Avionics Ground ??

Jeff Vaughan · #1 02-06-2006, 08:46 PM

One of the gizmos (Shurecheck traffic spotter thing)I am putting in my plane said not to connect the ground to the firewall (airframe) gound but to connect it to the avionics ground. Where is the avionics groung connected to? If I read the Aero Connection book correctly Bob K said to make a D-sub avionics ground connector and then run it to the firewall ground. I am confused. Please educate me.

Thanks
Jeff Vaughan
7A

Bryan Wood · #2 02-06-2006, 09:30 PM

Hey Jeff,

Your airframe ground will be typically brought up to the instrument sub panel and the terminations for your avionics, etc will terminate at this point. The goal is to prevent ground loops which are little electronic gremlins that will drive you and your avionics nuts for the rest of time if you don't terminate at a common point. This ground loop thing is close to impossable to troubleshoot, and if you bring in a professional later he or she will not usually be looking for this condition. Most shops look at problems as if they are a new occurance, not installation errors. Basically, you could end up with a noisy airframe electrically if you are not careful with the grounding points. Mooneys are famous for this and sometimes the airframes have to be degaussed to get rid of magnetic fields in the airframes. Our planes should never have this problem since they are missing the steel tubular frame of the Mooneys, but when you equiptment is on it will cause noisy places that can potentially disrupt your gear.

Regards,

rv9aviator · #3 02-07-2006, 06:46 AM

The way I read the book all the grounds from avionics simply go to a central ground block with flat blade connectors on the inside of the firewall. A brass bolt goes through the firewall and attaches to another ground block for all firewall forward grounds. This basically grounds everything at the same centralized location.

I have a related question. In the AeroElectric book it down plays the need for shielded wires for most applications but my wiring diagram for my PS Engineering intercom says to use shielded wires on everything but only terminating the shield at the intercom. The book says you can have more problems by improperly connecting the shield than not using one at all. I already have unshielded wire for this application. What did others use for most of the avionics wiring.

Ironflight · #4 02-07-2006, 07:38 AM

.....I'd do it the way PS Enginering says! My audio system has been perfect (done with shielded wires per their manual), with no whine or hum at all. Perfect, that is, until this weekend when I plugged in my MP3 Jukebox for the first time. You guessed it - got a hum! I can plug the stereo cable into the airframe jack, and no problem, but when I plug the other end into the Jukebox - Alternator whine (if I kill the alternator, the whine is gone)! It's not really that bad if I turn the music up, but that's not good for the ears!

Anyway, I talked to PS Engineering yesterday, to see if there is a "simple fix" (A filter or something), and alas - no dice. It's all magic, and probably has to do with a floating ground somewhere. Bottom line - do the very best you can to follow the installation manual to mimimize the chance of noise in the system. You're not going to know you have a problme until you are flying (you can't test it "all-up" until the wiring is finished), and at that point, fixing a wiring problem is WAY too hard....

I have much respect for Bob Nuckoll's and learned a lot from his book, but in the end, he doesn't have to fly your airplane - and you do!

I am going to check the easy stuff - the ground potential at the airframe jack - and then live with it (at least until I decide to upgrade to a stereo intercom...there's always more money to spend!

).

Paul

jcoloccia · #5 02-07-2006, 07:45 AM

Quote:

Originally Posted by rv9aviator

but only terminating the shield at the intercom. The book says you can have more problems by improperly connecting the shield than not using one at all.

All they're trying to say here is don't connect the shield to ground at both ends...you'll make a hairball for yourself. There are reasons connect both sides of the sheild on occasion (for high frequency stuff, for example) but it's a black art that even the "authorities" don't quite understand.

f1rocket · #6 02-07-2006, 09:54 AM

I've had some similiar problems. ALL my grounds terminate at the ground block on the firewall. My intercom was working perfectly until I upgraded my LightSpeed headsets from the 25 XL to the Thirty 3G. Now, I've got a slight alternator whine in the headsets. I know what's causing it though. My headset wires run very close to my alternator field CB wire. I think I'm getting some radiated noise as a result of that proximity.

At least it solved my problem with connecting my MP/3 player. I could never get the audio input jack to work on the PS Engineering PM3000 stereo intercom. The music volume was barely audible. I even sent the intercom back to PS for a field service change internally. Still no joy. With the new headsets, I just plug the MP/3 player into the headset and bingo, works great.

I think the key to the "avionics ground" issue, is to terminate all your avionic grounds at the same location. That can be on your common ground block or a separate ground block behind your panel that is then tied to your gound bus. Just as long as they all terminate together so there's no ground loops between different avionics gear.

Frank Smidler · #7 02-07-2006, 11:10 AM

This discussion got me thinking.

I am prepairing to install my avionics, including a PS3000 intercom. What I don't understand is how to connect all of the ground and shield wires at the intercom connection and then take them to my common ground block on the firewall. As I read the instructions PS Engineering recommends that all the grounds be tied together at the plug and then taken to ground.

Do you connect 3 pr 4 together in a butt splice then run a single wire from there into another group of 3 or 4 wires until you have one ground to go to the ground block?

Ironflight · #8 02-07-2006, 11:35 AM

Check out this article on Bob Nuckoll's site - I have learned LOT of neat fabrication techniques from his pictures....

http://www.aeroelectric.com/articles...l/pigtail.html

Paul

rv8ch · #9 02-07-2006, 12:23 PM

There are some other cool pics on Bob's site that deal with grounding here:

http://www.aeroelectric.com/articles/Grounding

I really thought someone sold a kit that makes this pretty easy, but I can't find that information.

rv9aviator · #10 02-07-2006, 01:25 PM

Hi Paul,
I know very little about wiring but did you use fiber washers to insulate the mike and headphone jacks from the panel and run a seperate ground wire from each jack back to the central ground? I'll bet 90 percent of all metal airplanes are grounded right at the jack to the panel. According to what I've read this is not good. I'm just curious how others did the grounding in that area.