"NO GP" message on LPV approach

[slowflyin]

Good morning,

The last couple of times I've flown the RNAV 4 approach into KBKT I've received a "No GP" message.  The GI275 displayed LPV.  The KFC 150 captured the localizer but not the GP.  A GTN 750 is navigating.  I fly this approach often and find this to be an intermittent problem.   Any advice would be appreciated.  

[flyboy0681]

Found this discussion on Beechtalk, sounds identical..

 

https://www.beechtalk.com/forums/viewtopic.php?f=21&t=124452

[PeteMc]

Any GPS or Approach NOTAMS?  Any of the other locals talking about it?

[slowflyin]

16 minutes ago, PeteMc said:

Any GPS or Approach NOTAMS?  Any of the other locals talking about it?

No other issues I can find.  No NOTAMS other than 5G and RA stuff.     

[slowflyin]

23 minutes ago, flyboy0681 said:

Found this discussion on Beechtalk, sounds identical..

 

https://www.beechtalk.com/forums/viewtopic.php?f=21&t=124452

Thanks, I'll check it out.

[Ragsf15e]

1 minute ago, slowflyin said:

His issue seemed to be persistent.  Mine works most of the time with intermittent failure.  Leads me to think it's not in the settings.  ???

And obviously you’ve tried lpv approaches elsewhere and not had intermittent issues?  Is it just this approach (local issue) or all lpvs (gtn issue)?

One thought with zero research and evidence (it’s the internet, get what you pay for), possibly it’s losing waas there? I’d think it would annuciate that? Is it is it reverted to LNAV which doesn’t need waas?

[slowflyin]

16 hours ago, slowflyin said:

Thanks, I'll check it out.

This thread was very helpful.  Mine was installed with "transition to approach" enabled.   I don't have the 600 AP, so per Trek's advice I disabled it.  I haven't had a chance to test but I'll report back.  Thanks again for the link.   I searched BT several times but missed it!

[slowflyin]

18 hours ago, Ragsf15e said:

And obviously you’ve tried lpv approaches elsewhere and not had intermittent issues?  Is it just this approach (local issue) or all lpvs (gtn issue)?

One thought with zero research and evidence (it’s the internet, get what you pay for), possibly it’s losing waas there? I’d think it would annuciate that? Is it is it reverted to LNAV which doesn’t need waas?

Ragsf15e,

Thanks for the reply.   I can only recall having the same issue somewhere else one time but I can't confirm the unit was annunciating LPV.   It's happened more at my home airport because I almost always fly the approach after a trip.  ATC won't clear me for the visual if the restricted airspace East of the airport is hot.  I'm hoping I may have solved the issue via the BT thread fix.  Thanks again.  

[David Lloyd]

Flying to a nearby airport, I loaded the LPV approach shortly after startup.  Got on the approach to that airport and no GP.  Subsequent approaches including the same approach were all okay.  Thinking back, that happened once before, same scenario, loaded an approach before the GPS had locked on to everything needed.

I could also see losing waas integrity even momentarily while the approach is loaded, preventing the GP from being used.

[PeteMc]

I thought the GPS decided as it got close to the IAF, but it may vary with manufacturer.

Might be worth shooting Garmin an email to see if this is a know issue and/or if the way you're loading the approach has anything to do with it.

[slowflyin]

7 minutes ago, PeteMc said:

I thought the GPS decided as it got close to the IAF, but it may vary with manufacturer.

Might be worth shooting Garmin an email to see if this is a know issue and/or if the way you're loading the approach has anything to do with it.

Upon reaching the IAF mine displayed LPV indicating it had made the choice.  Per the BT thread, Garmin setup defaults to "transition to approach enabled".  This only works if you have the Garmin 600 AP.   Per Trek with Garmin this is the problem.  I checked mine and sure enough it was enabled.  I have a King AP so I disabled it.  I'll test fly tomorrow.  The solution has worked for the BT guys.  I'm hopeful, fingers crossed.  Thanks for your thoughts.  

[Pinecone]

I had this happen with a Garmin 400.  Started an LPV approach and shortly after starting descent, got a no GP warning.  So I just changed to a LNAV approach.  It was VFR with a safety pilot, and I was above the step down fix, so not issues with terrain.

I always try to make sure I have an idea of what the LOC/LNAV profile is.  If I am not CLEAR, I climb and break off the approach.

[slowflyin]

14 hours ago, Pinecone said:

I had this happen with a Garmin 400.  Started an LPV approach and shortly after starting descent, got a no GP warning.  So I just changed to a LNAV approach.  It was VFR with a safety pilot, and I was above the step down fix, so not issues with terrain.

I always try to make sure I have an idea of what the LOC/LNAV profile is.  If I am not CLEAR, I climb and break off the approach.

Great comment.  This is my practice as well.   Even when the GS is active knowing the step downs offer increased SA.

[slowflyin]

Update- flew the RNAV 04 approach this morning with the same outcome.  LPV followed by NO GP.   I rechecked my settings to confirm "transition to approach" is not enable per the BT thread.  Now I'm wondering if VCALC or VNAV is the appropriate setting.   Frustrated

[RobertGary1]

On 6/23/2022 at 2:36 PM, Pinecone said:

I had this happen with a Garmin 400.  Started an LPV approach and shortly after starting descent, got a no GP warning.  So I just changed to a LNAV approach.  It was VFR with a safety pilot, and I was above the step down fix, so not issues with terrain.

