ILS Glideslope troubleshooting

[KDefran]

Hello All,

This is my first post, but I've been reading and learning a lot from this site for years. I have been having an issue with my ILS glideslope since I bought the plane ~2 years ago and still have not figured it out and I'm hopefully that some members might have some ideas that I haven't thought about yet.

The NAV/COMs in my airplane consists of a GNS 530W and a King KX-155. When I bought the plane, the ILS glideslope would be very weak. Sometimes it would work, sometimes it would not. This would occur on both the 530 and the 155. I can't clearly remember exactly what was happening since this was approximately June 2016

In about August 2016, I took it to an avionics shop. They did some troubleshooting and replaced the coaxial splitter that has 1 input from the antenna and provides 4 outputs to the 530 and 155 with both a localizer and glideslope signal. This fixed the problem initially, and I don't know what other troubleshooting they did since replacing the splitter initially fixed the problem.  After about a month, I could see that the glideslope was slowly starting to fail again. It failed gradually over the course of about another month or two where initially it worked, then it would only work in close proximity to the airport, and then it stopped working all together. Again this was for both the 530 and the 155, and they failed nearly simultaneously. 

Fast forward to September 2018, and I had a separate mechanic look at it since I had moved across the country from NC to CA. He shot all of the coax and said that a good signal was getting sent from the antenna to both the 530 and 155. His recommendation was that both the 530 and 155 had failed internally and that they should be sent out to get repaired. The minimum costs were quoted as about $1,500 for each instrument. I'm not a believer in coincidences; and 2 instruments failing in exactly the same way at exactly the same time is not likely. Occam's razor would suggest that there is something else that is causing this failure, but I do not know what it could be.

Recently, I tried replacing the coaxial splitter to see if somehow that was the culprit again. However that did not fix the problem or change anything. The problem is isolated to just the ILS glideslope since the 530 works normally while shooting an LPV approach. Also, when I tune the frequency to the ILS, I can hear the Morse Code signal which makes sense since the VOR and localizer functions works normally.

I'm running out of ideas what could be wrong. Now I'm thinking that there possibly could be something wrong with the antenna itself. Since the localizer and ILS frequencies are so different, maybe there is a chance that only part of the antenna is bad? I'm pretty sure the antenna is the original that is on the tail and has the "V" shape facing forward. During an annual, the sheet metal cover was off where the antenna is located and it looked like there was some tape holding the connections together. Since the localizer/VOR work I haven't thought too much about the antenna though.

Maybe the A&P is right and both the 530 and 155 just happened to fail at the same time. But before I send the units off I just want to see if anyone has other troubleshooting ideas that make more sense.

I feel like that was a lot of information, and I'm sure I forgot to mention something so please ask any questions that come to mind.

 

[carusoam]

Welcome aboard KD.

Congrats on the first post.

Are you an engineer, with a great memory? (Occam reference is usually somebody with a great memory, often an engineer)

Are you familiar with your ILS antenna?

Its cabling?

its connectors?

Its ground plane?

Might be worth planning some new wire if they are 40 years old... or some cleaned grounding or connectors...

Or joining the thread that denounces the ILS and only goes with WAAS.... 

PP thoughts only, not a mechanic.

Best regards,

-a-

[PaulM]

An avionics shop should have a 155 to swap in to remove the question of that unit.   They can also bench the two units directly with the LOC/GS test set.

You can bypass the splitter and just couple the antenna directly to the input you want to test.   Also, how did your AP test the cables?.. just with a meter?.. I have seen cables that will pass DC but not RF... and I've seen the opposite.  A dying unit could cross contaminate the other radio.. doing the direct tests without the splitter will confirm that.  

[Jerry 5TJ]

Using the horizontal “V” dipole for both localizer and glideslope is acceptable practice.  

It’s possiblw the antenna has degraded and now works ok at 110 MHz localizer but not at 330 MHz glide slope frequencies.  

You could disconnect the GS cable at the splitter from one radio and connect the radio to a GS-only antenna to test.  For test only you can build a test dipole cut to 330 MHz.  If it works well with the test antenna you can replace the combo VOR-GS antenna or install a GS antenna   

Here is an example of a commercial GS antenna. 

GS Antenna

The GS antenna can be mounted inside the windscreen on the steel cage tube.  The antenna is only about 17” from tip to tip. 

Here’s another on eBay with enough detailed images that you could build one of your own:

EBay GS Antenna

Good hunting. 

[CaptRJM]

I had nearly the exact same configuration in my 231 until upgrade 2 years ago. I’ve never had a problem with reception of ILS or VORs.  IMHO, Occam’s Razor indicates to me that there is a single point of failure affecting both radios. That might be the antenna, antenna coax to the splitter or possibly the splitter (even though it has been replaced). The odds of the 530 and the 155 failing the exact same way at the same time are near Zero. 

