Galen, one of our pilot readers, writes: Hello, I have always wondered why the left magneto is to the right on the keyswitch, and the right magneto is to the left.
Because I’m left-handed, my mother had a really hard time teaching me how to tie my shoelaces when I was little. Even to this day, I favor slip-on shoes, but oddly, I had never thought about the criss-cross applesauce nature of our mag switches until you brought it up.
The key position on the right is labeled left, and the key position on the left is labeled right. That does seem messed up at first glance.
But as it turns out, it might not be as back assward as it first appears. But for any explanation to make sense, first we need to do a quick 4-1-1 on how the magneto circuits work.
Limiting our conversation today to traditional magnetos — experimental airplanes can use electronic mags, and many certified airplanes are approved for one electronic and one traditional — mags are the independent, kinetically-powered spark-makers for your engine, providing the fire for internal combustion.
OK, OK, OK. Technically, they are the spinning magnets that provide the juice to the spark plug, which, in turn, actually makes the spark.
But moving on…The beauty of this somewhat primitive technology is that as long as the engine is spinning, you have spark. If your entire electrical system craps out, your engine keeps chugging along.
And as even further backup — as we aviators invented the concept of the worst case scenario — even though an engine will run just fine on one mag, we have a pair to back each other up. In most cases these are two completely separate units, although there are some aircraft engines that use a dual mag system that’s built into a single “box,” but either way, the important thing to remember is that we have a “pair.”
Which is why, ladies and gentlemen, we have the mag control switch in the cockpit. In daily operation, as part of the run-up, a pilot can turn off first one mag, and then the other, to ensure that both are functioning before flight.
Remember that I said the engine can run fine on one? Without the mag switch, you could fire up and fly off totally unaware that one of your mags had failed. At least totally unaware until the other one also failed and it got very quiet in the cockpit.
Oh, and the mag switch serves one other critical function. Most times when a mag fails, it just stops working. But sometimes, they go psycho and start firing at the wrong time, wreaking havoc in the engine. In cases like this, the pilot can use the switch to “kill” the crazy mag and continue to fly on safely using just the properly working mag. Or fly safely once his or her blood pressure returns to something like normal. I wasn’t kidding about the havoc.
Now, on to the hard part. Each mag is connected to a wire called a P-lead that goes from the mag through the firewall to the ignition switch. In the OFF position, correctly speaking, the P-leads “short” the coils of the mags to the ground, but I’m OK with you imagining it like a residential light switch: Turning it off interrupts the flow of power.
Now, in the BOTH position we get deep into the weeds of electrical engineering when it comes to what’s really happening, so let’s just keep it simple and say the light switch is now on.
Of course, this is no ordinary light switch, is it? It’s more like one of those found in the bathroom of a fancy high-rise condo, with controls for the overhead light, the lights around the mirror, and the fan.
With mag switches, the functions of BOTH and OFF are pretty clear, but things get interesting in the “R” and “L” positions. Of course, all pilots understand that using these two positions turns off one of the mags and leaves the other one running — and most of us don’t really care which is which as we happily cycle the key back and forth to assure ourselves that both are working.
But… deep breath… selecting the right-hand key position grounds out the right mag, essentially turning it off, while leaving the left mag running — so the right key position is marked with an “L.” Likewise, moving the key to the left (but not all the way to OFF) now disables the left side, leaving the right side working — hence the “R” label at the left key position.
Key position dictates what’s “off,” while the label shows what’s on. So there is a certain logic to it, once you embrace it.
For what it’s worth, mag switches didn’t always look like this. Many World War II airplanes have mag switches with left showing on the left side and right showing on the right side, but I have no clue about how they were actually wired. Did moving the switch to “L” leave the left side on or turn it off? Chime in, Warbird Crowd.
I also have no clue how we got to where we are today, and to proactively avoid being flamed too much in comments (there has been some talk of changing my call sign from Wildman to Lightning Rod), I need to point out that this subject is the source of a great deal of spirited late-night online hangar debate.
But really quickly, there are those who believe the position of the “R” and “L” has to do with the wiring of the switch, rather than some sort of logic, but I don’t believe that’s so. It wouldn’t take rocket science to design the switch with live left on the left, and live right on the right. Besides, mechanics I’ve talked to tell me that the P-leads are frequently wired backwards so that “L” kills the left and “R” kills the right. Bearing in mind that the wires come through the firewall and are waaaaaay up under the panel, it’s easy to see how they could become confused.
But logical or illogical, I’m at peace with the seemingly backward nature of it, and I think that either Right or Left in your political leanings, we can all agree that life is easier when some things are standardized. It’s good to know you can jump into any airplane and the switch will be the same every time.
