The International Cessna^® 170 Association

So I went looking for info on proper shielding of wire in aircraft. One of the first things I found is a white paper on aircraft wiring mistakes from Vertical Power a manufacturer of electronic equipment for the experimental market. http://www.verticalpower.com/docs/Top_1 ... stakes.pdf

In this document they pay some time to proper grounding and describe what a ground loop is as well as many other things.

On magnetos they say the following:

• Magneto grounding. The magneto should be grounded back to itself via the shielding.
  This is the best way to ensure a reliable ground.

Then they say the following:

• Shielding. Follow the manufacturer’s instructions for shielding. Avoid grounding the
  shield at both ends when the shield goes between the engine and the airframe.

As has been discovered the magneto ground lug is isolated from the the magneto switch body. So connecting the shield to ground at the mag and the other end at the switch to ground the mag to itself does not set up a ground between the engine and the airframe. In fact when the switch is open, as it is in normal operation, the shield is only grounded at the mag so it would be shielding the lead it surrounds.

Of course looking at the Cessna schematic Gary posted, the ground lug on the mag switch is grounded to airframe ground. which beings us back to ground loop possibility with unequal ground resistance:?

I back to the opinion that the mags should be grounded directly to the ground lug of the mag switch and the mag by the shielding of the P lead, and the shielding should be isolated from the airframe ground to avoid ground loops. At least until I find more evidence to the contrary.

We're having some fun now!

And from the EAA http://members.eaa.org/home/homebuilder ... nding.html

• .....
  
  Shielding
  
  The purpose of the shielding is to cause HF voltages to be induced in the shield rather than in nearby, or adjacent, units where they would cause electrical interference . . . like noise in your radios.
  
  A shielded cable is one whose entire length is encased in a metal braid.
  
  Some folks believe only one end of a shielded cable needs to be grounded but just as many, if not more, believe both ends should be grounded for maximum effectiveness. Who’s right?
  
  Well, it could be that both factions are right, sometimes.
  
  The shielding on your magneto wires should be terminated (grounded) at both ends to keep cables from emitting radio frequency noise.
  
  On the other hand, the shielded interconnecting cable used between a strobe light unit and its power supply normally is terminated (grounded) at one end only to reduce to a minimum potential radio noise due to RF radiation. Usually, it is more practical to ground or terminate the shield at the power supply end. However, do not terminate both ends. At least, that is the advice offered by the manufacturer of Whelen strobe light equipment.
  ........

And from Robert L. Nuckolls, III, AeroElectric Connection, 6936 Bainbridge Road. Wichita, Kansas 67226-1008

http://www.aeroelectric.com/articles/Ma ... ptions.pdf

second last paragrah:

• One final point. Note that I show shielded wire between the
  magnetos and the magneto switches. Spark plug wires and
  magneto p-lead wires are the ONLY wires in the airplane
  which are ALWAYS shielded. Further, p-lead shields should
  be grounded to the airframe/engine only at the engine end. The
  shield is used to provide a ground for the "GRD" terminals at
  the switch but no other connection to the shields should be
  made at the instrument panel end.

There are several schematics in the pdf and all of them show the P lead used as a ground between the mag and the switch and as he notes no other connection to the ground of the switch at the instrument panel.

Bruce Fenstermacher wrote: The shielding on your magneto wires should be terminated (grounded) at both ends to keep cables from emitting radio frequency noise.

This may be meant to apply to the high voltage leads between the mag and the spark plugs, rather than the P-leads.

sfarringer wrote:

Bruce Fenstermacher wrote: The shielding on your magneto wires should be terminated (grounded) at both ends to keep cables from emitting radio frequency noise.

This may be meant to apply to the high voltage leads between the mag and the spark plugs, rather than the P-leads.

Could be but I didn't take it that way.

Bruce Fenstermacher wrote:And from Robert L. Nuckolls, III, AeroElectric Connection, 6936 Bainbridge Road. Wichita, Kansas 67226-1008

http://www.aeroelectric.com/articles/Ma ... ptions.pdf

second last paragrah:

• One final point. Note that I show shielded wire between the
  magnetos and the magneto switches. Spark plug wires and
  magneto p-lead wires are the ONLY wires in the airplane
  which are ALWAYS shielded. Further, p-lead shields should
  be grounded to the airframe/engine only at the engine end. The
  shield is used to provide a ground for the "GRD" terminals at
  the switch but no other connection to the shields should be
  made at the instrument panel end.