I would find a reputable avionics shop in your area as a next step. Last I knew Garmin has a $900 flat rate repair on the 530. That at the current shop quoted you $1500 makes me skeptical, unless Garmin has taken a substantial increase in repair costs.

That you can shoot a LPV approach simply shows that the GPS reception on the 530 and presentation on your indicator is functioning fine and has no bearing on reception of ground based signals.

 

 

[KDefran]

15 hours ago, carusoam said:

Are you an engineer, with a great memory? (Occam reference is usually somebody with a great memory, often an engineer)

Are you familiar with your ILS antenna?

Its cabling?

its connectors?

Its ground plane?

Might be worth planning some new wire if they are 40 years old... or some cleaned grounding or connectors...

Or joining the thread that denounces the ILS and only goes with WAAS.... 

-Engineer yes, great memory no. Learned about it years ago and it just stuck.

I'm not as familiar with my antenna as I should be, but I'm going to become more familiar with it.

I'm definitely a fan of WAAS approaches so that's why I'm not in a big rush to fix the ILS.

15 hours ago, PaulM said:

An avionics shop should have a 155 to swap in to remove the question of that unit.   They can also bench the two units directly with the LOC/GS test set.

You can bypass the splitter and just couple the antenna directly to the input you want to test.   Also, how did your AP test the cables?.. just with a meter?.. I have seen cables that will pass DC but not RF... and I've seen the opposite.  A dying unit could cross contaminate the other radio.. doing the direct tests without the splitter will confirm that.  

I'll have to ask around to see if there are any 155s to check it with. I'm not sure of the process that he used. That will be a question for me to ask him as I never would have thought of that.

15 hours ago, Jerry 5TJ said:

Using the horizontal “V” dipole for both localizer and glideslope is acceptable practice.  

It’s possiblw the antenna has degraded and now works ok at 110 MHz localizer but not at 330 MHz glide slope frequencies.  

You could disconnect the GS cable at the splitter from one radio and connect the radio to a GS-only antenna to test.  For test only you can build a test dipole cut to 330 MHz.  If it works well with the test antenna you can replace the combo VOR-GS antenna or install a GS antenna   

Here is an example of a commercial GS antenna. 

GS Antenna

The GS antenna can be mounted inside the windscreen on the steel cage tube.  The antenna is only about 17” from tip to tip. 

Here’s another on eBay with enough detailed images that you could build one of your own:

EBay GS Antenna

Good hunting. 

This might the the cheapest/easiest way to test the units myself. I'm going to do some searching for a good internal antenna to buy/build and that should eliminate some variables.

15 hours ago, CaptRJM said:

I had nearly the exact same configuration in my 231 until upgrade 2 years ago. I’ve never had a problem with reception of ILS or VORs.  IMHO, Occam’s Razor indicates to me that there is a single point of failure affecting both radios. That might be the antenna, antenna coax to the splitter or possibly the splitter (even though it has been replaced). The odds of the 530 and the 155 failing the exact same way at the same time are near Zero. 

I would find a reputable avionics shop in your area as a next step. Last I knew Garmin has a $900 flat rate repair on the 530. That at the current shop quoted you $1500 makes me skeptical, unless Garmin has taken a substantial increase in repair costs.

That you can shoot a LPV approach simply shows that the GPS reception on the 530 and presentation on your indicator is functioning fine and has no bearing on reception of ground based signals.

I don't remember the exact prices, but I think the 530 was quoted as about $1,200 and the 155 as about $1,600. Both prices were more than I was will to spend until I could determine that was the actual problem.

 

Thanks everyone for the replies. As I continue to work on the issue I'll keep this thread updated with what I find.

[Marauder]

I don't remember the exact prices, but I think the 530 was quoted as about $1,200 and the 155 as about $1,600. Both prices were more than I was will to spend until I could determine that was the actual problem.
 
Thanks everyone for the replies. As I continue to work on the issue I'll keep this thread updated with what I find.

I always hated these kinds of problems. I had an issue with a brand new GNC 255B’s VOR reception. Just didn’t have the range it should have. Replaced the splitter and installed new RG-400 cabling and still saw the issue. Just about to send the new radio in when I decided to check everything a second time.

Piecing together the mystery, I concluded the original splitter’s BNC connection for this radio was bad but by changing out the RG-400 at the same time, we introduced a second variable. When a new temporary run of RG-400 from the new splitter was installed it worked great. Turned out the avionics shop made up a bad BNC connector on the new RG-400 and failed to check the cable.

Good luck on the hunt!


Sent from my iPhone using Tapatalk Pro

[Boilermonkey]

We had the same problem this year. Our G430W had just been overhauled for a different reason ($1000), so we replaced the cabling, connectors, and antenna. Works like a champ now.

