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

[Thread drift]

The alternative to the monkey story is to keep repeating the same old mistakes. If every new monkey climbed the ladder, the others may end up dying of hypothermia.

We have a group of monkeys that exhibits certain behaviour but doesn’t know why (and it keeps them safe). The alternative would be for every new monkey to make the same mistake and possibly harm the others. Of course, that doesn’t mean we shouldn’t re-examine the way things are done. The new monkeys might not mind getting soaked with cold water and so the avoidance behaviour would serve no purpose. What would be helpful for progress though is accurate and thorough record keeping so that newcomers can understand previous justifications and determine whether they are still relevant.

Fairoaks, United Kingdom

Flyer59, the A-7 switch I posted is a design produced around 70 years ago. A pretty good design BTW, many of them are still working. You mostly see them on little A and C series Continental installations, which have impulse couplings on both mags. AFAIK only Lycoming installed a single impulse coupling on some engines (?)

I think the ‘backwards’ mag switch explanation for engines that use single ignition for startup makes sense. The only question is why the airframe manufacturer didn’t request the impulse coupling be installed on the right magneto. My guess is that there’s a (possibly obscure) reason for that choice, and it was easier to make ‘backwards’ switches.

kwlf wrote:

Googling about shows up this switch, which would have been installed on a Convair CV-240 and therefore used about 1950-1970

Could the flight engineer be riding backwards and therefore right was left and left was right to him?

LFPT, LFPN

That’s an interesting idea!

I think Achim had it right the first time. I would go with the mechanically easier grounding of the right magneto for starting in that direction. So we have
OFF – L & R grounded
R – L grounded; R interrupted
L – L interrupted, R grounded
BOTH – L interrupted, R interrupted
START – L interrupted, R grounded

Now for the question why the left magneto has the impulse coupling? Well, some of the engines (e.g. Conti A75, C85, C90, O-300) do have an earlier timing on the left magneto in normal operation, so my guess would be that the impulse coupling on the left side became a convention because of this.

mh
Inside the sky.
EDXE, EDXF, Germany

… still L could be on the left side of the switch. I have heard that one before but i don’t buy it, because it would still work if L was left and R right …

OFF – L & R grounded
L – L grounded; R interrupted
R – L interrupted, R grounded
BOTH – L interrupted, R interrupted
START – L interrupted, R grounded

no?

Last Edited by at 17 Nov 17:28

Flyer59 wrote:

L – L grounded; R interrupted

No, the active circuit is open (interrupted), the inactive is closed (grounded).

Hmmm … I have to rethink what all this means … :-) Does that really make it necessary that “L” is on the right side?

No, it makes it easier to build the switch, if you have a coupler just on the left. The underlying question is then, why some planes had the coupler just on the left. I suppose the reason lies in different timings on some of the engines. The reason for that may be that on the early models one magneto fired the bottom plugs and the other the top plugs. Could be some other issue, though.

So if you insist on “OFF, L, R, BOTH, START” you would have this scheme with a left coupler:

OFF – L & R grounded
L – L interrupted, R grounded
R – L grounded; R interrupted
BOTH – L interrupted, R interrupted
START – L interrupted, R grounded

and with a right coupler:
OFF – L & R grounded
L – L interrupted, R grounded
R – L grounded; R interrupted
BOTH – L interrupted, R interrupted
START – L grounded, R interrupted

Last Edited by mh at 17 Nov 19:04
mh
Inside the sky.
EDXE, EDXF, Germany

Aviathor wrote:

Could the flight engineer be riding backwards and therefore right was left and left was right to him?

A good thought that had not occurred to me… but no, it seems to be in the overhead panel for the pilots, so facing forward.

A few other related threads:
https://www.reddit.com/r/flying/comments/2apc4q/why_generally_the_right_magneto_is_at_the_left/
http://www.pprune.org/archive/index.php/t-152122.html

Nothing definitive.

EGCW
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