GTN 750 not sending ILS glideslope to G5s

[DWillens]

We just got a new 750xi and two G5s in a C182. I am in SoCal but the install was done in AZ. I noticed that the G5s will not show ILS glideslope info (the NO GS flags are showing) although it is showing and tracking the localizer. The 750 is on VLOC, the Nav 1 is correctly tuned, and the G5s are sourced to NAV1. I do not believe this is a user error. I tried this on multiple different ILSs at different airports and the problem is consistent. RNAV glidpaths show fine.

When we took it back to the installer, he did some testing and cleaned the NAV antenna of some corrosion, tested it on the ground and it seemed to be working. We took it up and tried to shoot an ILS approach and again, no glideslope.

To make matters more interesting, the old 530 that is now Nav2 is picking up both localizer and glideslope on the G5s, which makes me believe it is not an antenna issue.

When we called the guy back and told him it still wasn't working, he basically said "there's no way it could be a configuration error, I don't know what it could be and I don't really have an answer for you at this point."

Without getting into the whole "you should demand the guy figure it out etc etc" issue, does anyone have any idea what might be the problem here? I am convinced that there is a data connection problem, or a configuration error in the 750 setup, or something. The installer says "that's not possible."

Also, I get it: very rarely would it be necessary to shoot an ILS when you have RNAV LPV, and if necessary we could use the #2. But the point is that it should work, it cost a lot of money.

Any tips would be appreciated.

Damon
KTOA

 

[Mongoose Aviator]

Also, I get it: very rarely would it be necessary to shoot an ILS when you have RNAV LPV, and if necessary we could use the #2. But the point is that it should work, it cost a lot of money.

Your money. 100%. You paid for a solution that is reasonable and should work. I would not even let that thought cross your mind. I absolutely want my ILS capability working and further, as you say, I want it working from both NAV sources.

Your mention of the antenna implies that you have one antenna that drives both NAV sources. Is this accurate or was that a misinterpretation? If you do have two antennas then could a low cost diagnosis be to swap the connection at the rear of the Garmin units to flip which antenna is the source and then see if the issues follows the Garmin unit or the antenna (and cable)?

 

[DWillens]

I'm not sure if there are one or two antennas. I got the impression that it was just one. I didn't think to ask that question. He did say something about splitters and cables and that was what made me think it might be a cabling issue. I've tried and failed to understand exactly how all of this information gets from the antenna(s), to the boxes, to the G5s, although I know that it involves a lot of cables and splitters and data connections.

 

[Floyd]

All the splitters I seen only have 1 GS out. (Could be wrong.) Is there an antenna wire going to the GS input of the 750?

 

[Doug Reid]

 

[Ryan Klems]

On the 750xi, go in to config mode. Go to navigation features, then vertical navigation. Check the “Transition to Approach” setting. This should be _disabled_ (no green highlight). If it’s enabled you will get the symptoms you describe. For whatever reason, enabled is the default, and it should almost always be set to disabled.

 

[DWillens]

All the splitters I seen only have 1 GS out. (Could be wrong.) Is there an antenna wire going to the GS input of the 750?

I think he said t

On the 750xi, go in to config mode. Go to navigation features, then vertical navigation. Check the “Transition to Approach” setting. This should be _disabled_ (no green highlight). If it’s enabled you will get the symptoms you describe. For whatever reason, enabled is the default, and it should almost always be set to disabled.

Ryan (and all): thanks. I will try to check this today. It seems weird to me that this would matter, since I would think that you should be able to just tune an ILS freq in NAV1 with nothing else in the 750 and it should work like a normal ILS receiver. But I don't claim to be an avionics expert or even a 750 expert. I will definitely let you know if this makes a difference.

 

[NealRomeoGolf]

ery rarely would it be necessary to shoot an ILS when you have RNAV LPV

Good luck on the fix. I've never understood the "I don't need an ILS" attitude. If you go into bigger airports, that's all they default you to. Sure you could ask for the RNAV, but when I'm the bugsmasher in the way of the jets, I prefer to go with what the big boys are doing so I'm not a nuisance. Just ran the ILS into KMEM a few weeks ago.

 

[DWillens]

On the 750xi, go in to config mode. Go to navigation features, then vertical navigation. Check the “Transition to Approach” setting. This should be _disabled_ (no green highlight). If it’s enabled you will get the symptoms you describe. For whatever reason, enabled is the default, and it should almost always be set to disabled.

Ok, so I checked that today and you were right, the "transition to approach" selection had been checked in the config mode. I unchecked it but I didn't have a chance to take it out to test it. I will hopefully know in a few days.

 

[Ryan Klems]

Ok, so I checked that today and you were right, the "transition to approach" selection had been checked in the config mode. I unchecked it but I didn't have a chance to take it out to test it. I will hopefully know in a few days.

My apologies, while the disabled setting is the correct setting unless you have a GFC 600, I don't think this will fix your glideslope issue. I just realized that I confused your "No GS" issue with the "No GP" issue with an LPV approach. The transition to approach setting can cause a "No GP" issue for an LPV approach when improperly set, but won't impact the GS for ILS.

When you say that the shop tested the ILS on the ground... do you know how they tested it? Did they use a signal generator box? If so, inside or outside of the plane? The GNS navigators have separate inputs for the VOR/LOC and GS signals, whereas the GTN navigators take the combined signal and do the signal separation internally... so you need to split the full bandwidth signal first (Garmin recommends this: https://www.aircraftspruce.com/catalog/avpages/garmin_11-17819.php but other splitters will work), and then you would use feed one of the full bandwidth split signals to a diplexor to feed the separate inputs of the GNS while the other full bandwidth output goes to the GTN. Most older avionics depended on the signal being separated externally, so depending on what you had in the panel previously, I'd expect that they'd probably have needed to add the splitter. Might be worth looking to see if your invoice or logbook entry mentions anything about a splitter.