Sent from my SM-G960U using Tapatalk

[Yetti]

Take the antenna out and clean everything up.  There is a balun that runs down the tail and makes the connection with coax that runs forward.  You can find instructions to make a new balun or buy from Spruce.  Heads up a balun in line will show a direct short on VOM. There is a thru hull BNC connector buy the port inspection port.  Don't ask me why port means a side of the ship and also place to check things.  Replace the coax going forward with RG400.  Use a VOM to test for shorts.

Edited December 30, 2018 by Yetti

[KDefran]

UPDATE:

Looked at the airplane today and found something weird. Coming out of the diplexer there are 4 coax cables, 2 VOR/LOC and 2 glideslopes. It appears as though the wires were switched. Basically the VOR/LOC output from the diplexer would go to the glideslope input for the unit. The opposite was also true with the glideslope output was routed to the VOR/LOC inputs. This was the case for both the 530 and 155. I switched the wires and noticed immediately that the localizer needle moved more forcefully however there was still nothing on either glideslope.

I decided to take it up for a flight to see if the glideslope would work while on final approach. I took it out to about a 4 mile final and the localizer worked normally, but there was still nothing from either glideslope. However at about 2 miles the glideslope started working on the 530 only. Still nothing on the 155 though. The working glideslope on the 530 provided correct indications based on the PAPI lights and worked from about 2 miles all the way to touchdown.

Questions that I have:

-I think I understand the basics of the diplexer in that it will only send the correct frequency for the VOR/LOC and G/S respectively, but what is it's actual purpose. Apparently for the past 2+ years my VOR/LOC has been working just fine while receiving the G/S output from the diplexer.

-Would the wires being backwards have any damaging effect to the units. Basically would 330hz signal going into the localizer be bad for the unit? I suspect not but I'm not sure. It seems like that there is some bleed through of the frequencies from the diplexer.

-Fixing this specific issue still has not fixed the problem. My plan going forward is to still remove the antenna from the equation based on Jerry 5TJ's recommendation. I ordered an antenna which should arrive this Friday. If I connect the antenna directly to the G/S inputs there the results should help point in the right direction.

Thanks everybody for the helpful advice so far and I look forward to finally fixing this.

[EricJ]

What are the inputs and outputs to the box/device you're looking at that you're calling a diplexer?   Unless there's a transmitter involved in the airplane somewhere, it shouldn't be a diplexer, just a splitter/filter.   The glideslope is at a higher frequency than the VOR/LOC (which are in the same band), so if the "splitter", or whatever it is, also has some filter functions it may matter what is plugged into which port.    Pics would probably help, and/or a part number of the device.

What is plugged into the antenna input side of the device may matter as well.

[KDefran]

4 minutes ago, EricJ said:

What are the inputs and outputs to the box/device you're looking at that you're calling a diplexer?   Unless there's a transmitter involved in the airplane somewhere, it shouldn't be a diplexer, just a splitter/filter.   The glideslope is at a higher frequency than the VOR/LOC (which are in the same band), so if the "splitter", or whatever it is, also has some filter functions it may matter what is plugged into which port.    Pics would probably help, and/or a part number of the device.

What is plugged into the antenna input side of the device may matter as well.

https://www.aircraftspruce.com/catalog/avpages/ramiav585.php

Here is the diplexer/splitter that I'm talking about. The coaxial from the antenna goes into the input, and there are 4 outputs; 2 VOR/LOC and 2 G/S.

I'm not exactly sure what the term diplexer means, but I guess it acts more as a splitter.

[EricJ]

14 minutes ago, KDefran said:

https://www.aircraftspruce.com/catalog/avpages/ramiav585.php

Here is the diplexer/splitter that I'm talking about. The coaxial from the antenna goes into the input, and there are 4 outputs; 2 VOR/LOC and 2 G/S.

I'm not exactly sure what the term diplexer means, but I guess it acts more as a splitter.

Yes, it's being used as a splitter.   If you were transmitting on both GS and VOR/LOC bands through the antenna port, i.e., the signals were going out rather than in, it'd be functioning as a diplexer.   Usually the specs for a diplexer are much different than for a typical splitter application, like it has to handle a lot more power, so they're not often labelled like that.  IMHO, it's confusing to do that, but not incorrect.

Anyway, if that whole system used to work and just isn't any more, I think you're on the right debug path.   If the GS antenna that you're waiting for works, then it's something in the other part of the signal path.   You could get a BNC barrel and connect the antenna directly to one of the GS inputs and test that directly just to make sure the antenna and cabling are okay, but nothing else would be expected to work during that test.   You could also get a known-good 3dB splitter (even a plebian home-cable splitter would work) and connect two things at once instead of just one.   If you can mess with it while flying (not necessarily recommending that, just saying if it is easy it's something to consider), swap 'em around and see what happens.   You should be able to isolate whether it is the splitter, or the cabling/antenna.   