Now, if I could just learn to tie my shoelaces…
Great topic, Left vs. Right.
I always felt good as a former engineer that there is an off for L and R magnetos and an OFF for the two with Both on. All those unique positions mean the operating person calls out the clicks, therefore is handling the correct switch, not mistakingly a nearby off / on switch and the intended action is trusted as completed for a desired action. Though it’s extra comforting for everyone to be safely aware anyhow with both mags not working, this is a positive practice and reminder towards that awareness that switches may not do as they are set to do. And all controls called out to their unique settings are especially good for hand starts. When out front the few times I turned a prop and times I did anything else near it, I trusted what airplane components were declared set as I asked them to be by the words called out and I called them out just the same from the seat.
And as this switch tells what mag is failing, we know there’s other engine troubles found when each mag can be off independently. That was cool to learn and use. However, thats where I did see the awkwardness of those switches at training in troubleshooting, but it became natural with something standardized for each engine on each airplane. Embrace it as you point out.
I don’t recall warbirds having different positions or operating differently at those switches. Though, my early days with a few of them, not a long time with them, were when they were doing postwar military and commercial flights. If I am correct, it’s possible those switches were modified to the standard before I got to them. I’ll find it interesting to see what the Warbird Crowd say.
For mixup of wires to wrong sides of the mag switch, yep, I agree it happens and can complicate things. All sorts of controls and components which are not separate units or otherwise have two or more pairs of matching connectors or fittings can be hooked up wrong. Disconnect without marking what goes where, or marking tapes fall off and things can go back on wrong. It was alway nice when the wire connectors and pipe fittings on things were reversed or different colours or sizes so they wouldn’t go together until they were matched to their functional and safe positions. Also was great when old wiring is fully removed so a dead wire with the same tape marker number and tip didn’t get connected as the right wire has fallen away. That’s hours of troubleshooting what additional problem has occured as part of the original problem or during installation and it turns out to be a mistaken hookup to dead wire. That kind of worry and actual mistake becomes a hangar discussion many times after bundles or tangles of wires are discovered.
I think I recall that dual magneto you pointed out has different or reversed connectors for each mag. I’ve not been a big fan of the dual mag though. It has both working independently in it, but both on a single drive system into that engine with a single gasket and in a smaller area. I think this can defeat the backup idea. And as both come off, maybe these are more expensive to replace and have repaired, both sides need proper procedures to finish the task for the past one’s failure.
Funny thing is that Velcro straps and metal snaps on shoes do up to the outer side of the shoe. I put my left foot up on my right leg to get a good grasp with my left hand, thereby tighten that left shoe at my right side, right shoe at the left. I also prefer slip-on shoes.
Continental Motors SB#583:
When magneto switch is on R, left magneto is grounded (inop) and right magneto is open to ground (running). When magneto switch is on L, right magneto is grounded (inop) and left magneto is open (running). Never trust that the P-leads are correctly attached.
Only a writer could make that explanation way too long, or just long enough to confuse me more, So to recap when the you switch to the Right mag ( labeling, not position) the Right mag is working?
Thanks for the reply. This has been hanger fodder for some time. We have not reached a full agreement yet either. Maybe this will always be unto each person to have an opinion. Suffice to say it is important to know they are backward to tell which mag you are having an issue on, if you have a failed mag.
Experience notes on mags: I was climbing out with a student in my Pitts S-2A. With just short of 8,000 hours in type, I’m more than just a little “connected” to the airplane. This time I can’t say I actually felt or heard anything. Something just wasn’t “right”. Landed, did a mag check. Totally fine. Then, for some reason did another one and the engine went totally crazy. ‘Can’t even describe what it sounded like. The rotator had broken loose in one mag and was firing what ever plug it felt like firing, including firing plugs on opposite sides of the engine at the same time (I think…it was horrible). Had I gone ahead and tried to take off after the first mag check and gone to full throttle, the engine would likely have self destructed before I could get the throttle back. So, now I always do two mag checks before launching.
Another mag subject: I’ve checked out three amputees in Pitts (so much for the mystique of taildraggers). TWO OF THE THREE LOST THEIR LEGS BECAUSE THEY LEFT A MAG ON AND THEY MOVED THE PROP! That’s why we ALWAYS do a grounding check on mags as we shut down.
Budd Davisson
Before the days of dual impulse couplings and electronic ignition, aircraft engines started on the LEFT magneto only. This is especially true for installations where a push button starter or hand-prop is employed in cranking the engine. Once started, the switch was turned to BOTH, located just to the right of the LEFT position. This avoided delay in running on one magneto in an effort to avoid fouling plugs on the inoperative mag, especially when the engine is cold and overly rich.