I always try to make sure I have an idea of what the LOC/LNAV profile is.  If I am not CLEAR, I climb and break off the approach.

If you were vfr you probably could have selected the visual to the runway and then you’d have glide slope.  

[PT20J]

1 hour ago, slowflyin said:

Update- flew the RNAV 04 approach this morning with the same outcome.  LPV followed by NO GP.   I rechecked my settings to confirm "transition to approach" is not enable per the BT thread.  Now I'm wondering if VCALC or VNAV is the appropriate setting.   Frustrated

I don't believe VNAV/VCALC should affect have any effect on GP intercept. Mine is set to VNAV and it will switch to GP on an LPV approach. You can disable VNAV in the Utilities screen in Normal Mode if you want to see if that is somehow interfering.

EDIT: I would call Garmin and find out what conditions cause a No GP indication. I cannot find this message listed in the Rev D GTN PIlot's Guide at all.

Skip

[PeteMc]

1 hour ago, slowflyin said:

Now I'm wondering if VCALC or VNAV is the appropriate setting. 

What @PT20J said....   Any VCALC or VNAV settings you have may work to get you down to an IAF, but then will be discarded once your on the Approach. 

And I just skimmed the posts again and I didn't see any mention of WHERE you're intercepting the Lateral Guidance?  I don't see this as being an issue for an LPV approach, but just curious if they're turning you in close to the FAF on the times it fails?  Auto switch from GPS to ILS can get messed up, so it's just an out of the box thought.

 

[slowflyin]

1 minute ago, PeteMc said:

What @PT20J said....   Any VCALC or VNAV settings you have may work to get you down to an IAF, but then will be discarded once your on the Approach. 

And I just skimmed the posts again and I didn't see any mention of WHERE you're intercepting the Lateral Guidance?  I don't see this as being an issue for an LPV approach, but just curious if they're turning you in close to the FAF on the times it fails?  Auto switch from GPS to ILS can get messed up, so it's just an out of the box thought.

 

Center gives me direct to the IAF YAKLU.   I was on a 355 heading and chose the no hold option.  Crossed at 3000.  KFC 150 captured the approach and the GI275 and GTN switched from terminal to LPV.   Pulled the power back to gear ext. speed and waited.  All seemed normal but no GS.  Then the "no gp" message about halfway to wovup.

rnav04.PDF

[PT20J]

I would definitely discuss with Garmin. The Pilot’s guide does not mention such an indication and suggests that the GTN should downgrade to LNAV. Garmin documentation is sometimes far from perfect, but in this case I would suspect that this error is not expected in normal operation. 

 

[slowflyin]

2 hours ago, PT20J said:

I would definitely discuss with Garmin. The Pilot’s guide does not mention such an indication and suggests that the GTN should downgrade to LNAV. Garmin documentation is sometimes far from perfect, but in this case I would suspect that this error is not expected in normal operation. 

 

Thanks for the reply.  I concur, I'll reach out to Garmin.  

[slowflyin]

Update

After changing my "transition to approach" to disabled things still didn't work.  I stumbled across another thread where an installer referenced the need to disable the CDI select on the GTN when utilizing dual GI275s.   I gave it try and flew the approach three times this morning utilizing three different entry methods, and all worked flawlessly!  Thanks for all your comments!   

 

[PeteMc]

1 hour ago, slowflyin said:

all worked flawlessly! 

Excellent!

 

[AIREMATT]

2 hours ago, slowflyin said:

Update

After changing my "transition to approach" to disabled things still didn't work.  I stumbled across another thread where an installer referenced the need to disable the CDI select on the GTN when utilizing dual GI275s.   I gave it try and flew the approach three times this morning utilizing three different entry methods, and all worked flawlessly!  Thanks for all your comments!   

 

Now that you have all the settings that work, write them down somewhere. When I have my GNX375 or dual G5 software updated, the update somehow changes or resets settings and the problems come back. This way you can ensure your settings are correct after any software updates or changes to your hardware going forward. Avionics shop spent some time on the phone with Garmin due to some of the problems that would occur—all due to unknown/unexpected setting changes after updates.

[PT20J]

2 hours ago, AIREMATT said:

Now that you have all the settings that work, write them down somewhere. When I have my GNX375 or dual G5 software updated, the update somehow changes or resets settings and the problems come back. This way you can ensure your settings are correct after any software updates or changes to your hardware going forward. Avionics shop spent some time on the phone with Garmin due to some of the problems that would occur—all due to unknown/unexpected setting changes after updates.

The GTNs have a configuration module and there is an option in configuration mode to update it. This should be done to save all the settings. This is important if you ever have to swap out the unit for repair.

There is a configuration log in the maintenance manual (available on Garmin's website) that should have been completed by the installer. You should not any changes on that and keep it with your airplane file.

Skip

[PeteMc]

43 minutes ago, PT20J said:

The GTNs have a configuration module

FYI...  There is also an internal SD Card (fixed in place) as well as the internal memory.   If you ever have issues with wacky settings coming back even AFTER replacing the Config. Module, you may have a corrupt file on that internal SD Card.  We kept fixing all the settings and saving to the Config. Module and the problem would come right back.  Finally did a Factory Reset and re-configured from scratch.  Later the Avionics guy found out from Garmin it was likely we had a corrupt file in that attached SD Card which was messing up both the internal and Config. Module settings.