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Glideslope Problem...


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Defect: My glideslope system using 51RV-1 or -4 or -5 series are showing fluctuation on pilot and co-pilot side upon captured. Ground check using test set cannot simulate the fault for months!! How my system works is as follows:

1 glideslope anthena input to 2 recievers, goes to my NIS tray and output to 2 isolated ADI, HSI and Flight Computers (FCS 105).

The following was carried out:

Glideslope antena changed twice.

Bonding check on VOR/ILS tray, glideslope antenna done.

NIS tray replaced.

Wiring on all glideslope system done.

Both receiver replaced.

However after few months, yesterday pilots did troubleshooting and found the fault is gone after all my Flight director CB are pulled. We found the wiring from NIS to the flight computer showing 1.5M ohms.

Is there any possibility that the wiring problem causing the fluctuation in ADI and HSI even though the glideslope signal is secondary for the flight director??

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Since I have no idea what model this aircraft is, it's hard for me to help.

You have no information in your profile.

So far, I only know that this is a 1978 or newer aircraft because you mention FCS-105. Does it have analog instruments? What is the specific tail number or Lockheed serial number?

The input to the FDC could be causing a problem if the inputs to the indicator are tapped off form those lines.

1. Does this aircraft have SCNS?

2. Is the G/S bug on the HSI or the ADI?

I may be able to help you out, but all the C130s are different. Please supply more information.

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Sounds to me like a corroded and/or loose wire or pin. The vibration from flying may be causing it to act up. Also, with the old instruments at least, there were times when one of the indicators was causing an excessive amperage draw and loading the other indicators. If that is the case, try disconnecting one of the indicators during the fail to see if the problem goes away.

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sorry its a C130H, still using analog HSI and ADI, USAF AF78-0813 to AF79-0473 series.

Fault was never able to simulate on ground. According to the pilot, the off flag was out of view and hence the signal strength was good. We are using the old INU Litton LTN-72.

Simulation run with full engine run up was unable to determine the fault.

My question is, the HSI, ADI and the flight computer tap from the same relay on the NIS tray. If the mega check on both flight computer input line was low (i.e. showing 1.55, 2.8 and 3.2 mega ohm), could it be the flight computer draining the glideslope signal from the HSI and ADI and hence causing the fluctuation?

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Trying to go through our wiring digrams is useless, since USAF aircraft have the ARN147 VOR, Self Contained Navigation System, MLS, GCAS, etc.

With the 51V() receiver installed, you must have a commercial model aircraft, at least I am guessing. ?? The only diagrams I can find for this year group in JEDMICS are totally screwed up,

Is there anything else that taps off of the GS signals, like a GPWS system?

The FDC could be loading it down, but it's hard to believe both went bad at the same time.

Of course, if something fried that circuit in one of them, and they were swapped, it could have fried the other one. Have you tried installing both flight directors on another aircraft to see if they work? That's one way you could rule them out. I wouldn't put any new ones into that aircraft if it is damaging them.

Like I said, it's hard for me to get a grip on how the system is laid out if I don't have a good diagram.

I think you can discount the antenna system. BTW, if the antenna cable is hooked up directly to test set, you should be able to dial in at least 60db of attenuation before the GS warning flag starts coming into view.

I thought maybe there were dampening capacitors for the GS up/down signals in your NIS like we have in our pre '74 NIRP panels, but I guess you don't.

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  • 4 weeks later...

i have cleared the wiring problem and the glideslope problem still persists. The next only thing i can suspect is the loading problem as well. However, the flight director system consist of so many components! It will be a headache. Furthermore, components with loading problem is difficult to detect in shop.

We do have resistor and capacitors on the NIS tray. They looks ok.

Could it be the VOR tray? The 2 VOR set shared the same tray.

Anyone have more recommendations? Appreciate it.

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It's kind of hard for me to help any further without wiring diagrams. I do have a couple of questions though.

Does the G/S bug on the HSI work, and is it just the ADI vertical pointer that is bad?

Does the G/S bug only work if the FDC breakers are pulled in flight?

On some aircraft, when you select an ILS frequency, There is a LOC/GS energize relay that is turned on by the VOR receiver. It tells the FDC you are in a LOC/GS frequency, so the information can be processed properly and displayed on the ADI. It should be labeled on the VOR connector as LOC/GS ENERGIZE.

It looks like the signal that goes to your FDC on pin 46 is the NAV/LOC MODE SEL. I can't figure out where it comes from without a proper VOR diagram.

Again, if I had access to an old H-2-13 or your schematics, I could help, because we got rid of the 51V() receivers about umteen years ago.

Where are you located? Maybe I could swing by...

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1) Both HSI and ADI fluctuated

2) Yes there is such a relay on the NIS tray. Its called Backloc and glidepath relay. However, both are changed.

3) I did not check the FCS105 mode selector panel. Is it possible? It is not linked to the glideslope system but only provide a command to the flight computer?

4) i am not in USA. Thanks for your offer!

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I can't think of anything else without looking at specific diagrams. Do your diagrams in the book have a small number on them in the lower right corner of the diagram? If it has more than one page, it will be on the last page. This is the drawing number. If I had those for the G/S, VOR, Flight Director & NIS, maybe I could look them up to see if I can help figure this out.

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