Proper Mag Grounding

I?ve been down for many months now, tiring to finish up all those little things that have been bugging me about my Cub ever since I bought it.

One of the last items on the firewall forward part, are the mag wires (P-leads). As is typical with my jalopy, they?ve botched up this item too. I?ve been burned a couple times with the ?monkey see?monkey do? thing (read Teflon tape) so I need a quick lesson on the proper way to wire the P-leads. Note: I don?t have any filters, but there also doesn?t seem to be much interference, so I?m guessing that I don?t need any filters?


Questions:

Ground points?
I?ve got the schematic for the system and it?s so simple that even I can follow it, but, I can?t find anything to direct me as to where to ?land? (place/attach) the final ground. I?m talking about the wire that goes from the ground stud on the switch to ????? Where are you supposed to ground the mags??

What is the proper gauge of wire?
Can somebody please explain to me why the P-leads, which are subjected to the most vibration that the aircraft can make, always consist of super small wires?

And lastly
About the terminal connections at the studs on the mags. When I was disconnecting the right mag, I discovered that the nut had vibrated off and was laying in the terminal boot, the ring terminal was just barely hanging on the stud. The insulating washers don?t allow you torque the nuts very tight, so what trick are you doing to keep the nuts from vibrating off? Are these supposed to be all metal lock nuts? Doesn?t seem to be enough stud there.

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The A&P school has all the answers to your "owner-maintenance" questions, Cavy.

I enjoy your funny posts, Cavy, and think I'd like you, so take this kindly: you shouldn't be doing stuff that isn't for you to do and that you don't have the knowledge to accomplish or direct supervision to keep you from screwing up.

take care.

Dave.

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Dave
No offense taken buddy, I understand and completely agree with what you are saying and encourage all to follow his advice on this point. If you?re going to make your own repairs, get it inspected before you go up and defy gravity!

Normally my A&P and I would talk about this type of repair before I get started. He then comes back and inspects my work and nods his head up & down or shakes it right & left. Unfortunately, he?s in the shop getting a frame repair (hip replacement) so this time, I?ll have to walk down the ramp and get the local IA to come over and inspect my work.

Diggler
You hit it right on the head. Check out what the last IA did.
1. At the right mag, the shield wasn?t connected to the body of the mag (no terminal on the shield)
2. The filters don?t exist, so the filter box is now a splice box. That?s ok by me, but this guy was really into leaving some slack in the wires?try 4-5 feet of slack (no kidding) that was just wadded up and zip tied to the firewall.
3. From the filter box to the switch, they?ve used a shielded - two conductor ? wire. Again, that?s ok, but you have to wonder why they soldered the shield to the filter box?on we go to the switch?Oh look, the shield is connected the ground post on the switch. I.e. the mags are soldered to the filter box?Even I can do better than that, and at this point, doing nothing was more dangerous then trying something, as the right mag was disconnected (loaded gun).

TJ
Thanks for the tips, I understand what you?re saying about positioning the ring terminal before tightening and I?ll check the terminals again to ensure that I have the correct washer stack.
Admittedly, I?ve only made a cursory inspection of the area that?s local to the switch, but it appears that the mounting tab for the fuel valve is the nearest hard point. I wouldn?t ground any wire that close to anything that involves fuel, so I can?t use that point. I see in AC 43.13 that it?s OK to sand the paint off of the frame and install a steel clamp as a ground point, but I don?t want to expose the frame. I was a bit surprised to see that there wasn?t a ground tab welded to the frame right near the switch.

I?ve removed the shield from the switch (and unsoldered it from the filter box) and landed the shields on the mag bodies, the firewall, and on one of the screws that mount the switch to the panel. I?ve temporarily jumpered the ground on the switch (jumper wire and alligator clips) to a ground while the plane is parked (so that the mags are dead) but I?m still searching for the correct place to permanently land the ground.

Thanks for the help guys.

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T.J. Hinkle said:

humped up like a show pony

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Now there's a Nikiski Alaska saying if I ever heard one!...

Yeah, those Nikiski folks........

And "...not a word a lie...", Diggler, on the statement that some A&P/IA may have done the work before Cavy.

It's tough to get anything done right, has it always been like that?

Ever hear the phrase: "...we don't have TIME to do it right the first time...".

Dave.

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There are "Certified Mechanics" in my neighborhood that I wouldn't let near my airplane. Being "certified" doesn't make you good. Do all the work you can, because your the one whose ass is on the line. Then have a good IA check your work and sign the book. My two cents.

Diggler, "..not a word a lie.." is a colorful way to agree with someone.

Since we were trading "sayings" I thought I'd use it to agree with you that some certified mechanics suck.

What's wrong with you that you would automatically think I was calling you a liar?

The other phrase I posted was coined by a guy working at a shop that hurried through things so often that they screwed lots of stuff up.

Take it easy.

Dave.

