Magnetos:† Starting A Piston
are two kinds of magnetos that you will encounter on piston engines: Impulse
Coupling & Shower of
are methods of creating a hot spark when the engine is turning over slowing
during a start.
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.
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!
the magic of the magneto Ė itís completely self-contained, and generates spark
all by itself, without any external source of electrical power.
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.
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.
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.
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.
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.
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.
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.
the engine starts, then manually select the right magneto on.
the engine has impulse couplings on both magnetos (I have seen that on TCM
IO-360 engines) then select both magnetos on for start.
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!
starting with shower of sparks, the right magneto is normal and should be
grounded out during cranking, just like impulse coupling.
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.
summary: you probably want to start on the left magneto only!