Starting on the L Mag

kontiki

kontiki

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Kontiki
I see recommendations for some engines that say start the engine on the L Mag, and, I understand impulse couplings are typically installed on the L mag.
So does having the R mag on too make it harder to start the engine or is the recommendation just written that way to make sure the Left Mag is on for sure?
 
Follow the checklist. That being said....


It's possible the magnetos or the ignition switch have been modified since that checklist was printed.



Many many many Lycomings are equipped with two magnetos and only one has an impulse. If the non-impulse mag is firing during cranking, its likely to cause prop/engine "kickback" which could break the starter or worse.


Most of the single impulsed cessna products have a "twist to start" ignition switch that automatically grounds the p-lead of the non-impulsed magneto anytime the key is in the "start" position.
 
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kontiki said:
I see recommendations for some engines that say start the engine on the L Mag, and, I understand impulse couplings are typically installed on the L mag.
So does having the R mag on too make it harder to start the engine or is the recommendation just written that way to make sure the Left Mag is on for sure?
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Yes, if you have a 'switches and button' system for starting if you turn on the non impulse mag, it could fire at the advanced 'normal' ignition timing rather than the retarded impulse timing making it harder for the engine to start. The key type systems do this internally. If you get in a plane that has the prop 'stall' at compression points during the cranking, this is often the result of the non impulse mag being hot as well which can indicate the wires being reversed, not following procedure, or the key mag/starter switch being faulty.
 
I'm with the previous posters.

If the non IC mag is hot during starting it is trying to spark at 20+ degrees (in most cases) before the piston comes up on top dead center. If it doesn't have sufficient momentum to carry through to the power stroke (as during start up) it will kick back. This usually breaks something in the starting system. This is why skytec starters have either a sheer pin or a slipper clutch in them.


This is one of the few problems that will get better with weak parts as it takes a good strong mag to spark at typical start up RPMs, but we want good strong mags so...
 
It's a non issue anyway with most single engines. You don't really have a choice.
 
Sac Arrow said:
It's a non issue anyway with most single engines. You don't really have a choice.
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Well, it is a troubleshooting tell though if you have a hard to start engine. Often times the P-leads get swapped and you end up with an engine that that cranks really slow with stalling points on compression and then fires right up when you let go of the key.
 
Skylane81E said:
it takes a good strong mag to spark at typical start up RPMs, but we want good strong mags so...
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This is true, and the possibility of kicking back is so remote it is a non issue in real life.
 
We actually have a Cherokee 235 with a O-540...left mag only impulse coupler. We recently replaced the starter (new Skytech) and now have weird issues starting the plane. Sometimes it will start on the left mag - sometimes you have to switch the key to 'both' (we have a push button starter - key only controls which mags) to get the engine to fire up.

I think we're just going to try replacing the left mag with a new one and see if that solves our problem. Often on the left mag it will crank and crank (like 8-10 blades) without even acting like it's firing - almost like you're cranking it with the mags turned off.

It's a mystery to us - we can't figure it out. One guy will get in and it'll fire right up on the left on 4 blades. I went out two days in a row this week - first day cranked it 5 blades on the left...switched to both, fired up. Went out the next day and it wouldn't start it all. Pulled my CFI (who also is an A&P) over from the hangar next door and he couldn't get it started up...neither could the old-timer who hangars across from us. We left, ate a hamburger, came back an hour later and it started up on 5 blades cranking on the left then switching to both mags.

So, we're kind of at a loss. Is there any advantage to switching to a dual impulse mag setup? Should that make starting easier? Is that something you can do without having to file all kinds of paperwork?
 
Sounds to me like you should figure out what is actually broken before you start slapping parts on it.

Bad mag switch? P leads chafing through and shorting out? Defective mag(s)? Bad plugs?
 
kenjr said:
We actually have a Cherokee 235 with a O-540...left mag only impulse coupler. We recently replaced the starter (new Skytech) and now have weird issues starting the plane. Sometimes it will start on the left mag - sometimes you have to switch the key to 'both' (we have a push button starter - key only controls which mags) to get the engine to fire up.

I think we're just going to try replacing the left mag with a new one and see if that solves our problem. Often on the left mag it will crank and crank (like 8-10 blades) without even acting like it's firing - almost like you're cranking it with the mags turned off.

It's a mystery to us - we can't figure it out. One guy will get in and it'll fire right up on the left on 4 blades. I went out two days in a row this week - first day cranked it 5 blades on the left...switched to both, fired up. Went out the next day and it wouldn't start it all. Pulled my CFI (who also is an A&P) over from the hangar next door and he couldn't get it started up...neither could the old-timer who hangars across from us. We left, ate a hamburger, came back an hour later and it started up on 5 blades cranking on the left then switching to both mags.

