Mag grounding check

nrpetersen said:
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.
Click to expand...
172M's came with ACS switches in them. I wish mechanics would get under that panel and see who made the switch. The ACS has an entirely different AD on it.
 
Arrow76R said:
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.
Click to expand...
It doesn't even need to be done fast. After all, if you let the engine die, the muffler has as much fuel-air mix in it as possible, and it doesn't go boom when you restart. At runup RPM there's a lot of flame or superheated gases coming out of the cylinders and they will set off raw fuel in the muffler if the mags go to off and on again. Not at idle. I never, ever had an afterfire on an idle mag check in all the years I flew or maintained airplanes.
 
wanttaja said:
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
Click to expand...
I used to shut my Stromberg down by turning the fuel off. Didn't want the cylinders charged, and since I didn't fly that airplane often I didn't want fuel in the carb drying out and forming varnish in there. It would idle for about 40 seconds on a bowlful of fuel.
 
With my old Stromberg carb that has mixture control but not a full cut off the mags are checked at each shut down... it’s the only way to turn her off...
 
James331 said:
Isn’t this what you do when you do a mag check at the start of the day?
Click to expand...
kinda....one at a time though and you really aren't confirming if the right mag is all the way off when the left mag is still on in that test.... The run-up mag check is more of a check to verify that the mags are working to fire the engine. This discussion is about checking to see if the mags will switch off cold

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 personally don't think you are missing much.

Seems the big concern is un-burned fuel making it's way in or past ...leading to backfire potential.
pulling the mixture sees to it there's not enough fuel left in the intake manifold, engine, or exhaust to support combustion.
but switching the key off would theoretically leave fuel ready to burn in the intake and some of that could be even sucked through...leaving the potential that the engine could restart with a hot mag if someone pulls the prop through...or I suppose even if a hot spot caused
dieseling

As far as I have seen, nobody has really addressed the issue here showing that the quick off then on isn't a sure way to check it.
To really verify it (except for the bad switch issue), it seems to me that it's needed to switch it off and give it time to shut-down

and I didn't catch anyone suggesting a procedure of what to do if the engine does shut down by switching key off....
leave it be?
re-start so that the mixture can be pulled to starve the engine?

seems to me that in the case of discovering you have a GOOD mag ground and the engine shuts down when you key it off
1) if an owned aircraft, or otherwise if you can be sure that nobody else is going to be around the aircraft turning the prop for a while, then with the engine already off, just pulling the mixture to idle cutoff and letting it be is probably fine.... the engine will cool and any remaining fuel will probably dissipate.... but how long of a "while" is needed before it's safe?
2) if it's a rental or uncontrolled, (or probably always) then perhaps it might be a good idea to either
a) pull mixture to cut-off and crank it through a few props using the starter, or
b) re-start the engine and kill it with mixture cut-off
what do you folks think?
 
Brad W said:
kinda....one at a time though and you really aren't confirming if the right mag is all the way off when the left mag is still on in that test.... The run-up mag check is more of a check to verify that the mags are working to fire the engine. This discussion is about checking to see if the mags will switch off cold


I personally don't think you are missing much.

Seems the big concern is un-burned fuel making it's way in or past ...leading to backfire potential.
pulling the mixture sees to it there's not enough fuel left in the intake manifold, engine, or exhaust to support combustion.
but switching the key off would theoretically leave fuel ready to burn in the intake and some of that could be even sucked through...leaving the potential that the engine could restart with a hot mag if someone pulls the prop through...or I suppose even if a hot spot caused
dieseling

As far as I have seen, nobody has really addressed the issue here showing that the quick off then on isn't a sure way to check it.
To really verify it (except for the bad switch issue), it seems to me that it's needed to switch it off and give it time to shut-down

and I didn't catch anyone suggesting a procedure of what to do if the engine does shut down by switching key off....
leave it be?
re-start so that the mixture can be pulled to starve the engine?

seems to me that in the case of discovering you have a GOOD mag ground and the engine shuts down when you key it off
1) if an owned aircraft, or otherwise if you can be sure that nobody else is going to be around the aircraft turning the prop for a while, then with the engine already off, just pulling the mixture to idle cutoff and letting it be is probably fine.... the engine will cool and any remaining fuel will probably dissipate.... but how long of a "while" is needed before it's safe?
2) if it's a rental or uncontrolled, (or probably always) then perhaps it might be a good idea to either
a) pull mixture to cut-off and crank it through a few props using the starter, or
b) re-start the engine and kill it with mixture cut-off
what do you folks think?
Click to expand...

I think you’re worrying about something that’s not really that much of a issue. Does the POH say to do this?
 
James331 said:
I think you’re worrying about something that’s not really that much of a issue. Does the POH say to do this?
Click to expand...

The following POHs do not specify checking the mags at shutdown (these are just the ones I have easily at hand):

C172N
C182Q
C182RG
M20J
PA28-151
PA46-350P
DA20
DA40
R-22
R-44 II

I am pretty sure I remember the DA20 and DA40 also not mentioning anything about checking the mags at shutdown.

That said, I do make it a habit to occasionally check the mags before shutdown.
 
Hang 4 said:
This is the AD
https://rgl.faa.gov/Regulatory_and_...C14960A415D956BD86256E520053A53E?OpenDocument

Supposed to be done every 100 hours
Click to expand...
That's for the bendix switch. Here's the one for the ACS: https://rgl.faa.gov/Regulatory_and_...B8ABD56539B4684886256A3E00759DBF?OpenDocument

Take a look at the list of airplanes you can expect to find it in, especially the long, long list of Cessnas. And then realize that just about any airplane with an Off-Right-Left-Both-Start switch could have one of these, since they're commonly used to replace bad Bendix switches.