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thoughts on the future of aircraft maintainence

On the subject of Aviation mechanics....I had a conversation with an Aviation tech school director about using my Cub as a project for the students to learn with.
He said that the students today weren't learning dope and fabric....but rather the emphasis was moving over to composite construction and repair.
Looks like between paperwork, liability issues and a lack of experience working on fabric aircraft....the maintainence headaches for Cub owners are not going to get better...only worse.
Randy

I found a neat way to do the p-leads on my cub. I have the box on the upper left side of my firewall and when I swapped to the slick mags it was time for new p-leads as well.
I used shielded wire and where I teminated each end for the shielding grounds i used a heat shrink style solder union to join the shielding to the wire used to attach the shield to the mag case and the firewall point. I will take some pic´s when i get home but it really made for a nice looking set of p-leads. The hot deal is the heatshrink tubing solder unions.
I will be home around the first of june and will send pics then if you are interested.
Scott

Try www.terminaltown.com and look for their solder shields. Makes a perfect way to ground shielded cables and P leads.

Thanks for the site- looks like I can do some restocking now. Their solder shields look like what I used to make my leads. Mkaes for a nice looking install.

AKCub said:

Thanks for the site- looks like I can do some restocking now. Their solder shields look like what I used to make my leads. Mkaes for a nice looking install.

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Apparently the same Mouldex terminals,etc, are available at McMaster-Carr for 50 to 70% less!

Do some price shopping first and then hit "CHECK OUT"!!

As of yesterday I have started getting what appears to be mag interference with the radio (tic, tic, tic, tackety tack etc.). Presumably something isn't properly shielded, but getting to the back of the mags involves swinging the engine, right? Big job (= $$$) for someone to do?

I have a 150 and am taking it across the English channel next week so anything to do with mags makes me a little nervous, but both mags seem to be working fine.

Forgive my ignorance, but any advice appreciated.

David

Switch mags and see if you get interference on each one individually or just one. If it is just one you probably broke a ground/shield wire at the mag. It is very common.

David
If you are only checking the connections, they are completely accessible by removing the upper and side cowls.
I have to open up my mags for my annual, but I haven?t had to do that yet, so I can?t offer any advice in that area. I?ll be waiting a while for my A&P to get back on his feet.

Regarding interference, you guys have got me thinking that maybe I do need the filters on the P-leads at the firewall.
When I first got my Cub, the alternator was wired wrong and I had so much RFI that even the intercom was unusable at anything above an idle. The improvement that came from rewiring was so astounding, that a little crackle was acceptable, but I still have to pull the throttle back to hear clearly. I?ve been thinking that it?s just cabin noise.

PS Flew for a couple hours today with the door open (my first time) what a gas!! :lol: But talk about cabin noise?jeess!! After we landed, my passenger looked like he just stepped out of a wind tunnel :roll: sorry Bud.

Thanks, chaps. Yes, I've been told it's probably a grounding lead problem, but haven't checked yet which mag it's coming from. Hopefully, if you can get in with just the cowlings off it shouldn't be too much of a problem. It sounds too as if it isn't flight critical? Again, forgive my ignorance, but if the grounding lead does eventually 'fail', then the mag will keep going, but just be always live right?

Cavy, get a decent voice-activated intercom like the Sigtronics SPA 400 and you can squelch out the wind noise and enjoy your open door flights a lot more.

David

David
Correct. No ground = no shut off, hence my statement above ?loaded gun?
No ground means that the prop is just waiting for some dummy (like me) to come by and lean on it (for that picture) and then be awarded the nick name ?lefty?

Understand that we?re talking about two components here, the shield and the primary lead (P-lead) The P-lead carries the ground to a good ground point to kill the mag, the shield carries external noise to a ground to eliminate radio noise.

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Thanks a lot, chaps. Very helpful.

David

The most important thing about the "shielding" is to make sure it will not contact the "P" lead at either end. If it does, you have a dead mag.

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May I ask for your opinions -- is this a likely thing to happen? It is going to be very difficult for me to get this fixed before an overwater trip next week and I'd rather do it on two mags than one.

David

Yes, and the insulation can break down between the wire and shielding anywhere along it's length. This usually causes a intermitent problem. Hook a meter up between wire and shield with the wire disconected at each end and shake the wire around like it would vibrate in flight. There should never be any continuity between shield and wire.

Also check the old rotary switches. They can "track" copper between the contacts, and intermittenty ground out, cleaning between the contacts cures it.

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Thanks a lot, fellas. Very helpful.

David

Guys, are those low-temperature solder/heat shrink connectors approved for a/c use? I doubt it.

I hesitated to mention this, but figured I would anyway.

..not to be a spoilsport, but to inform.

Dave.

Thanks, all. Turned out my problem was the radio and since it wasn't my radio anyway, it has been a very cheap and easy job -- put my radio back in!

Thanks again for the useful info.

David