MGL Avionics User Forum

Hi All,


Quick background, Rainer did post that some components when having air flowing across or through them without being well earthed, can have static electricity build up in them and then suddenly discharge electricity and voltage back into your aircraft's electrical system.

It has taken some time for me to check this out with a multimeter, why I don't know as I spent less than 20 minutes checking out OHMS reading of different parts of my aircraft components earthing back to the engine block to see what and how well things were earthed.

This picture below is of the oil cooler and is of the multimeter OHMS reading back to the engine block. This oil cooler had no earth strap to the engine block and with its rubber mounting is somewhat insulated from the engine block.

So it had a very high OHMS reading to the engine block and of interest, I found the Carby had an earth strap back the the engine block.

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I put a simple bridging test clip from the oil cooler to the engine block, to show how well the oil cooler can be earthed with an earth strap.

So as part of my search to fix my GPS issue, I will make up an oil cooler earth strap to the engine block, like the Carby already has got and see where this leads us.


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I finally got the type of aviation wire I wanted to use to ground the oil cooler to the engine block. It's the white wirer you see up behind the oil cooler. Looking at the sizes of the wirer loop, I am thinking it could be smaller, but I will leave it for now.

At the moment, I have not taken the plane for a fly to see what affect doing this may have had on my GPS issue. I will report the outcome, but regardless of the outcome, I think it's worth the effort in checking out and knowing how well things are grounded around your plane.

You may find that something is not grounded properly even though it does have a grounding strap, so you could take the strap off and clean the contact surfaces and put the strap back on and find the item is now grounded properly again.

As Rainier has said to us in many different posts, we should check for proper grounding of our oil coolers, radiators and other airframe parts for proper grounding.



Cheers
JimJab

Hi All,

Well, it's been a few weeks since I added the earth strap to my engine oil cooler and flown a few hours. Firstly, I am going to recap on a few things, if you would like to bounce over this recap go down to, Start Here.

I have a Jabiru 430/230 Gen 4 aircraft, it has dual Voyager EFIS's, one for the pilot and one for the co-pilot both sharing one RDAC. I had flown this plane for about 30 hours before I started having GPS dropouts.

The GPS dropouts started slowly and increased over the next 10 hours of flying. It was about 1 dropout per hour, then 1 or 2 dropouts per flight and then 2 to 3 dropouts per hour and then normally the GPS would be back in service in a matter of seconds. Then one day the GPS took minutes to get back on-line, so I started swapping things around between the two EFIS's, like the GPS antennas, power supply cables, SP units and installed 2 new GPS Antennas.

Those changes made very little difference to the number of GPS dropouts, so I swapped over the two Voyager EFIS's between the pilot and co-pilot sides, thinking I would then have a working GPS on the pilot side of the aircraft. To my surprise the GPS dropouts remained with the EFIS on the pilot side.

I then checked for any loose electrical connections and found nothing. But from doing this, I realized I had changed the order of the connected devices to the negative bus bar on the instrument panel, at around the time the GPS dropouts started happening. So I changed the negative bus bar order of connections back to what they had been, or as close as possible. In doing this, I found the EFIS GPS dropouts had moved from the Pilot EFIS to the Co-Pilot EFIS and with this there had been a huge reduction in the GPS dropouts nearly to nothing whilst flying.

This led me to think the problem was now EMI or static discharge as Rainer had talked about. So the next step was to check the plane's earthing system using a multimeter to see if there was anything electrically isolated that could be causing a static discharge. I found the engine oil cooler was electrically isolated from the earth system as it had no earth strap to it. So I made an earth strap and attached it between the engine oil cooler and the engine block.

Start Here.

After earthing the oil cooler and a few hours of flying, I think the earth strap from the oil cooler to the engine block has lessened the EFIS GPS dropouts in flight, but I must still have a small issue somewhere causing the odd GPS dropout.

Putting an earth strap on your radiator or oil cooler is not hard and costs very little and then you have one less thing to worry about that could cause issues. If you own an older plane and put an EFIS in it, I would take off all the earth straps and clean them up to make sure they don't have any electrical resistance.

Here is an interesting example where you think there should be a good electrical connection between two parts. But this pic below shows a rivet that has a very poor electrical contact between the rivet and firewall, the cause could be a small amount of oxidization or corrosion between the contact surfaces and this can happen for earthling strap connectors as well.


Cheers
JimJab


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JimJab wrote: Sun Nov 29, 2020 4:52 am Hi All,

I am having an issue with my Voyager EFIS, the GPS keeps dropping in and out. It's never done it before and just started.

I get an EFIS voice alert "GPS not available" and then a seconds or two later "GPS available" I can see on the EFIS screen the GPS has dropping out.

I also have a second Voyager EFIS in the plane and this is solid and does not dropout.

I am thinking the GPS module must be working and holding the required satellites, otherwise the GPS could not drop in and out so quickly.

So could it be the data from the GPS module to the EFIS mother board is being lost and the problem is the data cable and/or the connectors between the EFIS mother Board and GPS module?

Thanks for any experience or help anyone can give me.

