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PHOTO BY MEGAN VANDE VOORT
When I speak at various events, one of my favorite show-and-tell items is a magneto with an impulse coupling attached. It’s a great visual aid for a use- ful demonstration of an ignition system during engine start. More importantly, it demonstrates why a propeller should never be turned by hand in its operating direction if one wants to avoid serious bodily injury.
I recently saw a YouTube video from a security camera at an airport showing a
guy that hand-propped his Cirrus. If you haven’t seen it yet – spoiler alert – yes, the engine started and ran away with the guy frantically trying to get into the cabin to avoid the oncoming catastro- phe. The plane traveled a good distance before impacting a nearby hangar and significantly damaging the airplane. The impact did stop the engine, though, and no one was seriously injured, except for the ego and wallet of the aircraft owner. Of course, none of this would have been
possible if it weren’t for that ingenious bit of engineering inside the magneto called the impulse coupling.
There is a false conception among pilots that an engine starts better if the starter turns faster. When equipped with an impulse coupling, there is no need for much rotation speed. The impulse cou- pling retards the spark until after the pis- ton passes top dead center, then spins the magneto fast enough to generate a good spark at the plugs. If the fuel and
VOLUME 18, NUMBER 67
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COPA U CORNER