Mag grounding check

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Brad W

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SO how many of you regularly check the grounding of your mags?
....and how do you go about it?

I pulled a bonehead move in the rental the other day that I found interesting...and turned into a good thing
I didn't conscientiously do it, but at shutdown I turned the key to off rather than pulling the mixture...and the engine kept running. It sputtered and stumbled but kept running... I had just a moment of brain fade as I processed the event

Once the instructor realized what was happening, so did I. I was kicking myself but realizing the bonehead move was good in this case.

I then remembered that way back in the day I had started the habit of doing it at shutdown. I went back and looked at my old checklist for that type (172N) and I had even added it as a step for shut-down, prior to pulling mixture

But, what my memory is fuzzy on is how actually did I used to do the check. I recon it was much like a Rod Machado video I just found on the topic where he just momentarily went to off and back to both, just to check for faltering.

In this case, the engine would have faltered...but leaving the key in the off position would show the problem. So if you leave the key in off to see if the engine stops.... then if it does just pull the mixture to idle with the engine dead. Would there be anything wrong with that?

I think this story also points to an issue I have with checklists, but I suppose that's another thread.
 
Lots of grounding issues have been discovered this way. The other "test" I do is to engage the starter with the switch in the off position for a few propeller blades then switch to both while the starter is still engaged. Makes the plane easier to start as well since it acts like a primer.
 
Grum.Man said:
The other "test" I do is to engage the starter with the switch in the off position for a few propeller blades then switch to both while the starter is still engaged. Makes the plane easier to start as well since it acts like a primer.
Click to expand...
Most training aircraft won't let you do this. The starter is on the ignition following after "both" mags being on.
 
My primary instructor taught me to always do a ground check before shutdown. First time I did it during a rental checkout at a different FBO the CFI freaked out.

It's a little harder to do with the old style mag switch on my Hatz, it's a bit more difficult to turn than a modern key switch. But I do it from time to time, especially if I plan to to some maintenance afterwards.
 
I don't know what would happen if you went to key off and then pulled the mixture but I do the temporary key off about every third flight.
 
I do it every shutdown. Key off just long enough to verify combustion has stopped, then key back on for 5 seconds or so then idle cutoff.
 
Do you get any dieseling?
I was testing this yesterday after a Surefly mag installation.
It didn't just stop with the key off, it sputtered and eventually died ... tried 2 times.
IO540 with a key

BTW the FBO where I teach says to let it die and restart if you do this on a mag check, to prevent backfires and damage.
 
I put a mag check in my shutdown checklist. Just like runup, except I do it at idle. If each side shows a drop, you know they are turning off. It's a good safety measure in case someone has to move the prop, to hook up a towbar for example.

The reason to not shut both mags off is that the engine will continue to draw fuel from the carb and then pump that fuel out the exhaust. When you turn the ignition back on, that fuel will almost certainly combust, which can cause exhaust cracks, especially inside the muffler/ heat exchanger. That's the reason we kill the engine using mixture instead of just switching off the ignition off like a car. Cars don't have delicate heat exchangers in the exhaust, nor do they have heavy propellers hanging off the nose of the crank, so they don't turn long after shutting the engine off.

I once put spark plugs in the exhaust of a 78 Chevy pickup I had. You'd rev the engine and kill the ignition, and it'd shoot 6 foot flames. Good times.
 
Discovering an ungrounded mag is a rare event. Checking after inspections is about the only time I do this check. Teaching this to primary students is asking for a damaged exhaust.
 
I've posted this before a long time ago, but it is again topical to this thread...

When doing the mag check in the typical trainer equipped with a "Off Left Right Both Start" key switch, be sure to remove your hand from the key momentarily when the key is in the OFF position.

I was shutting down the rental PA-28 when my hand slipped off the key when it was OFF. The engine continued to idle normally. However, if I applied counterclockwise pressure to the key, the engine died. Holding the key at the counterclockwise stop, the ignition was grounded as it should have been. Releasing the key allowed it to move slightly off the counterclockwise stop into a position centered in the OFF position, which did NOT ground the mags. So unless you take your hand off the key, you have little assurance that the ignition is really grounded.

This is a known failure mode for the key switch. I believe it is brand name Bendix but that is from memory 20 years ago... caveat aviator!

This argues against Clip4's post in post #11 above, if you have the key switch in question.

-Skip
 
Jim K said:
The reason to not shut both mags off is that the engine will continue to draw fuel from the carb and then pump that fuel out the exhaust. When you turn the ignition back on, that fuel will almost certainly combust, which can cause exhaust cracks, especially inside the muffler/ heat exchanger. That's the reason we kill the engine using mixture instead of just switching off the ignition off like a car.
Click to expand...

and this sort of thing is why I'm trying to better remember the proper thing to do after a confirmed mag ground check.... if the engine shuts down when keyed "off"

Would it be better to restart right away and then kill by pulling mixture, or just kill the mixture and park the plane knowing that any fuel in the hot engine will evaporate as it cools?

