do you know which units and planes have this?

Thanks

Edited January 6 by jkeye