Cheers
JimJab

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You have two GPS antennas. Make sure they are a good distance apart - they are active antennas. That means they re-transmit the signal to some extent - this can easily lead to interference.

rainier wrote: Mon Mar 14, 2022 7:30 am You have two GPS antennas. Make sure they are a good distance apart - they are active antennas. That means they re-transmit the signal to some extent - this can easily lead to interference.

Hi Rainer,

Thanks for the heads up about needing to have the two GPS antenna's mounted a certain distance apart.

Roughly what would be your suggested distance between my two GPS antennas for our EFIS's?

I am guessing there is no hard and fast rule with this, as the distance could change depending how and where my GPS antennas are mounted and what metal is around and how the RF may be reflected around and interacts with my current GPS Antenna placements.

Thanks for your help.

Cheers
JimJab

I have only ever seen this with antennas mounted very close to each other - say 3 feet or less. The effect tends to be that one operates fine while the other struggles.
The GPS signals as received by the antenna are very weak - that's why antennas have a small amplifier built in and this is supplied with power via the cable (around 3V DC with ours - pretty much the standard at the moment for these things).
The amplifiers tend to be designed with around 20db gain (i.e. a factor of x100 power gain) - mostly to compensate for the losses in the cable leading to the actual GPS receiver in the EFIS.

Some of that amplified signal radiates from the antenna itself - it does not affect the GPS it's radiating from but any other GPS can be pretty badly affected - that is because, even though both receive the same signal - one inevitably receives the signal a little later as it has to travel just a bit further (and the amplifier also adds a little delay) - the wavelength is pretty short at these frequencies so you can see how the re-radiated wave can easily lead to cancellation of a signal at the other antenna - all depends on the phase of the signals in relation to each other - and there is more than one signal - each signal comes from a GPS satellite from a different location in the sky so the net effect becomes quite complex.

Thanks Rainer for the advice about the GPS Antenna distance and what the distance should be between two active GPS antennas.

In my aircraft the antennas are not quite that distance, more like 24 inches apart. I am very lucky that it will be pretty easy to move my two antennas further apart as they are Velcroed into position.

So I will move them further apart and see what the result is by monitoring the EFIS GPS signal strength page on each EFIS.

Cheers
JimJab

Hi All,

Well it's been some 4 weeks since my last post, I did moved my GPS antennas a little further apart, but not quite as far apart as Rainer indicated, but I needed to take a work trip.

I did about 5.5 hours flight time with a couple of landing, 2.5 hours for the first leg for a meeting and then about another 3.0 hour flight time home pushing a bit of a head wind and then we added a small diversion en-route to pick up some fuel to be on the safe side as there was also a small amount of weather around.

So what did bitching Betty have to say about my "GPS not being available" over these 5.5 hours of flight time. Well not a dame thing, not once did she tell me I had lost either of my EFIS's GPS signals. I am ecstatic about this, but am I 100% sure I've fixed the problem, well time will tell.

But what I can say, I don't think my GPS dropouts were being caused by just one issue when the dropouts were at their worst. I think I have reduced the occurrence of some static discharge issues below the threshold that makes the GPS lose satellite signal. I am pretty happy for now and we'll see what the next number of flight hours brings.

Cheers
JimJab

Well, great work and very useful posts and pics - I hope others will take note.
It's a bit of a mantra - I talk about static discharges and the possible effects on electronics whenever I come across a case where there is a chance this is behind a problem. I fear not many believe me...

The very first time I came across this dates back more than 20 years - we had started selling our Flight-1 instrument, our very first product. A two line 20 character LCD display and a processor with 256 bytes of RAM (of which much was unusable). Anyway - I packed quite a bit of functionality into this thing including the ability to measure coolant temperature on a Rotax 582.
One day I got a call from a builder saying that the instrument crashes as soon as he runs the engine. Obviously that must be power I thought. After a bit of to and throw I drove out to the aircraft (it was about 70km away). It was a ultralight fitted with a tractor engine, high wing - engine above the pilot. The coolant radiator was behind the engine serviced nicely by the prop blast. Sure enough - the instrument worked fine but once the engine started it would crash every few seconds. I could not understand what was going on and tried all sorts - like big capacitors on the power supply etc - nothing worked.
Day started to change to evening and as it got darker we started the engine for yet another try - now in the advancing darkness it was all too clear - a spark jumped at least 4 cm from the radiator to the grounded metal frame holding it - right over the rubber mounts. It discharged like that every few seconds crashing the instrument. Once a strap was fitted over one of the rubber mounts there were no more sparks and no more crashes.

Since then I still sometimes advise to run the engine at night with cowlings off - much easier to see a spark that way. Another method that is pretty good - use an old AM transistor radio, one that still uses those big ferrite rod antennas. Tune off station in the long wave or medium wave band and run the engine - then listen to the radio hiss. Any spark discharge results in a crackle on the radio. I had one case where there was no crackle but the static hiss would change and sound more aggressive - this took a bit of head scratching but the problem was found when dark - along the edge of a fuselage panel that was removable you could see a glow - elms fire it is called. Again a simple copper strap fixed that issue.

Rainer, if it is ignition interference, would moving the antenna to the rear of the aircraft help?