"Most, and perhaps all, key-operated ignition switches are labeled 'OFF-R-L-BOTH' across the top of the switch. Why is the R on the left and the L on the right?"
This question was in the TestPilot section of the July 03 AOPA PILOT, and has been bugging the heck out of me. The writer, who is a retired airline Captain, didn't know the answer.
Please excuse this not being oil related, I just thought it was interesting enough to post.....
Don
Magneto Switch Question???
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MolaKule
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How is the engine being viewd from the prop end?
The reason is the impulse coupler is on the right mag. this is the mag you use to start the engine and this is only mag with a coupler of the two. An impulse coupler is spring loaded to spin itself fast enough to generate a spark to fire the motor over. Also as it springs foward ![[Bang Head]](/forums/graemlins/banghead.gif)
it advances the timing to help stop a kick back of the motor. Hope this helps.
it advances the timing to help stop a kick back of the motor. Hope this helps.N2OIL, that sounds logical. So it's a safety issue? Since the Right mag has the impulse coupler, the R switch needs to be next to the OFF position.....Because in the event of a hand-prop start, this would reduce the chance of the wrong (higher timed) mag being selected, causing a possible prop kick back, and ruining someone's day....
I'll e-mail the author and see what he says about this.
i asked a couple of my pilot and a&p buddies. they remember the question but didn't know the answer. one of them came up with the idea that it is actually an "off" switch. so when you select "R" you are turning off your right mag, but your switch handle physically points to the left indicating the left mag is on. of course both still turns both on.
it makes sense in theory but i don't know if it really makes sense for operation. none of them are priviate aircraft mechanics.
i discussed it a bit more and we decided the "off" switch theory didn't make a whole lot of sense and go with the fact that you always start with the right magneto
I checked with the author about the above theories, and he said that they were not likely the reasons, according to Bendix and Lycoming.
It seems to me that Bendix should know the answer to this, since they produce the switches... ![[I dont know]](/forums/graemlins/dunno.gif)
It has not always been this way. Two different magneto switches are shown in "Aircraft Power Plants", Northrop Aeronautical Institute, 1948. They show OFF-L-R-BOTH and OFF-BAT-L-R-BOTH.
Neat question. 1) 100% of aircraft piston engines have the impulse coupling on the LEFT mag. 2) some (very few) engines were made with couplings on both mags, but 0% with a coupling on ONLY the right mag.
The switch has the L next to the BOTH position for hand propping. You prop with the switch on L, then go to BOTH when she fires. Nothing arcane.
AV8R is 100 % correct.
quote:I agree that single impulse coupling engines always use the left position, but they are not in the majority. The Lycoming O-235 C1 equipped Champion 7ECA I fly has a single impulse. (has toggle switches, tho) At work, our Lycoming IO-360-A1B equipped Partenavias also have a single impulse coupling. The majority of our piston fleet have two impulse couplings, i.e older C182's and new C-182's as well as new C172's, and an Aztec. The C-150-G I fly has two as well. I know of some older PA-140's with a single impulse. It is easy to convert to two impulse couplings on a Lycoming, you just need the spacer that goes between the mag and the accessory case. This is a popular upgrade when converting from Bendix (TCM) mags to Slick. The rebate just about covers the cost. You are 100% correct on the switch positions!
Originally posted by AV8R: Neat question. 1) 100% of aircraft piston engines have the impulse coupling on the LEFT mag. 2) some (very few) engines were made with couplings on both mags, but 0% with a coupling on ONLY the right mag. The switch has the L next to the BOTH position for hand propping. You prop with the switch on L, then go to BOTH when she fires. Nothing arcane.
Well, I stand on what I said, 'very few' engines had dual couplers.
Couplers can (and do) fail catastrophically, so it's better to have only one on the engine, instead of two, for reliability in the air.
