Small Continentals typically have impulse coupling on both mags.
Same on the C90 in the Vagabond. I am not 100% clear whether there are impulse couplings on both or not.
Hand propping the Super Cub (Continental C90) is done in “both”.
Maoraigh wrote:
PS For 7 years our O200 had the L and R key switch positions reversed, and nobody noticed. The Jodel was under the CAA, and it was only noticed when I had a bad mag drop when about to depart after her second Star Annual, and returned to the engineer for checking.
That’s interesting. My understanding is that one of the mags is an impulse magneto and only that one should be active when you start the engine. So when you turn the key to “start”, the non-impulse mag is grounded. If the L/R connections were reversed then the wrong mag would be grounded and you would have starting problems.
“When switching from L to R, there has to be some middle point where both are ON, or the engine would stumble.”
The switch earth’s the relevant mag to make it off. So in between neither is earthed – both are on. But it will be difficult to hold in exactly that position. Off earth’s both. Both earth’s none.
PS For 7 years our O200 had the L and R key switch positions reversed, and nobody noticed. The Jodel was under the CAA, and it was only noticed when I had a bad mag drop when about to depart after her second Star Annual, and returned to the engineer for checking.
Noe wrote:
I know every POH says it, I just never found a convincing reason as to why
The measured RPM diffs L/R/Both don’t change irrespective of how you play with your mags (they do change when I move mixture tough 
), I guess rpm diffs is what matters not how you get them….
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.
I know every POH says it, I just never found a convincing reason as to why
https://www.lycoming.com/sites/default/files/Magneto%20Drop-Off.pdf
Lycoming direct back to BOTH and let engine speed recover.
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)