My fuzzy memory is telling me that what I used to do was to just key off monetarily checking for a falter, and not long enough to stop the prop... but my recent bonehead experiment proved that this doesn't positively confirm a good mag grounding
 
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Skip Miller said:
Releasing the key allowed it to move slightly off the counterclockwise stop into a position centered in the OFF position, which did NOT ground the mags. So unless you take your hand off the key, you have little assurance that the ignition is really grounded.
Click to expand...
interesting.... in my case I did at least momentarily remove my hand but I don't remember if it was long enough to actually let the engine shut down...
 
Heed JimK in post #10. If you turn off the ignition as a mag test, do not turn it back on or you risk a damaging backfire.
 
Before every shutdown, a quick turn to off then back again.
 
Clip4 said:
Discovering an ungrounded mag is a rare event. Checking after inspections is about the only time I do this check. Teaching this to primary students is asking for a damaged exhaust.
Click to expand...
It is NOT asking for a damaged exhaust. We did this all the time, at the end of every flight, in all the years I was at the flight school. You will not get an exhaust afterfire at idle like you will at runup RPM if you switch the mags off and back on again. There is no flame leaving the exhaust at idIe to ignite anything in the exhaust, and at idle the mix is lean and the fuel flow low.

I was a mechanic as well as a flight instructor, too.

Turn the switch on long enough to see that the engine RPM falls, then back on and pull the mixture to idle cutoff. There is no risk to the engine or exhaust and it identifies a hot mag, a deadly risk that you won't catch otherwise unless you are savvy enough to notice no RPM drop on a mag during runup and realize what it means.
 
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Skip Miller said:
I've posted this before a long time ago, but it is again topical to this thread...

When doing the mag check in the typical trainer equipped with a "Off Left Right Both Start" key switch, be sure to remove your hand from the key momentarily when the key is in the OFF position.

I was shutting down the rental PA-28 when my hand slipped off the key when it was OFF. The engine continued to idle normally. However, if I applied counterclockwise pressure to the key, the engine died. Holding the key at the counterclockwise stop, the ignition was grounded as it should have been. Releasing the key allowed it to move slightly off the counterclockwise stop into a position centered in the OFF position, which did NOT ground the mags. So unless you take your hand off the key, you have little assurance that the ignition is really grounded.

This is a known failure mode for the key switch. I believe it is brand name Bendix but that is from memory 20 years ago... caveat aviator!

This argues against Clip4's post in post #11 above, if you have the key switch in question.

-Skip
Click to expand...

That fault is the opposite to the Bendix failure mode. They would fail if you forced the key a little past the off detent, unshorting one mag and letting the engine run.

And I have found so many log entries where mechanics have done this AD test for 25 years and found no fauilt, without ever getting under the panel and confirming that it really is a Bendix switch, and I find that it's an ACS switch that has a totally different AD against it and it's way out of compliance.
 
Domenick said:
Heed JimK in post #10. If you turn off the ignition as a mag test, do not turn it back on or you risk a damaging backfire.
Click to expand...
That there. It can crack mufflers, and it can also blow out intake gaskets at the cylinder, believe it or not. There's a bit of valve overlap that can let the shockwave do that. Leave the switch off and the engine die, put the throttle at idle and start the engine.

That problem arises often because pilots are holding the key by its edges. They are pinching it, and as they go to right mag they accidentally lose control of the key as the pinching snaps the key flat between their fingers, rotating the switch to off. Hold that key by the flats, not the edges.
 
Clip4 said:
Discovering an ungrounded mag is a rare event. Checking after inspections is about the only time I do this check. Teaching this to primary students is asking for a damaged exhaust.
Click to expand...
True,, I only do it on annual.
 
I check it at every run up before flight...
 
Stewartb said:
Do you do a mag check at run up? Do you get drops in both positions? Howz that happen? :)
Click to expand...

Seeing a drop on R and L at mag check shows a) both p-leads are intact and b) mag switch is OK at R and L. It does NOT check the switch at OFF, hence the common practice of turning it OFF momentarily before shutdown.
 
Domenick said:
Heed JimK in post #10. If you turn off the ignition as a mag test, do not turn it back on or you risk a damaging backfire.
Click to expand...
How do you ensure that they’re grounding than without momentarily turning off the ignition?
 
flyingbrit said:
Seeing a drop on R and L at mag check shows a) both p-leads are intact and b) mag switch is OK at R and L. It does NOT check the switch at OFF, hence the common practice of turning it OFF momentarily before shutdown.
Click to expand...

From what I observed over the weekend, it might also not show a partial ground.
Nor would the seemingly common mag ground check of quickly going to off to notice just a drop in RPM.
If I'd have done that in this airplane all would have seemed ok....the rpm dropped just like it was shutting down...but then just sorta kept faltering up and down a bit but never back up to a full smooth idle. So that's why I'm thinking bad ground....but not completely open
 
Okay... I misunderstood the question. A bad key switch is possible. The plane I fly most has rocker switches for the mags and a push button starter, which I greatly prefer over the key switch style.

