Ignition switch, what happen when Left or Right is selected

[Martin B]

Hello,

Here is my question:

On regular aircrafts, like C150, C172, PA28, when we perform the
magneto check where we select Left, than Right, while looking for RPM
drop... what is the switch doing?

When we select Left or Right, do we short that particular magneto or
are we shorting the other one?

I checked all my books and cannot find the answer.

Thanks in advance,

Martin

[Bill Zaleski]

You are shorting the magneto that is not selected, therefore
inhibiting it's operation.


On Tue, 17 May 2005 22:49:09 -0400, Martin B >
wrote:

>Hello,
>
>Here is my question:
>
>On regular aircrafts, like C150, C172, PA28, when we perform the
>magneto check where we select Left, than Right, while looking for RPM
>drop... what is the switch doing?
>
>When we select Left or Right, do we short that particular magneto or
>are we shorting the other one?
>
>I checked all my books and cannot find the answer.
>
>Thanks in advance,
>
>Martin

[Guillermo]

"Bill Zaleski" > wrote in message
...
> You are shorting the magneto that is not selected, therefore
> inhibiting it's operation.
>
And that's one of the dangerous things of a magneto system. They are hot on
their own, unless shorted. So if the wiring gets loose in the key switch, or
something else along the circuit gets disconnected, actually you may have a
hot prop, which is very dangerous.
If when you do a magneto check you notice NO RPM drop, then either the
grounding is not correct, or the other magneto is not working at all.

[BTIZ]

and now the other part of that question... for most key switches that have
"Off" "Left" "Right" "Both" "Start" positions.. what happens when you go to
the "Start" position.?.

BT

"Bill Zaleski" > wrote in message
...
> You are shorting the magneto that is not selected, therefore
> inhibiting it's operation.
>
>
> On Tue, 17 May 2005 22:49:09 -0400, Martin B >
> wrote:
>
>>Hello,
>>
>>Here is my question:
>>
>>On regular aircrafts, like C150, C172, PA28, when we perform the
>>magneto check where we select Left, than Right, while looking for RPM
>>drop... what is the switch doing?
>>
>>When we select Left or Right, do we short that particular magneto or
>>are we shorting the other one?
>>
>>I checked all my books and cannot find the answer.
>>
>>Thanks in advance,
>>
>>Martin
>

[Peter Duniho]

"Guillermo" > wrote in message
...
> And that's one of the dangerous things of a magneto system. They are hot
> on
> their own, unless shorted.

Well, yes and no. I mean, yes...a hot prop can be dangerous, depending on
the engine and airplane.

But compared to the hazard of having a broken wire disable your magneto,
it's a pretty minor danger. The "short the magneto" design is that way for
a reason, and I think most people would agree with that reason.

There are lots of "dangerous things" about airplanes. This just happens to
be one of them, and it's not really that big of a deal as things go. :)

> [...]
> If when you do a magneto check you notice NO RPM drop, then either the
> grounding is not correct, or the other magneto is not working at all.

Yup. Of course, it's easy enough to determine which it is, when you go to
test the other magneto. :)

Pete

[Peter Duniho]

"BTIZ" > wrote in message
news:uhzie.7310$Db6.649@okepread05...
> and now the other part of that question... for most key switches that have
> "Off" "Left" "Right" "Both" "Start" positions.. what happens when you go
> to the "Start" position.?.

Assuming just one magneto with an impulse coupling, the other magneto is
shorted and the starter is engaged (by electrical relay, generally...I'm not
aware of any airplane starter that runs the current straight through the mag
switch).

Note also that this is true only of systems that use an impulse coupling for
starting.

Of course, you could have just posted that information, rather than turning
into a pop-quiz. ;)

Pete

[Dave Stadt]

"BTIZ" > wrote in message
news:uhzie.7310$Db6.649@okepread05...
> and now the other part of that question... for most key switches that have
> "Off" "Left" "Right" "Both" "Start" positions.. what happens when you go
to
> the "Start" position.?.
>
> BT

Usually, the propeller starts turning and after a seond or two the noise
level increases. Sorry, couldn't pass that one up.

[BTIZ]

most correct answer...

"Peter Duniho" > wrote in message
...
> "BTIZ" > wrote in message
> news:uhzie.7310$Db6.649@okepread05...
>> and now the other part of that question... for most key switches that
>> have "Off" "Left" "Right" "Both" "Start" positions.. what happens when
>> you go to the "Start" position.?.
>
> Assuming just one magneto with an impulse coupling, the other magneto is
> shorted and the starter is engaged (by electrical relay, generally...I'm
> not aware of any airplane starter that runs the current straight through
> the mag switch).
>
> Note also that this is true only of systems that use an impulse coupling
> for starting.
>
> Of course, you could have just posted that information, rather than
> turning into a pop-quiz. ;)
>
> Pete
>

[BTIZ]

a good answer... but missing essential elements.. LOL

See Pete's answer
BT

"Dave Stadt" > wrote in message
m...
>
> "BTIZ" > wrote in message
> news:uhzie.7310$Db6.649@okepread05...
>> and now the other part of that question... for most key switches that
>> have
>> "Off" "Left" "Right" "Both" "Start" positions.. what happens when you go
> to
>> the "Start" position.?.
>>
>> BT
>
> Usually, the propeller starts turning and after a seond or two the noise
> level increases. Sorry, couldn't pass that one up.
>
>

[Greg Farris]

What's the logic of grounding the non impulse-coupled mag during starting?
(Not a "pop quiz" - I'd like to know).

G Faris



In article >,
says...