So, we're kind of at a loss. Is there any advantage to switching to a dual impulse mag setup? Should that make starting easier? Is that something you can do without having to file all kinds of paperwork?
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Run a continuity test on the key switch, they are known to go bad.
 
kenjr said:
We actually have a Cherokee 235 with a O-540...left mag only impulse coupler. ... Sometimes it will start on the left mag - sometimes you have to switch the key to 'both' (we have a push button starter - key only controls which mags) to get the engine to fire up.
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Make sure the P-leads on on correctly. "L" on the switch may be grounding the left mag, rather than leaving it hot.
Happens more often than you think.
 
yeah I think we need to change the switch. That said, my CFI/A&P pulled the ground off the left mag to get around the switch when it wouldn't start and we still couldn't get it to go. I'm not saying that completely takes the switch out of the equation...probably needs to be changed as it's 60 years old original.
 
kenjr said:
We actually have a Cherokee 235 with a O-540...left mag only impulse coupler. We recently replaced the starter (new Skytech) and now have weird issues starting the plane. Sometimes it will start on the left mag - sometimes you have to switch the key to 'both' (we have a push button starter - key only controls which mags) to get the engine to fire up.

I think we're just going to try replacing the left mag with a new one and see if that solves our problem. Often on the left mag it will crank and crank (like 8-10 blades) without even acting like it's firing - almost like you're cranking it with the mags turned off.

It's a mystery to us - we can't figure it out. One guy will get in and it'll fire right up on the left on 4 blades. I went out two days in a row this week - first day cranked it 5 blades on the left...switched to both, fired up. Went out the next day and it wouldn't start it all. Pulled my CFI (who also is an A&P) over from the hangar next door and he couldn't get it started up...neither could the old-timer who hangars across from us. We left, ate a hamburger, came back an hour later and it started up on 5 blades cranking on the left then switching to both mags.

So, we're kind of at a loss. Is there any advantage to switching to a dual impulse mag setup? Should that make starting easier? Is that something you can do without having to file all kinds of paperwork?
Click to expand...


Might be an OWT but that skytech might be cranking the engine so fast that the impulse coupling isn't working as designed.

I just converted our o320 to dual impulses. It starts a lot better than before. It involved a field approval, an impulse equipped magneto, replacing two studs in the accessory case, the magneto spacer, a different gear and removing a ground strap on the ignition switch which it doesn't sound like your switch would need that.

For better starting performance there's allways a starter vibrator or slick start system.
 
bnt83 said:
Might be an OWT but that skytech might be cranking the engine so fast that the impulse coupling isn't working as designed.
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In that case it would start normally at 20 + - degrees BTDC
 
Is there a GA product out there with electronic ignition for an internal combustion engine? Most of my students look at me with odd looks when I ask "Do you know what a distributor cap is?" to kick of the "how a mag works" discussion.
 
Unfortunately, if the left mag fails during a taxi reposition at a major airport, when you go to start it on a keyed type starting circuit that automatically grounds out the right mag for start, it will not start no matter what you do.

And then you get to stay overnight at a cold and dreary area.
 
Aztec Driver said:
Unfortunately, if the left mag fails during a taxi reposition at a major airport, when you go to start it on a keyed type starting circuit that automatically grounds out the right mag for start, it will not start no matter what you do.

And then you get to stay overnight at a cold and dreary area.
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Why not pull the P-lead? However, if the left mag failed, or in the case where you were doing a taxi reposition, aren't you spending the night anyway?
 
Henning said:
Why not pull the P-lead? However, if the left mag failed, or in the case where you were doing a taxi reposition, aren't you spending the night anyway?
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That'd be OK for a part 91 aircraft, but "Homey" doesn't touch a part 135 maintained aircraft unless under direct supervision. And, no, I had to reposition from one FBO to another because the one was out of avgas. So, no, I wasn't planning to stay the night originally.
 
Aztec Driver said:
That'd be OK for a part 91 aircraft, but "Homey" doesn't touch a part 135 maintained aircraft unless under direct supervision. And, no, I had to reposition from one FBO to another because the one was out of avgas. So, no, I wasn't planning to stay the night originally.
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Still the mag failure would have kept you there regardless, or would you have flown a 135 aircraft with a failed mag?
 
Henning said:
Still the mag failure would have kept you there regardless, or would you have flown a 135 aircraft with a failed mag?
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Who said he had a failed mag?

All he knows is, he has a fail to start, cause unknown.
 
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Henning said:
Still the mag failure would have kept you there regardless, or would you have flown a 135 aircraft with a failed mag?
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That is correct, I would not have have flown home with the aircraft, through miserable weather, IMC, possible icing, and over Lake Erie with one mag, even though the A&P came up with that idea after the fact. I was just irritated that I got stranded there in the first place. Had I not had to start it as many times as I did, I might have had one start left to get it home.:D
 
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