Magnetos:Starting A Piston Engine


There are two kinds of magnetos that you will encounter on piston engines: Impulse Coupling & Shower of Sparks.What are they, and why do you care?


Both are methods of creating a hot spark when the engine is turning over slowing during a start.


First, a little background:What the heck is a magneto, anyways?I strongly doubt you will ever encounter a magneto on a car.Cars use their batteries to power their electrical systems, including the high-voltage ignition system that powers the spark plugs.If a carís battery dies, so will itís ignition system, and hence itís engine.


This is not permitted on a certified aircraft.Flying along, you can flip the master electrical switch off, and the engine will continue to hum along quite nicely.You donít even need a battery in a light aircraft to start and run it!


Thatís the magic of the magneto Ė itís completely self-contained, and generates spark all by itself, without any external source of electrical power.

On the primary (low-voltage) side, all a magneto has is a wire (called a p-lead) running back to the cockpit, and if that wire is connected to aircraft ground, the magneto will not fire.This is a ďfail-safeĒ system design Ė if the wire breaks in flight, the magneto will continue to run, which is good, but you will have a ďhot magĒ on the ground, which is not good from a safety standpoint.


Anyways, for even more redundancy, certified aircraft engines have TWO magnetos, each driving their own spark plug in each cylinder.You can have a magneto fail in flight (happened to me, decades ago) and you wonít even notice it.



Now, there are two different systems that you will encounter as a pilot, which assist the magneto in producing a hot spark during start:Impluse coupling & shower of sparks.


Impulse coupling is probably the most common, and you can spot it when you open the cowlings - it appears as a mechanical shim about 1.5 inches thick, between the left magneto and the accessory case on the back of the engine that it bolts to. Usually there is no impulse coupling on the right magneto.


The impulse coupling is a neat mechanical trick, where at extremely low RPM it winds up a spring, then releases so that the magneto is spun rapidly, after the piston reaches top dead center. This both provides a hot spark, and delays the firing of the spark plug, both of which are good for starting.When the RPM increases, the impulse coupling disengages and is straight-through.


With an aircraft with impulse coupling on the left magneto only, you want the right magneto grounded during cranking. This is done automatically when the key is on the "start" position of the Bendix combined magneto/starter switch, but some airplanes don't have this fancy switch.


Instead they have separate switches for the magneto and starter. In this configuration, you as the pilot should ONLY select the LEFT magneto on for start - you want to leave the right mag off, to avoid a possible nastly kickback from the normally-advanced (weak) spark.


After the engine starts, then manually select the right magneto on.


If the engine has impulse couplings on both magnetos (I have seen that on TCM IO-360 engines) then select both magnetos on for start.


This is the sort of thing that pilots should really understand. It's probably buried somewhere in the POH, but ...



Now onto "shower of sparks". This technology was in common use on the incredibly ancient Ford Model T.Itís not used as much on modern piston aircraft as impulse coupling, but you might still encounter it. I do!


For starting with shower of sparks, the right magneto is normal and should be grounded out during cranking, just like impulse coupling.


However left magneto has an extra set of breaker points, which open after top dead center (called the retarded breaker points) and an external buzz box to provide the hot spark for starting.


Executive summary: you probably want to start on the left magneto only!


--†† Nov 2011