Magneto Check

[jayllamas]

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???

[dc3flyer]

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.

[jayllamas]

yes...there was a drop in the right.

[dc3flyer]

Was it a huge drop, or just the normal 100-150 RPMs?

[jayllamas]

around 150 i think

[bike21]

Sounds like it is not grounded and needs to be fixed stat.

[Berkut]

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.

[E6BAV8R]

So the L mag check on the ignition actually measures the R magneto's, and vice-versa?

[Berkut]

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.

[jayllamas]

Originally Posted by Berkut

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.

Actually that happened yesterday too.!!!

[Berkut]

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.

[jayllamas]

Thanks for all the help...you probably already saved my life!

[bike21]

Originally Posted by Berkut

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.

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.

[Timbuff10]

Originally Posted by bike21

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.

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!

[Berkut]

Originally Posted by Timbuff10

Which could totally happen at some flight schools where they level the prop after the flight...

Yep, and if the non-impulse mag isn't grounded during starting, its advanced timing could cause a kickback and damage the starter.

[VDEE7]

Originally Posted by Berkut

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.

mmmmmmm. I dont get it, care to explain a little more plz?

[taseal]

Originally Posted by Timbuff10

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!

always good to do a mag check! I can't believe how some people actually don't even know what that is!

and also irritates me when they move the prop in the wrong direction! that poor vacuum pump!

[Berkut]

Originally Posted by VDEE7

mmmmmmm. I dont get it, care to explain a little more plz?

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.

[n57flyguy]

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.

[bike21]

Originally Posted by n57flyguy

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.

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

[n57flyguy]

Originally Posted by bike21

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

OK....still makes me cringe though...

[ananoman]

Originally Posted by taseal

always good to do a mag check! I can't believe how some people actually don't even know what that is!

and also irritates me when they move the prop in the wrong direction! that poor vacuum pump!

The reason that people move the prop in the wrong direction, is so that the engine won't start if there is a broken p-lead on one of the mags. It is the only safe way to move a prop.

I woulden't worry about the vacuum pumps. It is true that you will kill a pump very quickly if you mount the incorrect pump for the engines direction of rotation, but rotating the prop by hand won't hurt anything. When I was at FSI the props on everything with a two bladed prop was moved everytime the airplane was fueled and there were never any problems with vacuum pump failure. Many other flight schools do the same thing. You can bet that no one would do this if it led to premature pump failure as this would cost them quite a bit of money.

[ctab5060X]

Originally Posted by n57flyguy

OK....still makes me cringe though...

Checking the mag switch grounding is SOP where I work. We were taught to check after stopping just before shutdown. With a normal mag switch (OFF-R-L-BOTH), it is a real simple click, click, click...engine dies, immediately return to at least one mag, then both (we have to be careful, our starters are also tied into our mag switches). All you are doing is shutting down the spark to the engine, nothing to cringe about.

[trafficinsight]

Originally Posted by VDEE7

mmmmmmm. I dont get it, care to explain a little more plz?

When you're running with the switch in the "R" position it's grounding the left mag.