Mags on BOTH
 

I was curious to know what would happen if you had the master switch turned off on a C-172/C-182 but the key was still in the ignition on BOTH for a long period of time. It might be a simpler question than I thought, but what effect would that have on the plane if you left it on all night? Not to say it happened to anyone I know;).

I'm no expert but what I remember from my a/c systems class at WMU if the key is left in the "both" position and the prop gets spun by anything it could ignite the engine and cause the prop to kick a few times before dying again. despite the obvious dangers of that happening I don't think it would hurt the a/c itself at all.

Nothing unless you turned the prop, then bad things might happen. Mags generate a spark, they don't use any power.

Nothing would "happen". What did you think would "happen"?

If someone primed the engine, advanced the mixture to rich, and hand propped the engine, it would start without the master switch being on.

Earlier posters have it correct.
Nothing adverse would happen to the aircraft in the situation described in the original post describing the keyed ignition switch being left in the "both" position with the electrical master switch off.

This comes with major caveats described above though.

Important points to take from this thread-

Most (all that I know of besides experimental and certified A/C with electronic ignition systems)(Porsche Mooney?) do not require an alternator or generator driven electrical system independent of magnetos to provide spark to their ignition systems. In other words- all tin cans you are gonna strap on run their spark for ignition from magnetos. Old school. Mechanical. They spin, and provide spark. Many reasons for this going back to early days, Lindbergh times, magnetic theory, and a whole bunch of science you might read up on. In effect you have been gifted with an independent, mechanically driven system which provides spark to those two spark plugs per cylinder which motivate your winged craft around.

Think about it. Your electrical system, ie- battery, alternator (generator), voltage regulator, they drive your starter, lights, radio, flaps (maybe), and a few other odds and ends you don't really need to get from A to B if you are able or wanting to aviate 1920's style.

Like a coupled GPS, or glass cockpit. Don't rely on those little electron driven bastards, okay? For too long anyway if your alternator fails.

Your magnetos, and the wires they run to their associated spark plugs, run your cylinders, your lifeblood over stormy Kansas or North Atlantic waters Charlie L. style.

Which would you rather fail? Alternator, Generator, or magnetos?

Good first flight magneto checks on run ups will save you much heartache. They do fail. They do go out of time. Spark plugs take a dive. FIX IT. OR GET IT FIXED BEFORE YOU VENTURE OUT.

Again, magnetos- physical- think of somebody's grampaw driving a tractor- did he need a radio, nav system, or even an electrical starter? Nope. Old school tractors were single magneto sparked. Crank that model A or International Harvester with a bar attached to your arm. Ka-blam! Alright, she's runnin'! Now we're plowing. Or driving down the new road in our buggy.

Magnetos. They spin- like your prop. They are physically geared, attached internally through the engine case to your prop. Your prop keeps turning, your magnetos keep burning. Fuel and air. Gap, timing, spark.

Things not to do- never ever turn a prop with important body parts or valued loved ones in the arc. Even with the switch in the off position. EVEN WITH THE SWITCH IN THE OFF POSITION. Remember magnetos are ALWAYS hot unless they are properly grounded. Do you know if they are properly grounded? No.

Never pose for photos with your hand on the prop. Learn about impulse couplings and ground leads. Teach your students, passengers, and yourself about the following thing. NEVER EVER TURN A PROP WITH SOMETHING IMPORTANT IN ITS ARC EVEN WITH THE IGNITION SWITCH IN THE OFF POSITION!

Ugh, I'm done, consult your local trusted A+P for a tour under the cowling.

Great post!

When a magneto is switched "off," it is electrically grounded and rendered impotent. When a mag is switched "on," the connection to ground is broken and the mag is now functional.

You CAN have a bad wire to the mag switch which can cause it to be 'live'. It's best to treat all props like they could be alive at any moment.

Very common with GA recips, you usually notice it when you do the mag check. I'd have to use my toes to count how many times it's happened to me.

Also since the carb bowl still contains fuel, there is absolutely nothing to prevent an engine from running with a broken mag lead. If it's hot and the crank's in the right position, it can start with only a slight bump.

Always, always, always treat a recip prop as a hazard zone.

The question I've always wondered is: why is the switch ordered off, R, L, Both, Start? Shouldn't "L" be on the left and "R" be on the right?

maybe the engineer had Dyslexia:D

It's a hold over from when you normally hand propped an engine. The impulse coupler is on the left mag and if you started on LEFT you wouldn't want to go through RIGHT to get to BOTH.

Just like to add, that this is why most aircraft, at least to all of my knowledge, you turn the aircraft off using the mixture, not just turning off the key. This then insurers that all the fuel in the cylinders is burned off. If you ever shut a plane down with just the key there will be fuel left in the engine. If this happened, better to keep the mixture out, then turn the key to start. The prop will spin a few times then come to a stop, but just imagine if you didn't do that. Some poor SOB could come along to push it and turn the prop, engine start, and then someones head/hand is gone.
The
WildSmurf

P.S. If you ever have a student, this could and has happened.

The CJ-6 and J-3 actually shut down using the mags - both for different reasons. The CJ calls for a pre-shut down run up to 1900rpm for 10 seconds and bringing the throttle and mix full aft (full rich) and then turning the mags off. It's weird...it pnuematic discharge and 'shower of sparks' for start.

The caveat is that if you don't have enough air for a start (in the bottle), you probably don't have enough air to operate your brakes - so hand propping requires a tie down or something.

As a radial it also requires the prop turned through about 9-10 times checking for hydraulic lock.

Thanks for the posts. I didn't think it would do anything to a C-172 other than the risk of starting the propeller when you least expect it. I've heard of one story of someone leaving the mags on a twin engine overnight and he killed the battery or left the mags too warm for too long and it did some sort of damage. Now I'm just trying to figure out the difference of turning on mags between lighter vs. larger aircraft.

There's no way.

Magneto's in any airframe, large, small, russian even etc.. work the same. They have no, none, nada, zip, zilch effect on the battery. Now if someone left the Master switch on, then yes, that's the battery switch. Master switch is not the Mag switch, and never have they been connected to my knowledge in the 100's of GA planes I've flown.

Mooney Shutdown Procedure
 

We pull the mixture slowly til the cylinder temps rise and then full cut off. This is supposed to prevent plug fouling on the "lean loving" close cowled fuel injected IO360 in the Mooney E Model.

Very interesting, I figured that there was some aircraft out there that would call for something other than pulling the mixture out.
"Pneumatic discharge and 'shower of sparks' for start"?:confused: I have an idea how it works, but just might Google search to see exactly what that is.

WildSmurf

all i have to say, is if you ever leave the mags on and an A&P comes to work on your plane, you will get an ear full. I always check before turning the prop but there is always the chance of me forgetting to check (swiss cheese model). This could easily result in my death, especially if someone turned the engine off by the mags.

and yes, the above posters are correct. the mags have absolutely, positively nothing to do with the battery or electrical system. they are an independent system for a reason.

The labels are for the mag that is active. So when you're in the R position, you're grounding the Left mag, therefore the Right one is operating.

So R, L, Both is the correct order (some don't have the start position :))

They do it that way to confuse the Russians!

The reason it's labeled OFF-R-L-BOTH is due to the nature of grounding. When you turn the key, you're connecting a circuit; when you turn to R, you're completing the circuit (AKA connecting the P-Lead and grounding the left mag). This is in the left position, and the manufacturers logically chose to display which magneto would be running in this position instead of the one that is grounded.