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THE question I could never find a good answer for (labelling of the ignition switch)

So, I just came across this thread. Is it still a mystery or someone can provide a definite answer to the question? If the mag switch was rotated 180 degrees, then why not to put impulse coupling on the right magneto?

LKHK, Czech Republic

I suspect Robert Nucholls’ paper above contains the best clues, and the rest is just tradition.

Shoreham EGKA, United Kingdom

I just thought it was because on some types (Grob 115E for example) we start on Left only and then as engine fires select full rich and mags to Both. You wouldn’t want to have to go through isolating Left mag and onto Right only during the start sequence, so the switch is designed so you go straight to Both from Left.

Now retired from forums best wishes

No particular reason IMO. It’s just a switch, and a particular convention got stuck. Lots of planes have have single throw switches for each ignition and a push button for the starter, even old ones. The Pawnee comes to mind.


And why do people (and POH) insist to go back to both in between left and right?

And why do people (and POH) insist to go back to both in between left and right?

To make an equal (start from higher RPM) test for both mags, I presume .

LKHK, Czech Republic

But, if you start on both, to go L, see a normal drop, then go to right, you can also compare really well the drop between L and R.

I tend to think that, the least you do, the least might go wrong. The POH method both has more moves, and sometimes the move back to both is 1 click, sometimes 2 clicks, which could lead to you just going to L when you wanted to go back from R to Both.
I also find it a better way that you guarantee that you don’t go back to L after having done Both → L → both.

I can’t think of a reason why clearly not to use any of the two methods, but I’ve been asked to go back to both when I did the Both → L → R → Both method. It isn’t something I really want to argue about, but keep wondering if it isn’t just some “tradition” that is being kept for no reason.

To be clear, the method is essentially:
Both: Note RPM
L: Check small drop, well within range
R: Check no significant drop from L.

If L were to drop close to max drop and R even more, having to check Both → Right to see the absolute drop. But in hundreds of these checks, in many airplanes, that has never happened. (Either drops were small, as expected, or very large – never on the fence)

Last Edited by Noe at 09 Sep 07:59

Lycoming direct back to BOTH and let engine speed recover.

Now retired from forums best wishes

I know every POH says it, I just never found a convincing reason as to why

It’s a bit funny. On the Rotax iS there is no drop at all. I have never seen any. But what you look for is also the engine management and monitoring systems. They are divided between the two “lanes”; A and B. So it is also a check to see that both lanes are working (both ECUs with their own generator) and that they monitor what they are supposed to monitor. Each ECU has it’s own ignition system, and each ECU (“lane”) monitors slightly different sub-systems.

There is no mention of testing any particular lane first, but you have to go back to both lanes, due to how the switches are set up. If both are turned off, the engine stops.

This makes me think of how the “traditional” combined ignition switch actually work. When switching from L to R, there has to be some middle point where both are ON, or the engine would stumble. The only logical way it should be made (IMO) is L-BOTH-R (+ OFF in any position left of L or right of R or rather both). The most logical is to have two switches, one for L and one for R.

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