Questions about Keyed Ignition Markings

[SeanB]

Hello all. I'm in the process of wiring my panel. I chose to use the ACS Keyed Ignition (Bendix copy).

https://www.aircraftspruce.com/catal...gswitches3.php

I've noticed this before and wondered, but now since I'm actually wiring, am really curious! Why is the placard for the keyed ignition: OFF R L BOTH START ? Why are R and L backward? Is it because when I do a ground check for "R" I'm actually observing how the "L" side of the engine runs and the "R" side does not?

On top of that, I need to install test buttons for the dual P Mags. My thinking is to install them to match the "reverse" order of the keyed ignition placard. So, "R" PMAG test will be just above the "R" position on the keyed ignition switch, then "L" to follow suite.

Kind of confusing to me. Thanks for any thoughts.

[SeanB]

I was sent this article from a helpful support guy at ACS. Helps me understand, but the placard still appears backward to me. As long as I understand what's happening, then all is good.

https://www.aopa.org/news-and-media/...-magneto-check

After reading...my understanding:

OFF = Both P-Leads grounded...mags won't run if wiring is good.
Switch to R = Grounds L magneto. R ignition is verified running.
Switch to L = Grounds R magneto. L ignition is verified running.

As you can tell, my flight training didn't go into great detail on this subject. We of course checked for rpm drop during run-up by switching mags, and checked the OFF position on shutdown before leaning to kill the engine to verify grounds. Building a plane calls out your hand more than only operating systems. Happy to continue learning.

[Ralph Inkster]

The article tells most of the story, quickly scanning it I didn't see it mentioning the starting switch sequence.

Basically it's handling the two mags, for 'Mag Checks' as well for Starting. I assume you have mags on your engine.

Normal running both mags will be running and triggering the plugs at the same timing mark, usually 25 degrees before top dead center. During the start sequence it would be very hard to start the engine at this timing mark so one of the mags (the left one) is an Impulse Coupled mag, it will retard the ignition mark close to top dead center for that mag only when the engine is turning over very slowly for start. You would want only the Inpulse coupled mag live during start, firing both mags at this time would be counter productive as the mag with the most timing advance will light the fire first & most likely fight starting. After the engine catches & goes over a certain RPM, the Impulse Mag reverts to the 25 degree timing mark.

The Ignition Switch has Off-R-L-Both-Start positions are probably set out this way because of the circuit layout internally in the switch... what you don't see is that the Start position will ground out the right mag during Start (Note - the instructions that came with the switch shows a little jumper plate on the back of the switch terminals, be sure to follow those instructions.) So when you are holding the switch in the Start position & turning over the starter, only the Left mag is operating, as soon as you let go of the key the switch reverts to the Both - now both magnetos are firing at the 25 degree BTDC timing mark.

Hope this helps