Cessna 172N P-lead length

[Kevin Holbrook]

I have to travel to replace p-leads on a C172N which has damaged p-leads. Another mechanic has noted them to be unrepairable and the aircraft therefore unairworthy.

Time is not of of the essence, but I would like to only make one trip. For cost, availability, and quality customer wants to use Bogert p-leads instead of the Cessna product.

Does anyone know the length of the p-leads on this plane? Are the right and left the same length?

Thanks!

 

[Tom-D]

they are usually built for raw materials.

 

[Timbeck2]

Unrepairable? As Tom says, they are usually built from raw materials which is a wire, a tiny "washer" from a pop rivet and solder.

 

[Kevin Holbrook]

According to my customer, the mechanic who saw the airplane stated there are multiple breaks in the shield, leading to intermittent failure to ground the mags. I know that they can be fabricated in the field, my customer wants new prefabricated p-leads.

Even if I were to fabricate them myself, this is to be a quick in and out job at a remote airfield and I would prefer to build in my shop ahead of time. But again, my customer wants new factory fabricated p-leads.

 

[Doug Reid]

The shield has nothing to do with grounding the mags...the center conductor does that. The shield is to prevent noise.

 

[Bell206]

my customer wants new factory fabricated p-leads.

FYI: if I recall Bogert leads are not TSO/PMA and does them as an owner-produced at the direction of the owner's request which technically require a separate write up by the owner. FWIW: in the past, I've simply taken a roll of shielded wire, solder sleeves, and ring connectors and repaired on site at whatever length needed regardless of location.

 

[Tom-D]

I know that they can be fabricated in the field, my customer wants new prefabricated p-leads.

But again, my customer wants new factory fabricated p-leads.

ask him to show us some.

 

[bnt83]

I have to travel to replace p-leads on a C172N which has damaged p-leads. Another mechanic has noted them to be unrepairable and the aircraft therefore unairworthy.

Time is not of of the essence, but I would like to only make one trip. For cost, availability, and quality customer wants to use Bogert p-leads instead of the Cessna product.

Does anyone know the length of the p-leads on this plane? Are the right and left the same length?

Thanks!

First thing you need to know is what engine and magnetos are in it. Does it have the O-320-H2AD and do you have terminal kits for the D-2XXX/D-3XXX magneto on it? Has it been converted to O-360 with some other mags?

I've always made my own using M27500-18TG1T14 wire.

The wiring diagrams show the length or wire in inches if you can find the cessna 172N service manual. 6 feet each should be more than adequate, route wires, cut to ength, strip and crimp on terminals. While under there verify which ignition switch is installed, they all have ADs on them.

 

[Dan Thomas]

The wiring diagrams show the length or wire in inches if you can find the cessna 172N service manual. 6 feet each should be more than adequate, route wires, cut to ength, strip and crimp on terminals. While under there verify which ignition switch is installed, they all have ADs on them.

The manual is here: https://www.redskyventures.org/doc/cessna-maintenance-manuals/Cessna_172_1977-1986_MM_D2065-3-13.pdf

I checked the two wiring diagrams for the P-leads, but no lengths are specified. As you say, six feet should be plenty. The panel, IIRC, is 19 inches aft of the lower firewall, and so the upper firewall will be about three inches closer than that. The wire bundle exits the top left corner of the firewall, so the leads need to reach from the switch and up about a foot and forward 16 inches and then across the front side of the forewall to the dual mag or separate mags on an O-360.

The switch is almost certainly an ACS, and there should be a diode on the starter contactor. A least half the ACS switches I came across were out of compliance with the AD. Some mechanics think all ignition switches are Bendixes.

 

[Kevin Holbrook]

The manual is here: https://www.redskyventures.org/doc/cessna-maintenance-manuals/Cessna_172_1977-1986_MM_D2065-3-13.pdf

I checked the two wiring diagrams for the P-leads, but no lengths are specified. As you say, six feet should be plenty. The panel, IIRC, is 19 inches aft of the lower firewall, and so the upper firewall will be about three inches closer than that. The wire bundle exits the top left corner of the firewall, so the leads need to reach from the switch and up about a foot and forward 16 inches and then across the front side of the forewall to the dual mag or separate mags on an O-360.

The switch is almost certainly an ACS, and there should be a diode on the starter contactor. A least half the ACS switches I came across were out of compliance with the AD. Some mechanics think all ignition switches are Bendixes.

OP here...


Thank you! I had gone through the maintenance manual and didn't find lengths either.

The lead times on the prefabricated p-leads from Bogert are pretty long in this COVID era.

I reminded my customer that I was already responsible for about half of the wires and connections on his airplane so we'll be flying up with a few yards of wire and some tools.

Thanks for all the input!