Aircraft Magneto P-Leads -- Three Types

P-Leads are critical to aircraft safety. The P-Lead is the shielded wire that connects the magnetos to the mag switch in the cockpit. If this wire breaks, the ignition may always be "HOT" with the engine ready to fire. This is extremely dangerous for anyone moving the propeller even a little bit. If this lead shorts to the engine or airframe or shorts between the center conductor and the shielding, the ignition will be "KILLED" and the magneto will not work. Either way, it's a bad situation. Replace worn P-Leads with new if the shielding is frayed or the insulation is cracked. Richard Bogert talks about the three main types of P-Leads commonly found in service on Bendix and Slick style magnetos and how to tell them apart. Thanks to Quality Aircraft Accessories for some of the magneto images.

[@Windtee]

An excellent description and demo on the variety of magneto models and types. P-Lead wiring is just as important as the mag itself.

[@richardbogert7675]

It's amazing to me the number of people that tell me that they were never taught anything about p-leads in tech school. I wasn't either that I can remember but that was a long, long, long time ago. You are absolutely correct that the p-leads are very important and mostly overlooked. Glad you liked the video.

[@Windtee]

Richard Bogert I enjoyed it as much as a mag is "hot". Thanks for taking the time in creating a very well explained and clearly illustrated video-production.
I already shared it with a student-pilot friend of mine.
Continued success in your business.

[@ukrsindicat]

Thank you for discussing single drive mags. I always was wondering about them

[@CH67guy1]

Good video! I learned something. I was only familiar with the round nut. Thank you.

[@Bogibar]

These three types take care of the majority of aircraft that are in service today. There are several other types of P-Leads that I encounter once in a while. These are found on really old and basically obsolete magnetos used before we had shielded ignitions (Bendix and Eisemann, CASE). They might still be used on antique aircraft and restorations. The other place you may find something different is in radial engines and WWII "V" engines.

[@Bogibar]

I've seen a lot of p-leads that are in pretty rough shape. The insulation is cracked, the shielding lead (ground) is broken or patched together with butt connectors. It is important that the shielding is connected! That is the direct ground path. It's part of the system and needs to be taken seriously.

[@CH67guy1]

Would you believe it if I told you that in my A&P school, P leads were not discussed, beyond saying that a complete circuit to the mag switch is necessary to kill the mag. No discussion of types of P leads or hands on removal of a single solitary P lead! I mean they are simple enough to figure out on our own in the field, but I'm just saying!

[@cehupper]

What does the 'P' Stand for and why? Thanks

[@BogertAviation]

The "P" P-lead comes from the primary winding in the magneto's coil.

[@sblack48]

The 2 different bendix types have either a hex nut that is drilled for safety wire, which suggests that it needs some torque, or a round knurled nut that is not drilled for safety wire implying that finger tight is good enough. That is very puzzling. Does the round nut type use a tapered thread? Is the hex nut just overkill? Anybody know?

[@Bogibar]

Maybe that's why so many P-Leads are in bad shape. People replace engines, exhaust systems, mags, alternators, hoses etc. and hook it all up to the same crappy wiring that was in the plane from it's birthday. The wiring is important. Trouble free flying is not an accident... it's on purpose.

[@frankdavidson9675]

most of my flying was c47 during the korean war they ran so smooth i was i air borne so never landed in one that i can remember prob. one of the safest planes made

[@blueskylazer]

How do I isolate the bad p-lead or bad magneto? My cherokee 140, when I use the ignition switch to shut the engine per AD does not shut down like in the past. It will continue running, but not smoothly,... not like it was operating on one good magneto. It barely runs, but keeps kicking over, where I have to use the mixture to shut it down. I checked the grounding at the ignition switch at all positions and checks out with a multitester. I've bypassed the ignition switch completely with two ground wires from the mag p-leads and tried to shut the engine by grounding both wires... same result, barely running, but rough. I've changed the ignition switch to verify and it still has the same problem. My next attempt will be to try to bypass the p leads completely with separate wires, at least, for testing, to see if I get the same results. I don't know if this will give me more information as I don't have spare p-leads for testing. The fact that during mag checks the mag rpm drops are proper with about 75 rpm each mag, the engine operates normally at all normal operation with BOTH mags, I can't understand why the engine continues to run... but roughly, if I ground both mags.

[@richardbogert7675]

Blueskylazer,
I hope you called us or sent us an email about your problem. I have to admit that I don't check out the comments made on our You-tube videos that often. I certainly care about what is written and about our customers,we just don't have time to review all the videos for comments You might have noticed that we have a lot of videos and we have videos for our other product lines as well.

[@richardbogert7675]

Blueskylazer - Sorry I didn't reply back to you. Did you get your issue figured out? We have so many videos that I can not check them all to see if someone left a comment or question. Sorry

[@doncridlebaugh6790]

I know this is old, but I would have been curious to know what the mag drop was on single mag operation at idle.

[@doncridlebaugh6790]

And how fast the engine would run at an above idle setting with the ignition turned off.