Note that there are several things that need to be done on the initial task, including installing a diode on the starter contactor. Found those missing, probably never installed when the switch was installed, or the mechanics just assumed it was a Bendix. Most seem to know nothing about the ACS switch AD.
 
Brad W said:
kinda....one at a time though and you really aren't confirming if the right mag is all the way off when the left mag is still on in that test.... The run-up mag check is more of a check to verify that the mags are working to fire the engine. This discussion is about checking to see if the mags will switch off cold


I personally don't think you are missing much.

Seems the big concern is un-burned fuel making it's way in or past ...leading to backfire potential.
pulling the mixture sees to it there's not enough fuel left in the intake manifold, engine, or exhaust to support combustion.
but switching the key off would theoretically leave fuel ready to burn in the intake and some of that could be even sucked through...leaving the potential that the engine could restart with a hot mag if someone pulls the prop through...or I suppose even if a hot spot caused
dieseling

As far as I have seen, nobody has really addressed the issue here showing that the quick off then on isn't a sure way to check it.
To really verify it (except for the bad switch issue), it seems to me that it's needed to switch it off and give it time to shut-down

and I didn't catch anyone suggesting a procedure of what to do if the engine does shut down by switching key off....
leave it be?
re-start so that the mixture can be pulled to starve the engine?

seems to me that in the case of discovering you have a GOOD mag ground and the engine shuts down when you key it off
1) if an owned aircraft, or otherwise if you can be sure that nobody else is going to be around the aircraft turning the prop for a while, then with the engine already off, just pulling the mixture to idle cutoff and letting it be is probably fine.... the engine will cool and any remaining fuel will probably dissipate.... but how long of a "while" is needed before it's safe?
2) if it's a rental or uncontrolled, (or probably always) then perhaps it might be a good idea to either
a) pull mixture to cut-off and crank it through a few props using the starter, or
b) re-start the engine and kill it with mixture cut-off
what do you folks think?
Click to expand...

The engine idles at around 600-700 RPM. You switch off and the RPM will drop if everything is fine, and if you turn back on at 300 or 400 RPM it will pick right back up. It's NOT a precision, highly-skilled, carefully-timed operation. Anybody can do it.

It's always best to starve the engine for shutdown. Any fuel in the cylinders at all is an accident waiting to happen. If it dies with the switch, close the throttle (it should already be closed) and restart, then pull the mixture. After 47 years I think I can assure you that it will be fine.
 
smv said:
The following POHs do not specify checking the mags at shutdown (these are just the ones I have easily at hand):

C172N
C182Q
C182RG
M20J
PA28-151
PA46-350P
DA20
DA40
R-22
R-44 II

I am pretty sure I remember the DA20 and DA40 also not mentioning anything about checking the mags at shutdown.

That said, I do make it a habit to occasionally check the mags before shutdown.
Click to expand...

I’d just go off the POH or ...

07-AA9743-7-CAE-4-B71-8-DFE-98317-E712618.gif
 
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smv said:
The following POHs do not specify checking the mags at shutdown (these are just the ones I have easily at hand):

C172N
C182Q
C182RG
M20J
PA28-151
PA46-350P
DA20
DA40
R-22
R-44 II

I am pretty sure I remember the DA20 and DA40 also not mentioning anything about checking the mags at shutdown.

That said, I do make it a habit to occasionally check the mags before shutdown.
Click to expand...

Does the POH say anything about grounding the airplane before fuelling? Or about making sure it's tied down in a high wind? Or shutting the engine off before you let someone out of the airplane? Or not backing the tail into a snowbank? Or not landing the with the brakes on? Some newer POH/AFMs might mention some of these because if they don't someone will do it and sue the OEM when they get hurt or dead. Common sense is expected, or at least it used to be.

Most of the fleet consists of old airplanes that, in my experience, need a lot more care than they're getting. The number of broken or frayed wires I have found is simply amazing. Have fixed alternator failures caused by ancient wires that finally broke that should have been caught about five annuals previously.

I once bought and flew home a Citabria in which the trim wouldn't stay put. I found the trim cables wrapped around the top elevator cable, and all three were frayed. This airplane had been through about 15 annuals or 100-hour inspections since those cables had been installed. Later bought another Citabria that had been restored ten years previously, and the same problem was found in it, too.

Simply amazing, and very concerning.
 
The other thing that a mag check after a flight discovers is if you have a rough mag that will only show up when the mags have heated up and the coil opens. I had a bad coil that would be undiscovered if I didn't check the mags at the end of the flight. Only showed up after 45 minutes of flying time during the summer when things heated up. The engine was running fine under power, but at idle with the mag check it would run rough on one mag. So it's not just hot mag but HOT mag....
 
skyking3286 said:
The other thing that a mag check after a flight discovers is if you have a rough mag that will only show up when the mags have heated up and the coil opens. I had a bad coil that would be undiscovered if I didn't check the mags at the end of the flight. Only showed up after 45 minutes of flying time during the summer when things heated up. The engine was running fine under power, but at idle with the mag check it would run rough on one mag. So it's not just hot mag but HOT mag....
Click to expand...

Yup. And mag switches aren't just for checking mags. If a mag goes rogue, such as happens when the plastic distributor gears fail and the rotor start sparking the wrong cylinders at bad times (like on the intake stroke), you lose a lot of power (maybe most of it if the spark sets off the charge in the whole intake system) and might not be able to continue to stay aloft. The mag switch shuts off that bad mag and everything smooths out on the remaining mag and you fly home. If the P-lead has failed to that mag, you cannot shut it off and now you're in trouble.
 
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