I can't think of any way to test it other than the key off test you describe, which I don't like for the reasons outlined above, or checking with a multimeter, which isn't very practical. Seems like a key switch failing in that particular way would be pretty rare, but I really don't know.

How common is dieseling in aero engines? That would certainly produce the symptoms you describe. I've got an old gas powered forklift that will occasionally run for twenty seconds with the ignition off if it's a warm day and I've been working it hard.
 
Isn’t this what you do when you do a mag check at the start of the day?
 
Grum.Man said:
Lots of grounding issues have been discovered this way. The other "test" I do is to engage the starter with the switch in the off position for a few propeller blades then switch to both while the starter is still engaged. Makes the plane easier to start as well since it acts like a primer.
Click to expand...
That’s a radial engine technique.
 
Jim K said:
Seems like a key switch failing in that particular way would be pretty rare, but I really don't know.
Click to expand...

It's not usually the switch that fails. It's the P-leads themselves where they connect to the mags. Vibration fatigues the wires and their strands break one at a time until the lead gets so floppy that it breaks altogether. This normally happens in flight, after the runup mag checks, so the mag check is no guarantee that the system is still shutting things off after the flight. So the quick switch-off at idle is to make sure those leads are still intact and working and the mags are both dead.

During inspections I have frequently found P-leads ready to break off, their strands all frayed and a couple or three left (out of 15 or whatever).
 
The time when a hot mag malfunction is deadly is after the flight is over and before restarted (engine off and hangaring/hangared). That’s the only time when I have an opportunity to be in the way of the prop. I have no idea after a flight if vibration changed anything with my mag system. So I check it at shutdown rather than the next day at runup after I’ve had my body in the way of the prop (turn prop, attach tow bar).
 
Jim K said:
I can't think of any way to test it other than the key off test you describe, which I don't like for the reasons outlined above, or checking with a multimeter, which isn't very practical. Seems like a key switch failing in that particular way would be pretty rare, but I really don't know.
Click to expand...
Can’t be that rare if the FAA issued an AD on the keyed switches. Unless it’s been rescinded, the AD requires you to test the switch every so many hours.
 
Dan Thomas said:
It's not usually the switch that fails. It's the P-leads themselves where they connect to the mags. Vibration fatigues the wires and their strands break one at a time until the lead gets so floppy that it breaks altogether. This normally happens in flight, after the runup mag checks, so the mag check is no guarantee that the system is still shutting things off after the flight. So the quick switch-off at idle is to make sure those leads are still intact and working and the mags are both dead.

During inspections I have frequently found P-leads ready to break off, their strands all frayed and a couple or three left (out of 15 or whatever).
Click to expand...

That's what I thought.... and what I'm checking for by switching the mags off one at a time before shutting down. The only thing that wouldn't check is the key switch itself.
 
There is a Bendix mag switch AD that address this same issue.
That one hit our 172M years ago. I'd swear the IA used a pliers on the key to get it to go left past the OFF enough to get the engine to relight.
ht.
 
I have taught my flight students from day #1 to always do a "hot mag" check just prior to engine shut-down. The only caveat is to make sure the engine is at idle rpm and the check is done FAST to avoid backfire. I tell the students that this is different than the mag check on engine run-up which is really to check mag and plug operation.
 
Clip4 said:
Discovering an ungrounded mag is a rare event. Checking after inspections is about the only time I do this check. Teaching this to primary students is asking for a damaged exhaust.
Click to expand...

Not if you teach them correctly. LOL.
 
Brad W said:
SO how many of you regularly check the grounding of your mags?
....and how do you go about it?
Click to expand...

Every flight. I turn the key to "Off."

The Stromberg's mixture control doesn't have enough range to starve the engine to kill it. So I shut the engine down with the mags. If the grounding is broken on one or both, I'll need to shut off the fuel valve to kill the engine.

Ron Wanttaja
 
Jim K said:
The reason to not shut both mags off is that the engine will continue to draw fuel from the carb and then pump that fuel out the exhaust. When you turn the ignition back on, that fuel will almost certainly combust, which can cause exhaust cracks, especially inside the muffler/ heat exchanger. That's the reason we kill the engine using mixture instead of just switching off the ignition off like a car. Cars don't have delicate heat exchangers in the exhaust, nor do they have heavy propellers hanging off the nose of the crank, so they don't turn long after shutting the engine off.
Click to expand...

I had a '46 Willys Jeep as a teenager. One of the clamps that held the exhaust pipes to the muffler wasn't too tight. I'd turn off the ignition while the car was rolling, then turn it back on. Most of the time there'd be a big backfire. Most of the time, it would blow the exhaust pipe out of the muffler and I'd be driving with straight pipes.....

Ron Wanttaja
 
I think perhaps I am missing something. Why does the mag check require a quick ignition off-on? Why not simply turn it off?
 
Domenick said:
I think perhaps I am missing something. Why does the mag check require a quick ignition off-on? Why not simply turn it off?
Click to expand...
I like to burn all the fuel so even if the mags are switched on accidentally it's less likely to start if you move the prop.
 
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