An "upgrade" to dual couplers would take more than a spacer, it would take a mag switch that starts on both mags. All of this, and you're getting into FAA Form 337 territory, and field approvals are very hard to get these days, especially after the FAA inspector asks "what's wrong with it the way it is?"
Guys, I stick with auto's HDD's and industrial applications but I am also a professional pilot and this question has driven me nuts !
So I contacted a friend and professional,Darrell Morgan of FirebirdAviation.com . Darrell is a A&P and IA and a EAA tech counselor. Here's his response ;
"Terry,
We have been kicking around that mag switch question for a couple of days now and have come up with a consensus.
If you look at the old switches, they were labeled , Both, Left, Right, and Off, read from the 8 o'clock position counterclockwise to about the 4 o'clock position. The label/placard on the bottom. Then the switch lever would rotated from Off to Both clockwise. This is the way you would expect it to read. Right mag on the right and left on the left.
Take a look at any picture of old instrument panels.
Somewhere along the line someone decided to put the label/placard on top. Rather then re-engineer the entire switch, just label it backwards as you see them today, Off, Right, Left, Both, Start, and put the label/placard on top.
It might help to visualize this if you draw a circle, put the old on the bottom and new on top. You will see the lever action, or key, is the same.
Hope this helps,
Darrell"
Opinions ?
Terry
Well, that's surely an interesting observation. It seems that the only question left (sorry, I couldn't resist) is "why is the retarded magneto on the left?" I suspect it may have been an attempt to disguise a pirated British design originally having a retarded mag on the right. Having the right mag on the left should confuse a British intelligence officer, I would think, unless, of course, he owned a Cessna 337.
Now for an even more arcane question, why do the FAA and engine manufacturers discourage the use of anti-seize compound on spark plug threads? (I have an inside track on this one.)
Terry, to be serious for a moment, the 'L' has to be next to 'BOTH', otherwise you have to switch through a previously-dead right mag and its set of possibly fuel-fouled plugs on the way to the 'both' position, if you're hand-propping the airplane. The 'push to start' feature of the Bendix key switch starts on 'left' and goes directly to 'both' when key pressure is released, for electric starting. The thing about "upgrading to dual couplers" in just BS, since nobody makes a switch to start on both mags. About putting the placard below the switch, should be great for aerobatic airplanes. (oh, sorry again)
AV8R, your theory seems to be the most logical. I thought that it must be a safety issue, and related to the impulse coupler. It makes sense to not go through a previously dead mag/sparkplug (that has advanced timing), to get to both mags. There would be too much chance of a backfire or misfire, in the case of a 'slow to advance' impulse coupler.
Uh....as for anti-seize??? My brain cell has come up with 3 possibilities. One, the anti-seize insulates and prevents a good sparkplug ground (not likely). Two, the lead in it contributes to plug fouling. Three, the anti-seize might cause the plug to loosen during the heat/cool engine cycles.
???
In response to your "left handed comments" AV8R.. ![[Big Grin]](images/icons/grin.gif)
Darrell, states;
There is an incredible number engines that have dual impulse coupled magnetos. For example the Continental C-85 on my Cessna 140 has a dual set up.
The anti-seize compound question. He obviously has never removed a seized spark plug. Champion's Aviation Service Manual dated April 2001, page 11, specifically states to use it.
Don't shoot the messenger.....
Darrell, states;
There is an incredible number engines that have dual impulse coupled magnetos. For example the Continental C-85 on my Cessna 140 has a dual set up.
The anti-seize compound question. He obviously has never removed a seized spark plug. Champion's Aviation Service Manual dated April 2001, page 11, specifically states to use it.
Don't shoot the messenger.....quote:the only other think i can think of is the compound getting on the center electrode and fowling it. i know GE recommends using milk of magnesia on their turbine engine spark ignitors.
Now for an even more arcane question, why do the FAA and engine manufacturers discourage the use of anti-seize compound on spark plug threads? (I have an inside track on this one.)
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