Peter Dohm wrote in message ...
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.

What does a single impulse coupling have to do with liability and
the risk of injury? Most of the singles in the fleet have a single
impulse coupling on the left 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!

It's really a safety feature? That's a new one on me. As far as I
know, the impulse coupling retards the spark to enable the engine to
start at low rpm. It also gives the mag a quick "spin" to increase
the power of the spark. If you have no coupling, you get no start
(unless the plane is equipped with a "shower of sparks" system).


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 wouldn't say that "most" C-172s are equipped with the dual
magneto. The ones that have it are noted by a "D" in the forth
position of the engine identifier (e.g. O-320-H2AD). Dual mags are
not that popular because they have a common point of failure. The
drive gear. This goes against the notion of having two redundant
ignition systems. They are not common in the fleet.

snip
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.

I don't understand. What makes dual impulse couplings safer. If
you have a single coupling and it fails, you get no start. The normal
spark timing is to far advanced and the mags are turning too slow to
provide a strong enough spark at the proper time.

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!

I must be missing something here. The only airplanes I've ever hand
started had a single coupling on the left mag. This is the most
common configuration for piston singles. Could you elaborate on why
you think this is dangerous?

John Galban=====N4BQ (PA28-180)