When a P-lead breaks on your magneto...
- Thread starter EdFred
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It won’t ground and stays hot.
bflynn
Final Approach
Same thing that happens if you don't turn the ignition key off. The mag is "on" and if you turn the prop, it will spark. If there's residual fuel, the engine might kick over.
Be very careful not to have anything in the prop arc.
Pull the fuel shutoff and make sure the mixture is full lean.
Be very careful not to have anything in the prop arc.
Pull the fuel shutoff and make sure the mixture is full lean.
timwinters
Ejection Handle Pulled
Reel 'em in Ed!
Hang 4
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Hang 4
Ok, if it's a trick question, then the answer is nothing. At least while flying. As others have pointed out, after you shut down, life can get interesting.
EdFred
Taxi to Parking
Except when it doesn't stay hot.
Depends WHERE and how the P-lead breaks and on the type of magneto.
Story:
Was climbing through 6000 and noticed the MVP50 said "R Mag out" but the engine felt perfectly fine, and the run-up was normal. Precautionary landing anyway. Land, get off runway, go to idle. Ground the right mag, engine runs without an RPM drop. Ground the left mag - engine quits. Look inside cowl expecting to see shrapnel. All looks fine, except the P-lead on the right mag is broken. Put the plane away, wait till Monday. Head out to airport, and the mechanic says, "Oh I bet that it's not pushing that grounding plate inside back far enough and it's killing the mag." So we pop the back off and sure enough the end of the P-lead that presses the plate back is bouncing around inside the (non moving side) magneto.
So we got some new wire, made a couple soldered connections/shrink wrap (connection outside the mag) and put it back together. Back to normal. Here's my crude drawing of what it looks like going into the magneto (case and rest if internal wiring not drawn):
So, sometimes a broken P-lead will kill the magneto and it does not stay hot.
I have had one break and it did stay hot, but it broke outside the case and the internal bushings/washers/etc all stayed in place keeping the magneto hot. In this case the P-Lead broke at the washer - solder joint intact wire just broke near it. Close enough that the vibration caused the washer and rearmost bushing to fall and relieve the pressure against the grounding plate. When that happened it grounded the mag out and it goes cold.
Much cheaper fix than an OH/ new mag.
Edit: I don't have the model number of it, but it's a Bendix.
Edit 2: Went out and looked at it tonight, S6LN-21
Depends WHERE and how the P-lead breaks and on the type of magneto.
Story:
Was climbing through 6000 and noticed the MVP50 said "R Mag out" but the engine felt perfectly fine, and the run-up was normal. Precautionary landing anyway. Land, get off runway, go to idle. Ground the right mag, engine runs without an RPM drop. Ground the left mag - engine quits. Look inside cowl expecting to see shrapnel. All looks fine, except the P-lead on the right mag is broken. Put the plane away, wait till Monday. Head out to airport, and the mechanic says, "Oh I bet that it's not pushing that grounding plate inside back far enough and it's killing the mag." So we pop the back off and sure enough the end of the P-lead that presses the plate back is bouncing around inside the (non moving side) magneto.
So we got some new wire, made a couple soldered connections/shrink wrap (connection outside the mag) and put it back together. Back to normal. Here's my crude drawing of what it looks like going into the magneto (case and rest if internal wiring not drawn):
So, sometimes a broken P-lead will kill the magneto and it does not stay hot.
I have had one break and it did stay hot, but it broke outside the case and the internal bushings/washers/etc all stayed in place keeping the magneto hot. In this case the P-Lead broke at the washer - solder joint intact wire just broke near it. Close enough that the vibration caused the washer and rearmost bushing to fall and relieve the pressure against the grounding plate. When that happened it grounded the mag out and it goes cold.
Much cheaper fix than an OH/ new mag.
Edit: I don't have the model number of it, but it's a Bendix.
Edit 2: Went out and looked at it tonight, S6LN-21
Last edited:
bnt83
Final Approach
I'm pretty sure those magnetos are obsolete now and all new Bendix/TCM or whatever they call themselves now are like these, a ring terminal on the p-lead attaches the wire to the capacitor.