There are several schematics in the pdf and all of them show the P lead used as a ground between the mag and the switch and as he notes no other connection to the ground of the switch at the instrument panel.

This information from Mr. Nuckolls is either incorrect...or out of context .... as the number of shielded wires in airplanes are so numerous they defy numbering. (Think of each radio antenna lead, each strobe power lead, most nav-lite cables, etc. etc.) And, when he says "airframe/engine"....WHICH does he intend? (Hint: They should be grounded to each-other. In several places. With heavy conductors.)

Ground-loop faults are (usually) only significant in audio circuits where whine, pops, and static can be detected. This is not an uncommon matter found when new intercom systems are installed.
Some data circuits are also affected, but generally speaking, most technicians "worth their salt" will carry good practices with them throughout their work-product, and this is where many come to believe that grounding shields at both ends is a "no-no".
It's not true in all cases, especially where the shielding itself is used for a dedicated "data-line" communication, i.e., P-leads.

I doubt the writer confused the two magneto circuits (primary vs secondary) since he appeared to use the terms "spark plug" and "p-lead" .... which seems to show he differentiates them.

In any case, this discussion certainly points out the confusion which predominates and explains how so many "hot magneto" accidents may occur. Just because YOUR mags die when you switch them OFF.... doesn't mean they are reliably "cold". Having the p-leads connected at both ends assures the matter.

Also.... it is not exactly true that our airplanes have only "one ground". When the main ship's battery negative terminal is connected to the firewall and/or engine and/or ground-service plug (as many are) that constitutes a multiple-ground circuit. (Think about this, if there were only a single ground... then everything...and I mean everything.... would stop working if the battery went dead or developed in internal dead-cell, and that clearly is not what happens with aircraft in flight. At least, we don't WANT that to occur, right?)

The multiple-ground refers to the many circuits sharing a common-ground....the airframe... and that is also shared by audio systems, where "ground loop faults" can be irritating. (Don't worry about ground loop faults if you don't hear them in your audio and if your Cessna-170 computerized equipment doesn't fail.)

George if you read it again Nuckolls says the P-leads and Spark Plug wires were the only wires always shielded. He didn't say there weren't many others. But this statement is wrong if taken literally. My Cub and thousands like it flew with no shielded wires, including P-leads and spark plug wires. Many still are. My Cub P-leads still are not.

I did not take his comment to mean they MUST be shielded. He stated that the spark plug and p-leads are the ONLY wires shielded in an airplane.... Clearly he is wrong on that matter....if that is what he actually meant to say.

"Spark plug wires and magneto p-lead wires are the ONLY wires in the airplane which are ALWAYS shielded."

It helps to read the whole sentence.

David Ross has it correct. The magnetos are and should be completely independent of the electrical system. All the P lead does is short the mag to itself through the switch. Cutting either of the wires to the switch gives you a hot mag that you cannot shut off. Many old planes did not have electrical systems and were hand propped to start. And you can do the same today when your battery is totally dead. The mag may or may not be grounded to the airframe and to rely on airframe ground to the mag leaves room for possible bad connections. Fixed a Hot mag yesterday and found the problem when I removed the P lead filter and it came apart in my hand. One end had separated and caused a internal break in the circuit. Hope this helps how you think of a magneto

Here is the response I got from Champion Aerospace. It doesn't mean much without my note to them, so I also include it. I wish he had distinguished between the center P-lead wire and the shield.
Open Quote"==========================================================
Bill, Your IA is correct, we do want the mag end ground floating and the P-lead should be grounded at the ignition switch.

From: Bill Garnett <interstellardust@yahoo.com>
To: slicksupport@champaero.com,
Date: 04/04/2014 02:41 PM
Subject: Proper grounding for Slick 6364 Mags

I own a 1956 Cessna 170B with a Continental 0-300-A engine equipped with Slick 6364 magnetos. During my annual inspection I discovered that the engine runs fine with the switch in the off position. In replacing the (original) P-leads I discovered there are many opinions about how the mags should be grounded. Can you please tell me what the proper method is and or point me at the appropriate technical bulletin/wiring diagram? I am not an A&I but there is disagreement among those I have consulted.

