Peter Dohm wrote:
I am sure that there are airplanes still in service that have engine(s) with
impulse coupling one only one of two magnetos; but I doubt the wisdom due to
liability and real risk of injury.

I'm pretty sure most of the fleet has impulse couplings on one mag only.


The purpose of the impulse coupling is to retard the spark; the fact that it
will usually also "fire a weak mag" is only a beneficial side effect. It is
really a safety feature to assure that the engine will start forward rather than
backward--since "kicking back" tends to remove the starter drive or the
lineman's fingers!

Not sure in what context the impulse coupling will "fire a weak mag". The
impulse coupling only comes into play at very low RPM, like cranking speeds.


Many, if not most, of the C-172s use a "dual magneto", which is really two
magnetos on a common drive assembly including a single impulse coupling which
retards both of the magnetos for starting. If the engine has two separate
magnetos with impulse couplings, you can frequently hear the distinctive double
click when the engine is spun over to start.

I don't think "most" C172s have a dual magneto on a common shaft, but then I'm
not a Cessna owner, so what do I know. Maybe the "new" 172s have this
arrangement? I think "most" C172s have dual magnetos, each with its own drive
shaft, and with an impulse coupling on just one magneto.


The POH is a good source for documentation on what was standard, and what was
available when your airplane was built. The depot manual for the engine is
another good source, and one which may shed some light on possible modifications
to alternate "dash numbers". If I had an engine with impulse coupling on one
mag, I would locate any necessary documentation and make DAMNED SURE it had dual
impulse coupling before it ran again.

OK, I'm not aware of any STCs to convert to dual impulse couplings. Anyway I
wouldn't spend any money on one if it were available, but that's just me.


If you are not sure, or believe that your particular engine has impulse coupling
on only one magneto; then DON'T EVEN THINK ABOUT HAND STARTING IT!


Well, I seldom think about hand-starting it anyway, but having a single impulse
coupling doesn't enter my thinking on the subject.

Dave

Remove SHIRT to reply directly.