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Cessna 182: VOR/LOC/GS issues

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Cessna 182: VOR/LOC/GS issues

Old 24th Jan 2017, 16:30
  #1 (permalink)  
Thread Starter
Join Date: Apr 2012
Location: London
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Cessna 182: VOR/LOC/GS issues

Hi everyone,

I have a 2001 Cessna 182T in which I have installed a Garmin GTN 750 and 650 as my nav/comm units. They're great, but I'm having a problem with the ground-based navigation that my mechanic and avionics guy have been unable to solve. I'm hoping that someone here might be able to point me in the right direction.

Here's the issue: when I tune in a VOR or ILS, the needles oscillate. The closer I am to the station, the more the oscillation is. We've checked all the antenna wiring and it's fine. This occurs with both nav/comms, although strangely the 650 is less susceptible than the 750. It's particularly bad when flying an ILS - the needles do a "hula hoop" dance around the central circle.

The theories so far (and the reasons why they are hard to believe) are:
- Antenna problem (The plane is a 2001 and that's not really very old in aviation terms)
- Alternator issue (charging system seems fine)
- Noise supressor (yet I'm not hearing anything on the intercom and the com radios are clear)
- Antenna wiring breakout (this apparently only fails catastrophically according to my avionics guy)

So we're all stumped. Does anyone have any ideas?
mthocker is offline  
Old 24th Jan 2017, 16:51
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Join Date: Jan 2008
Location: Canada
Posts: 601
Assuming that the avionics are working properly, which seems a fair assumption, it sounds like an electromagnetic interference (EMI) issue. A good avionics shop will have a handheld unit which can artificially generate the signals to test your indications (I have witnessed this testing being done before). Start with that - with a solid test signal, you should get a solid indication.

Next, consider that wiring and antenna wiring may be in conflict - has ANY wiring been changed in the fuselage since the 'plane was new? Look there first. It has happened that the addition of entirely unrelated wiring has interfered with presently installed systems which worked prior. The new wire does not have to be connected to interfere, proximity can be enough.

Have a read through chapters 2 and 3 of AC43.13-2B, and you'll have a greater understanding of how to proceed.

9 lives is offline  
Old 24th Jan 2017, 17:33
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Join Date: Apr 2008
Location: Hampshire
Posts: 365
Spiking due to minor fluctuations in the electrical supply to the two instruments ? A multi meter reading might confirm.
Capt Kremmen is offline  
Old 24th Jan 2017, 18:11
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Join Date: Nov 2001
Location: UK
Posts: 946
Have you got anything such as a USB power supply in the cigar lighter socket??
Johnm is offline  
Old 25th Jan 2017, 00:51
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Join Date: Jan 1999
Location: north of barlu
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I would be inclined to put a high voltage insulation tester on the antenna line ( after disconnecting it from the radios and antenna).

If the antenna line is OK, check the splitter box that splits the antenna line for the NAV boxes, these are the only things common to both radios. It is very unlikely that both radios are faulty.
A and C is offline  
Old 25th Jan 2017, 01:44
  #6 (permalink)  
Join Date: Aug 2005
Location: Glens o' Angus by way of LA
Age: 56
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Or take the splitter out the equation and plug the cable directly into one of the nav boxes then test with the other box turned off along with any other breakers not needed to perform the test.Also I assume the antenna cable is the existing one that went into the old box I would check there is not "Thicknet" size stepdown connector to a "thinnet" type coaxial cable just prior to entering the new nav box.
piperboy84 is offline  
Old 25th Jan 2017, 08:04
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Join Date: May 2007
Location: Southwest France
Posts: 9
You can eliminate the alternator by checking if the problem persists with the alternator switched out.
High Winger is offline  
Old 25th Jan 2017, 12:30
  #8 (permalink)  
Join Date: Mar 2004
Location: Oxfordshire
Age: 49
Posts: 470
Is the 'needle' being displayed on the Garmin kit or on a 'steam driven' dial?
glum is offline  
Old 25th Jan 2017, 13:19
  #9 (permalink)  
Join Date: Jan 2017
Location: Blue Marble
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Any home-brew ADSB equipment on board (Stratux, Pilotaware or other tinkerPi stuff)?

Cable types checked for compliance with requirements of dual installation?

Two VOR antenna, or one with Diplexer/Triplexer (checked they are ok)?
Wuniform is offline  
Old 25th Jan 2017, 13:49
  #10 (permalink)  
Join Date: May 2006
Location: North West
Posts: 191
I would suspect some sort of on board interference which is affecting the IF stages of your nav receivers. These are simple AM type receivers, largely designed before modern EMC requirements.

The IF stages work at a constant frequency, independent of the frequency you are tuned to. Hence it will likely affect wherever you are and whatever you have tuned.

As you get closer to the ground station the receiver will adjust its gain taking account of the stronger signal, which may give an interfering signal more influence, hence more oscillation as you get closer. ILS circuitry is normally more sensitive given the greater accuracy required.

Test the system on the ground with an avionics test set..

How do the idents sound? Distorted in anyway?

Switch alternator off in flight and observe and change in the behavior.

Switch off any accessories, transponder, un needed electronics etc one at a time and all together, look for changes.
horsebox is offline  

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