BNC inside (female-female) barrel connector, should be obtainable at nearby stuff store:
https://www.amazon.com/CNE41862-Barrel-Connector-Coupler-Female/dp/B00IPZBL7I

That's an "inside" barrel, or female-female.   The male-male is an "outside" barrel.  

Edit:  To answer your earlier question, it shouldn't hurt anything to connect the GS output to the VOR input or vice-versa.

Edited January 4, 2019 by EricJ

[G Miller]

Where are you located? I am in the Seattle area, am a retired Boeing antenna engineer, and have a piece of test equipment called a network analyzer that can be used to determine the health of cables, power splitters, diplexers and antennas. I would be happy to make the measurements if you were in the local area. My Mooney is currently in for it’s annual inspection so I am essentially grounded for another week or two.

[skykrawler]

So was this problem resolved?

 

[carusoam]

Seeing if @KDefran is visiting....

-a-

[KDefran]

Problem is still not fixed. I tried to do some troubleshooting with a glideslope antenna inside the cockpit and the splitter removed from the equation. It appears to confirm that both the 530 and KX-155 are not processing the glideslope signal. Next annual I am planning on sending either the 530 or the 155 back to the manufacture to repair. If the manufacture fixes one, then the other one is probably fixable too. I still find it hard to believe that two relatively independent units failed in the same way, but that is what the evidence is saying.

[Bob - S50]

I don't know if this will matter or not.  When we first installed our GTN650 we had a similar problem.  LOC worked fine but the GS would not work until we were within a mile or two of the airport.

I don't remember exactly what the problem was, but it had to do with the splitter.  All the cables were hooked up in a manner that should work according to logic.  However, it was not the way it was supposed to be connected according to the GTN installation manual.  When we switched it to be in accordance with the installation manual it started working perfectly and has done so ever since.

I would start by looking at the GNS530 installation manual.  Make sure all the connections, including number and type of splitters, are as specified.

[KDefran]

I just wanted to follow up on this topic. I recently got the airplane back from its annual inspection. While it was there, I made the decision to send the GNS530 to Garmin for refurbishment. This has appeared to fixed the problem. I've taken it on a few flights so far and have had no issue with the ILS glideslope at a couple of locations. Since this fixed the problem, I will likely send the KX-155 for refurbishment next time the plane is down for a while. After all was said and done, I was wrong that it was something to do with the splitter, but I'm glad it is working.

[0TreeLemur]

 

47 minutes ago, KDefran said:

I just wanted to follow up on this topic. I recently got the airplane back from its annual inspection. While it was there, I made the decision to send the GNS530 to Garmin for refurbishment. This has appeared to fixed the problem. I've taken it on a few flights so far and have had no issue with the ILS glideslope at a couple of locations. Since this fixed the problem, I will likely send the KX-155 for refurbishment next time the plane is down for a while. After all was said and done, I was wrong that it was something to do with the splitter, but I'm glad it is working.

Good thing to send the KX-155 in for work.  They  are notorious for having leaky capacitors that cause decreased performance over time.    After I bought the plane, the KX-155 in my a/c was really weak and noisy according to the staff in the local tower.  So, I sent it to a shop in Wichita specializing in rebuilding them.  They replaced a bunch of leaky caps, re-balanced the radio, swapped out a volume knob, and now it is like new.   All that for about $750.   Worth it.   The KX-155 is a great bulletproof radio.

[DualRatedFlyer]

11 hours ago, 0TreeLemur said:

 

Good thing to send the KX-155 in for work.  They  are notorious for having leaky capacitors that cause decreased performance over time.    After I bought the plane, the KX-155 in my a/c was really weak and noisy according to the staff in the local tower.  So, I sent it to a shop in Wichita specializing in rebuilding them.  They replaced a bunch of leaky caps, re-balanced the radio, swapped out a volume knob, and now it is like new.   All that for about $750.   Worth it.   The KX-155 is a great bulletproof radio.

What was the name of the shop?

[0TreeLemur]

5 minutes ago, DualRatedFlyer said:

What was the name of the shop?

In 2018 the shop I used was Bevan Rabell Inc.   Since then the name has changed to Bevan Aviation.  I haven't dealt with them since the name change, so I don't know what else has changed.

[KDefran]

It appears as though my glideslope woes have been fixed. I sent the KX155 to Bevan Aviation. They were great and talked me through my options and were able to get it fixed for a reasonable price. Today, I flew the plane and both of the glideslopes worked as expected! In the end, it appears as though there was not a common cause and both units independently had failed glideslope receivers. Thank you everybody for the help and guidance!

[MartinRanch]

KDefran:

 

Thank you for posting all your issues and troubleshooting with your glide slope. This thread enlightened me on how to attempt to correct my own glide slope issue. 

[carusoam]

Note for @KDefran above…

Its been a while KD!

Best regards,

-a-