View Full Version : Key Switch & Magneto Handing

14th Nov 2004, 17:15
OK was up and about yesterday and got to wondering why when the Left Magneto is on the left side of the engine as I look forward & the right Magneto is on the right side, why the switch is wired in the opposite sense??

In other words you turn the key from left to right but select the right magneto first then the left then both??

Nobody I have asked seem to know why, does anybody else?

Just curious.

14th Nov 2004, 17:57
It IS logical!

A Magneto is a self generating piece of equipment, it requires no external power to generate a spark. To switch it OFF you earth out the field coils. By moving the switch to detent 1 / position R you are actually earthing the left magneto. :8

14th Nov 2004, 18:45
Interesting point and if the earthing mechanism fails the mag is live - a "wrong sided" failure - yet another reason for always treating a prop as live.

Not sure about this but the left mag is often the impulse mag and when you turn the key to start its only the left mag that is operating until you release the key - any engineers/techies choose to comment?

14th Nov 2004, 19:52
Not sure if it's the left one but it probably is on most single switched setups as it's the furthest position to the right, which would suggest the right mag is earthed.

Only one mag works at start. The impulse mag works by delaying the spark (retarding the ignition point) and then "flinging " the magneto innards round more quickly than the engine speed would suggest. It then plays catchup so the a fatter spark occurs at start cranking speed.

If the other mag worked too and it managed to make a spark at the normal point, which would be too advanced for the low engine speed, it might cause a kickback, which can damage people if it was a "hand swing" start and possibly the crankshaft, due to reversal of loading.

14th Nov 2004, 21:08
There isn't really any logic to it as far as I can see. The position that the key is at (be it R, L or Both) indicates the mag that is live. Probably an American thing like having all the switches working the wrong way round to!

As for the starting mag, it is often (particularly on Lycomings) that the left mag is the one fitted with the impulse coupling but some engines (like most C152 O-235s) have impulse mags on both sides.

A jumper on the back of the key-type ignition switch causes the right mag (on a single impulse set up) to earth out when 'start' is selected.

Incidentally, 152s often start when you let go of the key in the start position because the battery turns the engine over so fast that the impulse coupling disengages (but not fast enough for the mag to produce a fat enough spark to ignite the fuel). As the engine slows down (key by now at 'both') the engine fires.

15th Nov 2004, 03:56
Ummm, there is certain logic to it.

Switch rotates through "Off" "R" "L" "BOTH".

When testing "L" the right magneto is earthed and vice versa. Otherwise the sequence would be Off L R Both.

Of course if I was standing facing the propeller, which turns clockwise seen from behind, then it would appear I was turning the key anticlockwise because I was looking at it from ahead - as a propeller puller. Now if I was in an aircraft with a hand swung pusher propeller it would be completely different wouldn't it?

15th Nov 2004, 07:40
I can see two lines of logic appearing here! Either you are checking that the left and right magnetos are working or are that they are 'earthing out'.

Surely you are checking they are working: 'Both' = both working; 'L' = left mag is working; 'R' = right mag is working; 'Off' both are earthed out ie off.

The only lack of logic here is that when you look at the switch, R and L are not as we see left and right (or right and left!) when viewed from the pilot's seat.

How confusing can we make this thread.....?!!

15th Nov 2004, 10:28
I'm not at all confused and have no intention of becoming so.

I've taken advice on how to deal with such situations from my young daughter.

"I'm not listening, I'm not listening! ;)

15th Nov 2004, 11:16
The only lack of logic here is that when you look at the switch, R and L are not as we see left and right (or right and left!) when viewed from the pilot's seat Ok, so following on from this logic, what about the position marked "both" should that mean both workin g or both shorted out? My guess is that "both" means neither mag shorted and therefore it follows that "L" should mean the right mag shorted and "R" the left mag shorted.:ouch:

QNH 1013
15th Nov 2004, 12:30
Gypsy Major engines have the impulse mag on the right, and normally toggle switches for each mag.