The older styles will be found in service for decades to come.
The older styles will be found in service for decades to come.
Last edited:
EdFred
Taxi to Parking
Oh, the mag looks ancient, but it's been OH'd so on the inside it's like a Timex. Just more of a heads up because I was also always told a broken P-lead equals hot mag.
Bill
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Why I do a mag ground check at least once every trip.
I think I now know why the Ushuaia Flying Club, southern tip of Argentina, does a mag check before shutting down at the end of a flight. That was the first time I had seen that done. The plane I DID NOT rent failed the post flight check, just one bad spark plug, but the maintenance was done immediately, so the next user had a good plane.
There is very little terrain there where a successful off airport landing can be made.
There is very little terrain there where a successful off airport landing can be made.
My mechanic learned that the ignition switch was bad in my plane when the magneto fired into her hand while she was working on it. "I hate it when that happens" was her approximate words.
Tom-D
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Tom-D
only for the old Bendix, Those the ceramic insulator and nut must be remove to ground the Primary circuit.
wilkersk
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I kill my engine with the mag switch also. "Idle-cutoff" is not a thing on Zenith carbs apparently....
zodiacflyguy
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I usually say other stuff in as many languages as I can think of.
Sent from my SM-N960U using Tapatalk
Sent from my SM-N960U using Tapatalk
Dan Thomas
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I had that carb. I turned the fuel off and waited for it to die, about a minute at idle or maybe ten seconds at 1500 RPM.I kill my engine with the mag switch also. "Idle-cutoff" is not a thing on Zenith carbs apparently....
The mixture control on those old carbs doesn't control the fuel flow directly. It governs the air pressure in the float bowl by admitting some suction from the venturi, which holds back on the fuel and leans the mixture. At idle there's no venturi suction, so you can't use it as an ICO.
wilkersk
Pattern Altitude
Yep. Idle-cutoff is not a thing on Zenith carbs.
Sometimes I close the fuel valve as part of the pos-landing checklist. It takes about a full minute and a half to burn all the fuel remaining in the line.
Radar Contact
Pattern Altitude
@EdFred , thanks for the lesson.
I just lost my left engine left mag P-lead 2 weeks ago. I have Bendix that I do the 500 hour IRAN/OH on. Mine was the more typical. Just never grounds out and stays hot. I was with them while they were trying to find the bad spot. Ended up just running a new line from the mag to switch.
Good to know different failure options.
I just lost my left engine left mag P-lead 2 weeks ago. I have Bendix that I do the 500 hour IRAN/OH on. Mine was the more typical. Just never grounds out and stays hot. I was with them while they were trying to find the bad spot. Ended up just running a new line from the mag to switch.
Good to know different failure options.
kyleb
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Drake the Outlaw
Doesn't the mag check at runup address the issue?
Dan Thomas
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It doesn't catch the P-lead that breaks during the flight. That's when they usually break, after all. Vibration.
Tom-D
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Tom-D
A completely disconnect P lead, shows no drop between mags, it will run is if there is nothing wrong, until you turn the mags off, then it will continue to run.
Next question, when you do this check, and one mag continues to run, which mag is running?
kyleb
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I've never been in an aircraft with no mag drop. The lack of a mag drop would be a pretty clear indication. And if one mag continued to fire in the either/or/both scenario, I don't see why it would be hard to identify the failed p-lead. Turn off mag A and no drop - bad lead on A... Turn off B and get a drop, good lead on B.
Gary Austin
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I think what Tom-D is getting at and the question: is Mag A the left or right mag, just because it says left, the P-leads could have been unknowingly swapped at some point.
kyleb
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I'd probably pop the hood and pull out the multi-meter to check wire continuity, assuming the broken lead wasn't visible. Ain't that hard.
Tom-D
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Tom-D
When you have a hot mag, the mag switch will be on OFF, and the engine will continue to run, how do you know which mag is hot?
You will of course kill the engine by the mixture. leaving one mag hot, which one?
Tom-D
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and when both P leads read good what then?