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.

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!

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.

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.

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!

Peter

Andrew Gideon wrote:

I posted this to the mailing list for the club to which
I belong, and I thought people here might appreciate it too.

When I joined the club a few months ago, a major motivation was to learn
more about the "operations" side of aircraft. To my mind, this meant
maintenance. As a renter, that side of things - outside of required
inspections and such - was a black box to me. I hoped to learn more
participating in the club.

I'm a lucky fellow, I guess.

A club 172 wouldn't start this morning. I got to watch it tugged to MD's
hanger, and peer over the shoulder of Tom as he looked at it "naked" (I
cannot remember the last time I looked closely at a plane sans cowling).

I learned that one wires the frame to the engine when trying to start
the engine with the cowling top off to prevent the frame from hitting
the spinner. I learned that there is a single "starter" magneto
(although I think I actually knew that already). I learned that the
engine won't start if something's amiss with the single starter magneto.

I learned that it might be something like "a wiring problem", or that it
might require replacement of the magneto. I learned that, when leaving
an airplane with maintenance, one doesn't use the cover or brakes, but
one does put the control lock into place.

I learned that airplanes look rather unhappy when left with maintenance
with the cowling off.

I also had the opportunity to call the person with a booking later today
to give him the Bad News. I didn't really have the full experience of
that, though, as I had to leave a message with his voicemail. I did
also send what I think was a "text message", just as a backup.

I'm getting what I said I wanted, right?

One of the notes I received in response was:

Welcome to the world of airplane (co)ownership.

- Andrew