My current A&I, (no pun intended) insists that the P-lead shields should only be attached at one end, either to the ignition switch ground terminal, or to the ground screw on the mag casing. This seems wrong to me since new P-leads (ordered from Aircraft Spruce) come with ring connectors at both ends for both the P-lead and the shield. It would seem the intent is to use the shield to ground the switch directly back to the mag. Several mechanics I poled agree.

A little history. The airplane came equipped with Bendix mags. The original equipment P-leads had an insulated center wire surrounded by a woven metal shield. the shield was connected to the ignition switch at the G terminal and to the hex nut fitting at the mag end. There was no ground wire from the G terminal of the switch to the airframe. In other words, the shield did indeed provide the ground from the switch back to the mag.

Apparently, when the Bendix mags were first replaced with Slicks in the 70s someone (not me) just clipped off the hex nut connection, skinned the woven metal shield back an inch and reused the original P-leads. This should not have worked as the switch itself is not grounded and the shields were no longer attached to the mag. It looks like it only worked as the metal shields had been zip tied to the instrument panel bracing which provided the ground. Now that's a scary thought!.

My A&I installed a ground wire from the G terminal of the ignition switch to the back or the instrument panel. The shields are attached to the G terminal as well. He actually cut the Shield connectors off at the mag end so they could not be connected. This setup works but depends on multiple connections from the switch to the instrument panel to the firewall to the ground strap around the Lord bushing to the engine mount, then via ground wire around the forward starboard isolation mount to the engine block to mag case. That's a lot of connections.

What is the correct method?

Bill Garnett
Cessna 170B N2974D SN: 26917
End Quote"===============================================================

I should point out that while the woven metal shields on my original P-leads were very robust and could certainly have grounded the primary back through the switch to the mag ground screw, my new certified P-leads seem extremely wimpy by comparison. I'd be surprised if the shields are even 22 gauge wire. Just looking at them I don't trust them.

While I doubt if it would hurt to add another wire from the G terminal of the switch back to the mag ground screw, it would only be providing a redundant ground path

Bill, as I pointed out that advise is contrary to Slicks own install instructions that say to ground the shielding at the mag. What their instructions don't say is what to do with the shield at the switch. But we are really beating a dead horse.

How did Clyde want us to wire the mag switch? One infallible reference is the 170 parts catalog electrical wiring diagrams items number 35 and 36. Items 35 and 36, if you interpret the location of the semicolon as starting another thought to follow, reads "RC863 (shield) (Connects to ignition switch)" and next column "RC363 (shield) (Connects to magneto)".

johneeb wrote:How did Clyde want us to wire the mag switch? One infallible reference is the 170 parts catalog electrical wiring diagrams items number 35 and 36. Items 35 and 36, if you interpret the location of the semicolon as starting another thought to follow, reads "RC863 (shield) (Connects to ignition switch)" and next column "RC363 (shield) (Connects to magneto)".

Mag to Magswitch wire.jpg

I suspect this was accurate at the time and is exactly the way my original equipment Bendix Mags were configured. The debate arrises with conversion to Slick mags.

Bruce found Slick installation instructions that indicate the shield should be attached at the mag.

Champion Aerospace (who now own and manufacture Slick) recommend connecting the shield only at the switch end even when I pointed out that this leaves a screw labeled grd on the mag housing unused.

Others favor using the shield to complete the circuit from the mag via P-lead through the off switch grd back through the shield to the grd screw on the mag case vs. using a wire from the grd on the ignition switch to the airframe and leaving the shield disconnected at one end or the other.

It seems to me that either method depends on having the mag case grounded to the crankcase through the mag attachment bolts, then around the ...

Bruse is right, we (or maybe it's just me) are probably beating a dead horse.

The real message is check to make sure your mag switch is working correctly... often.

PS: I measured them and the shields are indeed 18 gauge. I stand corrected.