Magneto Check

jayllamas

jayllamas

Well-Known Member
When you are doing a magneto check for a C-172 and there is no drop in the L position...does it mean there is a problem with the left or right?...Or is there a problem at all?...I talked to the mechanic about it and he said there's no problem if there's no drop...I thought it's the other way around???
 
Is there a drop on the R?????

If there is no drop at all when you switch to L, that usually means that only the left mag is working and the engine should die when you switch to R. If the right side checks okay, then I would assume (bad to do) that there is no problem.
 
It sounds like the right mag is not grounding when you switch to L. The first thing I would try is shutting the engine down by switching the ignition to off rather than by the mixture. If it keeps running in the off position, you've got a problem that MX needs to fix.
 
When you switch to L you are only running on the left magneto.

When you turn the key to L, the ignition switch connects the right magneto's P-lead to ground, preventing the right mag from firing. This leaves you just running on the left mag. Since the right mag is no longer firing, you get an RPM drop.

Now consider if the right magneto has a broken P-lead. When you turn the key to L, the switch grounds the right mag's P-lead. But since the P-lead is broken it can't ground the right mag, so it keeps firing and you don't get a drop.

The easiest way to check for a broken P-lead is to switch the ignition to OFF while the engine's running. The ignition switch will try to ground both the left and right mags, but if one has a broken P-lead it will keep firing and the engine will still run.

Here's some ignition switch trivia for you: You ever wonder why an aircraft ignition switch has the R and L positions backwards, like OFF-R-L-BOTH? Normally a switch is configured so that it completes a circuit to turn the device on. But in the case of an aircraft ignition switch, we complete the circuit (P-lead to ground) to turn the magneto off. So R is on the left because that position actually completes the circuit for the left mag, turning it off. Same thing for the L position.
 
Berkut said:
It sounds like the right mag is not grounding when you switch to L. The first thing I would try is shutting the engine down by switching the ignition to off rather than by the mixture. If it keeps running in the off position, you've got a problem that MX needs to fix.
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Actually that happened yesterday too.!!!
 
Then that airplane needs to be fixed before it hurts someone. There also might be a risk of engine damage depending on how the starting aids are configured on that particular 172.
 
Berkut said:
When you switch to L you are only running on the left magneto.

When you turn the key to L, the ignition switch connects the right magneto's P-lead to ground, preventing the right mag from firing. This leaves you just running on the left mag. Since the right mag is no longer firing, you get an RPM drop.

Now consider if the right magneto has a broken P-lead. When you turn the key to L, the switch grounds the right mag's P-lead. But since the P-lead is broken it can't ground the right mag, so it keeps firing and you don't get a drop.

The easiest way to check for a broken P-lead is to switch the ignition to OFF while the engine's running. The ignition switch will try to ground both the left and right mags, but if one has a broken P-lead it will keep firing and the engine will still run.

Here's some ignition switch trivia for you: You ever wonder why an aircraft ignition switch has the R and L positions backwards, like OFF-R-L-BOTH? Normally a switch is configured so that it completes a circuit to turn the device on. But in the case of an aircraft ignition switch, we complete the circuit (P-lead to ground) to turn the magneto off. So R is on the left because that position actually completes the circuit for the left mag, turning it off. Same thing for the L position.
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Great explanation!

Also we should point out the main reason for getting it fixed as you might know is that it could actually start up if someone moves the prop even though the ignition is off.
 
bike21 said:
Great explanation!

Also we should point out the main reason for getting it fixed as you might know is that it could actually start up if someone moves the prop even though the ignition is off.
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Which could totally happen at some flight schools where they level the prop after the flight... I still have this habit after each flight even though no one else does it at the school I am at now... Primacy!
 
Timbuff10 said:
Which could totally happen at some flight schools where they level the prop after the flight...
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Yep, and if the non-impulse mag isn't grounded during starting, its advanced timing could cause a kickback and damage the starter.
 
Berkut said:
Here's some ignition switch trivia for you: You ever wonder why an aircraft ignition switch has the R and L positions backwards, like OFF-R-L-BOTH? Normally a switch is configured so that it completes a circuit to turn the device on. But in the case of an aircraft ignition switch, we complete the circuit (P-lead to ground) to turn the magneto off. So R is on the left because that position actually completes the circuit for the left mag, turning it off. Same thing for the L position.
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mmmmmmm. I dont get it, care to explain a little more plz?
 
Timbuff10 said:
Which could totally happen at some flight schools where they level the prop after the flight... I still have this habit after each flight even though no one else does it at the school I am at now... Primacy!
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always good to do a mag check! I can't believe how some people actually don't even know what that is! :eek:

and also irritates me when they move the prop in the wrong direction! that poor vacuum pump!
 
VDEE7 said:
mmmmmmm. I dont get it, care to explain a little more plz?
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I'm not sure what else I can say about it. Did you have a specific question? I know I have a hard time expressing myself clearly sometimes. That's why I'm not a teacher. :)
 
Not to thread hijack, but speaking off magnetos, what is the purpose of doing mag switch grounding? Personally, I hate it, feels like I'm making the old engine work harder. I can see if it is for the safety of the flight, but isn't that why you check both magnetos in the first place?

I'm flying a PA-38-112, if that helps.
 
n57flyguy said:
Not to thread hijack, but speaking off magnetos, what is the purpose of doing mag switch grounding? Personally, I hate it, feels like I'm making the old engine work harder. I can see if it is for the safety of the flight, but isn't that why you check both magnetos in the first place?

I'm flying a PA-38-112, if that helps.
Click to expand...

You are ensuring the magnetos are indeed grounded...aka p-lead not broken. So the airplane can't start itself as mentioned earlier.
 
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