Can you post your G5 ARINC 429 settings, as well as the VOR/LOC/GS settings from GTN and the GNS 530? Other than a signal issue, about the only thing I can think of would be improper SDI settings causing some confusion in the G5 between the outputs of the GTN and the GNS.

Also, absolutely this should work to be considered a complete installation, that's not being picky.

 

[DWillens]

Ryan, I greatly appreciate your assistance.

I watched them test it. They did use a signal generator box sitting on the ground near the tail. I watched them switch some things that made the GS go from full up to full down deflection, depending on a setting in the box. It worked fine. What you are describing sounds like what he was explaining to me about one full signal running to the GTN but being split for the GNS, although some of it went over my head. I believe I saw a splitter entry on the invoice but I need to go back to the plane and look at it. I'll find it. I will also get the other configuration settings you mention.

I don't mind making them fix it, I was just hoping to avoid having to take the plane back to AZ and have it sit there while they figure it out, if it was a simple config setting that someone forgot to check or uncheck.

 

[Pinecone]

Good luck on the fix. I've never understood the "I don't need an ILS" attitude. If you go into bigger airports, that's all they default you to. Sure you could ask for the RNAV, but when I'm the bugsmasher in the way of the jets, I prefer to go with what the big boys are doing so I'm not a nuisance. Just ran the ILS into KMEM a few weeks ago.

The ILS is what is default at big airports as most airliners don't have WAAS.

Just ask for the RNAV, it is almost the same approach, so you won't mess them up.

We just at Mooney Max and one comment struck me. End of the day, one of the attendees stated, I learned something important today. GA pilots take what ATC gives them. Airline pilots tell ATC what they want and are going to do.

 

[DWillens]

My apologies, while the disabled setting is the correct setting unless you have a GFC 600, I don't think this will fix your glideslope issue. I just realized that I confused your "No GS" issue with the "No GP" issue with an LPV approach. The transition to approach setting can cause a "No GP" issue for an LPV approach when improperly set, but won't impact the GS for ILS.

When you say that the shop tested the ILS on the ground... do you know how they tested it? Did they use a signal generator box? If so, inside or outside of the plane? The GNS navigators have separate inputs for the VOR/LOC and GS signals, whereas the GTN navigators take the combined signal and do the signal separation internally... so you need to split the full bandwidth signal first (Garmin recommends this: https://www.aircraftspruce.com/catalog/avpages/garmin_11-17819.php but other splitters will work), and then you would use feed one of the full bandwidth split signals to a diplexor to feed the separate inputs of the GNS while the other full bandwidth output goes to the GTN. Most older avionics depended on the signal being separated externally, so depending on what you had in the panel previously, I'd expect that they'd probably have needed to add the splitter. Might be worth looking to see if your invoice or logbook entry mentions anything about a splitter.

Can you post your G5 ARINC 429 settings, as well as the VOR/LOC/GS settings from GTN and the GNS 530? Other than a signal issue, about the only thing I can think of would be improper SDI settings causing some confusion in the G5 between the outputs of the GTN and the GNS.

Also, absolutely this should work to be considered a complete installation, that's not being picky.

So I finally had a chance to get back with this and check a few things and I found something VERY interesting. I flew a couple of approaches with the VLOC #1 and I noticed that the glideslope indications on the G5s were intermittent, as if it was losing the signal. I actually was able to couple the AP in approach mode and it started to fly the ILS approach with the GS, until it lost the signal and the "NO GS" flags came into view, then it worked again for a few seconds, then lost it again. Something is glitching. Antenna or cable connection? It seems like there's no way this would be a GTN setup issue. Or maybe a bad splitter? Anyway, I am taking it back out to the installer next week but maybe this will help troubleshoot.

 

[Ryan Klems]

Based on what you are describing, I'd suspect there is some improper signal splitting occurring that has made the signal marginal or there is an internal issue in the GTN. It seems that somehow it's being fed a signal with the GS freq's strongly attenuated. It's possible they wired it up like nav antenna -> 4 port diplexer -> 2 port diplexer in reverse (as a combiner) -> GTN ... and in that case there will be some extra loss, as well as the potential that the cable for the GS signal between the 4 port and 2 port has a fault. The better way is antenna -> 2 port splitter -> GTN. Did you ever find the log book entry or invoice mentioning what splitter or diplexers they used?

 

[DWillens]

Ryan: right. You reminded me I wanted to check the invoice, which is in the plane. I will be there later today. Also, I took photos of the GTN screens you mentioned, but not sure if it matters at this point.

 

[DWillens]

Based on what you are describing, I'd suspect there is some improper signal splitting occurring that has made the signal marginal or there is an internal issue in the GTN. It seems that somehow it's being fed a signal with the GS freq's strongly attenuated. It's possible they wired it up like nav antenna -> 4 port diplexer -> 2 port diplexer in reverse (as a combiner) -> GTN ... and in that case there will be some extra loss, as well as the potential that the cable for the GS signal between the 4 port and 2 port has a fault. The better way is antenna -> 2 port splitter -> GTN. Did you ever find the log book entry or invoice mentioning what splitter or diplexers they used?

Well here is the W&B revision and I don't see anything that jumps out at me. I didn't see any new port splitters listed on the invoice either.