A new engine is currently being fitted into my plane. The engineer has been complaining that the new plug leads are too long. To get over this problem he has doubled the lead back on itself with a very tight radius and then returned again back to the original point, thereby effectively shortening the lead.
For a low voltage cable, like the engine analyser, I know this is not a problem, but what about a Plug Lead?
I am sure I have read somewhere that this is not good practise as the plug leads should not be put through such sharp turns. I can’t remember why I think this is not good practise, maybe it increases resistance, or leads to heat build up, or Eddy currents. I am not sure.
Can anyone tell me if this practise can cause problems in Plug Leads.
Definitely not good, because it compresses the high voltage insulation and that creates points where the dielectric stress (in volts per metre) is greater than it should be.
The problem is that the lead could be permanently damaged if it was kinked hard enough.
The insulation could even just split…
If somebody did this to me I would bin the harness and fit a new one. A few hundred quid… but with a new engine always fit a new harness (and new plugs, usually).
I would have at least 2cm radius.
The only reason I can think of is a chance to damage the leads. However, new high-quality leads should not be very susceptible to breakage by mere folding in half (unless severely kinked), and it may well be safer to have them this way rather than dangling all over the place. On the other hand, one peculiar thing about a broken plug lead is that you don’t initially notice it – at first it works almost as well as an intact wire, but deteriorates as the broken location gradually burns up. At the bottom line, I agree with Peter, a 2 cm radius seems a reasonable minimum figure for a bend.
Thanks Peter,
I knew this wasn’t a good idea, but couldn’t think why.
Everything is new on the installation, plugs, leads, magnetos’ , governor etc. Zero time engine and new prop.
A problem has arisen for me with a single plug lead giving RT interference. At least we think that is the issue. In flight it can be traced to one mag and the interference is in marmony with RPM. Do I (1) test for which lead and how or (2) fit new harness or (3) can anyone suggest a different cause.
Thanks
It’s possibly a dodgy lead, and you might be able to find it with a multimeter (if there’s a broken lead, it might work fine – because the high voltage when in use will flash over the break and cause your radio interference, but at low voltage will be an open circuit you can measure). And if that’s the case, it’s probably time for a new harness on that side. Last time it happened to me, it was a lead that was failing (the harness was old enough I did both sides).
In flight it can be traced to one mag and the interference is in marmony with RPM.
It could be several things: one or more of the HT leads, the capacitor (“condenser”) on that mag, something with a spark plug (if you use the old Champion ones with the old dodgy resistors).
Thanks for the answers. Peter how does one test the capacitor on a mag or who can test one for me. How do I find the correct one with which to replace it just in case. The answers presumably will be in some engineering manual somewhere but where? It is a very confusing scene to try and source such details!!
To test a capacitor you need a capacitance meter. They can quite cheap nowadays, but they test at a few volts at most and in this case I would try to test it at the rated voltage, because it could be breaking down at higher voltages. Yes the capacitor type should be in the overhaul manual for the magneto. They are cheap – chuck them out with every mag overhaul, or inspection if something is suspected.
I also wonder if this could be poor grounding. The shields on the ignition leads are connected to the magneto case, which is connected to the magneto body, which is connected to the engine block, which is supposed to be well connected to the airframe, not least because the starter motor current usually returns that way.
If it was me I would replace the whole ignition harness. It’s probably a few hundred quid but if it is some years old, it may not be in a great condition, especially if it has been clamped badly somewhere.
Is the spark timing slightly different for each of the 2 plugs in a cylinder and how is this achieved with the magneto setting? If so what is the timing difference and is this possibly where electronic ignition scores over the magneto??
