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I just learnt, mostly by reading about something else, that magnetos are the primary sources of ignition in planes and other types of engines. The article I read said that it was important that a P-lead was working properly because if it wasn't, then moving the blade or propeller could instantly start it up, and there would be no way to shut it off until it ran out of fuel or oxygen (the latter of which would be hard to do). Or maybe cutting the wires would do it.

I also found a question on Aviation SE and posted a comment but thought I'd ask here just in case.

I kind of understand how grounding works, but I don't have a full understanding of it. I know that one type of grounding is to send excessive electrical build-up into the ground's surface via a grounding rod or pipe. The other end of the grounding wire would be connected to the metal casing of the appliance. I imagine it like a damb having an emergency valve that would open if the flood was too severe, and it was about to burst. Another type of grounding is used as a reference point to measure voltage.

Is there another type of grounding that magnetos use to stop the flow of electricity to prevent them from starting the engine?

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    \$\begingroup\$ Not that diesel engines do not use electric ignition and luckily we can still switch them off :-) "I kind of understand how grounding works" Sorry but no, from your description you do not understand grounding. You can not "send excessive electrical build-up into the ground's surface" A magneto is part of an electric circuit and to stop the ignition you break the circuit, just like you do with the light switch in your house. \$\endgroup\$
    – Oldfart
    Mar 28, 2020 at 15:33
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    \$\begingroup\$ Lawn mower ignitions often short the magneto so that no spark can be generated at all. An open circuit is an invitation for high voltage to go some place you didn't expect. \$\endgroup\$
    – JRE
    Mar 28, 2020 at 15:44
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    \$\begingroup\$ @JRE this is also how kill switches on an outboard motor work. The operator is tied to a piece of plastic between 2 contacts across the breaker switch. If he falls overboard pulling that plastic out, the "kill switch" grounds the magneto so that when the breaker opens, magneto still shorted, no spark. He still has to climb aboard though... \$\endgroup\$
    – user16324
    Mar 28, 2020 at 16:16

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"P-lead" is not an electrical engineering term. It appears to be exclusively a term used with aircraft magnetos. In electrical engineering, we would call the "p-lead" connection a short circuit. It provides a low resistance path that allows current to flow past an area where you want to prevent it from flowing. For an engine with a magneto connecting the "P-lead" forces the current from the magneto to flow around the spark plug and back to the magneto.

In electrical engineering, we try to reserve the word "ground" to refer to an electrical connection to the earth. The word "common" refers to a conductor or series of conductors that is connected to one side of the power supply and is usually connected to "ground" at just one point. In a vehicle, the "common" is often connected to all of the metal parts of the vehicle and sometimes called "ground." Since engine spark plugs have current conducted through an insulated piece to the spark gap, then across the gap as a spark to the part that is threaded into the engine block, the "P-lead" that prevents that spark provides a parallel path from the insulated part of the spark plug to the engine block. The engine block is the common point or "ground" for the magneto (power supply). With the "P-lead" connected, current generated by the magneto flows from the magneto coil through the high-voltage wire, through the "P-lead", through the metal parts of the engine, and back to the magneto. That forces the magneto voltage to nearly zero and prevents a spark across the spark gap.

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The accepted answer is correct from an electrical engineering point of view, but may be confusing from an aviation standpoint which seems to focus on the P-Lead.

I am an ATPL instructor for this subject, so please allow me to clarify.

The P-Lead (Primary Lead) in aviation is not just a "short-circuit", it is a physical component - a wire - that goes from the Primary winding of the magneto (attached to the engine) to the ignition switch in the cockpit. As such it is fairly long and has to pass through the firewall between the engine compartment and the cockpit. It is therefore vulnerable to being broken (open circuit) or suffering some chaffed insulation, which may cause it to short out to some extaneous metalwork that forms part of the common ground of the aircraft fuselage.

The switch and the P-Lead together bypass the contact breaker points which are designed to open and close to produce the spark.

An AC waveform is produced continuously in the primary circuit while the contact breaker is closed, but this is a fairly low voltage and is not strong enough to produce a high enough voltage in the secondary to produce a spark.

Typcially the secondary circuit will produce a pulse of around 25,000 - 30,000 volts at the exact moment that the contact breaker points open, which causes the magnetic field in the primary circuit to suddenly collapse. This rapid collapse of the stored energy in the soft iron core is what induces the high voltage in the secondary coil, but it only happens when the contact breaker opens.

When the ignition switch is closed, it bypasses the contact breaker so that the circuit can never be broken. In this way, a small AC voltage is continuously induced by the alternating primary current in the secondary, but not enough to produce a spark.

It is designed this way to be fail-safe in flight. Should the P-Lead be broken, the contact breaker points will continue to open and close to produce the spark to keep the engine running. It is not, however, fail safe on the ground as an open circuit on the P-Lead would prevent the mageneto from being shut down by turning the ignition off. (This is why engine shut-down is usually achieved by shutting off the Idle Cut Off (ICO) valve in the carburettor and starving the engine of fuel).

The other failure mode - chaffing of the insulation, is less likely but if a ground short did occur in flight, then the magneto would stop working. To cover this eventuality, two independent ignition systems are used with two spark plugs in each cylinder. One set is driven by the "Left" magneto and the other set driven by the "right" magneto. Normally both magnetos are used to improve combustion efficiency and combustion speed. If one magneto were to fail, the other magneto and its associated spark plugs would continue to keep the engine running, although a drop of around 100 RPM may be noticed due to the impaired combustion efficiency.

The whole system should be checked by the pilot on the ground before and after flight according to the checklist in the aircraft flight manual.

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So on lawnmowers the magnetos have two coils. One of them is high tension with very fine wire and a huge number of turns, the large number is required for high enough voltage to ionize air and cause a spark, which ends up being a tiny current due to high coil resistance and inductance. The spark pulse is cause by induced voltage pulse caused by a field pulse generated by the revolving flywheel permanent magnet.

The other coil is with a much lower number of turns with a heavier gauge of wire to allow a much higher current than the high voltage coil because of lower resistance and inductance. It generates a low voltage pulse because of the low number of turns. The lower voltage is for safety.

Both of these coils are grounded on one terminal. So when the ungrounded terminal of the low voltage coil is open it does nothing. But when it is grounded (say by a switch) Lenz's law is key to understanding what is going on here. The current induced in the now shorted low voltage coil opposes the magnetic field that is inducing it. It does this by generating a magnetomotive force in opposition to the flywheel generated magnetic pulse. This results in a much reduced magnetic field pulse which then cannot generate the required ionizing voltage, and the spark is quenched, just as it would be if the high tension coil were itself shorted.

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There is an excellent Q+A with a nice image of the ignition system here: aircraft high-tension magneto ignition system.

There are a couple of things a pilot can do to ensure the aircraft engine will not start if the prop is spun by hand. Let's assume we are talking about a Cessna product with a carburetor.

  1. Live mag check. Before shutting down the engine, the ignition switch is turned to the off position for 1 second, then turned back to the both position. If the switch / wiring / GND connections are OK, you'll hear the engine RPM drop. If the engine keeps running in the ignition off position, you have a problem somewhere in that part of the system.

  2. After the live mag check comes the normal shutdown procedure. With the engine at idle RPM, the pilot pulls the mixture control knob which shuts off the fuel to the carb. Once the engine "dies" due to the lack of fuel, the ignition switch is then turned to off.

If the pilot follows the aircraft's checklist, and all the systems are working correctly, the engine should not start if someone was to spin the prop by hand.

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@VE7JRO provides a great illustration and explains the aircraft procedure, but doesn't explain the magneto. Which we should do here on EE.

The magnetic field passing across the coil is not moving fast enough to create a high voltage spark. The spark is produced by having the points open in the middle of the cycle, after the coil has been charged. With the points closed, the coil primary is a closed circuit, and the moving magnetic field induces a current in this coil. Once this current is flowing the coil is "charged". Opening the points interrupts this current flow instantly, and the field that it produced collapses, causing a very steep voltage spike on the output.

If you keep the points from doing their job, you will kill the spark. In this instance, "grounding" simply means, "connect to ground". It merely short-circuits the points, so they can't open the circuit.

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  • \$\begingroup\$ I answered this part of the OP's question: "The article I read said that it was important that a P-lead was working properly because if it wasn't, then moving the blade or propeller could instantly start it up, and...". The magneto part of their question already has a good answer, and I provided a link to it in my answer's first paragraph. The OP's third paragraph suggests a complete mis-understanding about what ground is, with regards to home appliances? If that is the case, then this may help them: youtube.com/watch?v=xn4az8EuNO0 \$\endgroup\$
    – VE7JRO
    Mar 29, 2020 at 1:14
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The "P" in P-lead stands for Primary. The p-lead is connected to the capacitor of the magneto primary circuit. When the ignition switch is turned to "off", both magnetos are grounded and can't fire the park plug (secondary circuit). The P-lead grounds the magneto when the ignition switch is selected to do so. When ignition switch is turned to "both", both magneto primary circuits are ungrounded. It is the removal of ground that allows the primary contact points to control the primary circuit. The primary contact points open and close. When the point open, the primary circuit collapses rapidly and induced a high voltage current in the secondary circuit. The secondary circuit is made up of distributor, high tension leads and spark plug. When high voltahe current is induced in the secondary circuit, the spark jumps acriss two places, across the distributor finger to electrode and across the elecrode of the spark plug. If the ignition switch is turned to "both" the magneto can fire the spark plug. If the P-lead is broken, the magneto can fire the spark plug (secondary circuit) It's more complicated than this becuase there is also a magnetic circuit that produces the AC voltage in the primary circuit. But that's another discussion

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