>Assuming just one magneto with an impulse coupling, the other magneto is
>shorted and the starter is engaged (by electrical relay, generally...I'm not
>aware of any airplane starter that runs the current straight through the mag
>switch).
>
>Note also that this is true only of systems that use an impulse coupling for
>starting.
>
>Of course, you could have just posted that information, rather than turning
>into a pop-quiz. ;)
>
>Pete
>
>

[Tauno Voipio]

Greg Farris wrote:
>
> In article >,
> says...
>
>
>>Assuming just one magneto with an impulse coupling, the other magneto is
>>shorted and the starter is engaged (by electrical relay, generally...I'm not
>>aware of any airplane starter that runs the current straight through the mag
>>switch).
>>
>>Note also that this is true only of systems that use an impulse coupling for
>>starting.
>>
>>Of course, you could have just posted that information, rather than turning
>>into a pop-quiz. ;)
>>
>>Pete
>>
>>
>
>
> What's the logic of grounding the non impulse-coupled mag during
starting?
> (Not a "pop quiz" - I'd like to know).
>
> G Faris
>
>

The non-impulse magneto will fire earlier, if at all, and it may
back-fire the engine, as the cranking speed is slow.

--

Tauno Voipio
tauno voipio (at) iki fi

[Happy Dog]

"BTIZ" > wrote in message
news:uhzie.7310$Db6.649@okepread05...
> and now the other part of that question... for most key switches that have
> "Off" "Left" "Right" "Both" "Start" positions.. what happens when you go
> to the "Start" position.?.

Already answered. Why are 'L' and 'R' often reversed on the switch?

moo


>
> BT
>
> "Bill Zaleski" > wrote in message
> ...
>> You are shorting the magneto that is not selected, therefore
>> inhibiting it's operation.
>>
>>
>> On Tue, 17 May 2005 22:49:09 -0400, Martin B >
>> wrote:
>>
>>>Hello,
>>>
>>>Here is my question:
>>>
>>>On regular aircrafts, like C150, C172, PA28, when we perform the
>>>magneto check where we select Left, than Right, while looking for RPM
>>>drop... what is the switch doing?
>>>
>>>When we select Left or Right, do we short that particular magneto or
>>>are we shorting the other one?
>>>
>>>I checked all my books and cannot find the answer.
>>>
>>>Thanks in advance,
>>>
>>>Martin
>>
>
>

[Ron Natalie]

Guillermo wrote:
> "Bill Zaleski" > wrote in message
> ...
>
>>You are shorting the magneto that is not selected, therefore
>>inhibiting it's operation.
>>
>
> And that's one of the dangerous things of a magneto system. They are hot on
> their own, unless shorted. So if the wiring gets loose in the key switch, or
> something else along the circuit gets disconnected, actually you may have a
> hot prop, which is very dangerous.
> If when you do a magneto check you notice NO RPM drop, then either the
> grounding is not correct, or the other magneto is not working at all.
>
>
Or things are severely mistimed.

But the fact there is a drop on L and R doesn't tell you things
are working properly in the OFF position.

[Ron Natalie]

BTIZ wrote:
> and now the other part of that question... for most key switches that have
> "Off" "Left" "Right" "Both" "Start" positions.. what happens when you go to
> the "Start" position.?.
>
Depends on the switch. Some switches ground out the RIGHT mag, some
leave both on. It also provides a contact for the starter solenoid.

[nrp]

Some more thoughts on selecting the correct mag for starting -

The impulse function on a mag provides a both a substantial timing
delay and a velocity enhancement for starting purposes. Both are
important. The timing delay of the impulse prevents an engine from
kicking back. That's why starting is done on one (the impulse) mag
only. Most aircraft have only one mag with an impulse, but even a
non-impulse mag may generate a spark at cranking speeds if the mag is
in good condition.

Two things to consider are

1) Be sure the only correct mag is selected if hand propping - and
never put your fingers over the back of the blade. I didn't once on a
TriPacer and still have a fortunately small scar on the tip of my index
finger when it kicked back mightily.

2) Consider that possibly the mag p leads might get reversed in
maintenance. The starter may be able to handle the cranking on the
wrong mag but it could be a hand propping accident waiting to happen.

That's why I won't hand prop a strange airplane for the first time.

[OtisWinslow]

"Guillermo" > wrote in message
...
> And that's one of the dangerous things of a magneto system. They are hot
> on
> their own, unless shorted. So if the wiring gets loose in the key switch,
> or
> something else along the circuit gets disconnected, actually you may have
> a
> hot prop, which is very dangerous.
> If when you do a magneto check you notice NO RPM drop, then either the
> grounding is not correct, or the other magneto is not working at all.
>

That's a good thing to pay attention to. You can also check it at
idle before shut down by briefly turning the switch to "OFF" and
see if the engine starts to shut down.

[Guillermo]

what is a non-impulse coupled magneto? (also a question, not a quiz)

"Greg Farris" > wrote in message
...
>
> What's the logic of grounding the non impulse-coupled mag during starting?
> (Not a "pop quiz" - I'd like to know).
>
> G Faris
>
>
>
> In article >,

> says...
>
> >Assuming just one magneto with an impulse coupling, the other magneto is
> >shorted and the starter is engaged (by electrical relay, generally...I'm
not
> >aware of any airplane starter that runs the current straight through the
mag
> >switch).
> >
> >Note also that this is true only of systems that use an impulse coupling
for
> >starting.
> >
> >Of course, you could have just posted that information, rather than
turning
> >into a pop-quiz. ;)
> >
> >Pete
> >
> >
>

[Dave Butler]

Guillermo wrote:
> what is a non-impulse coupled magneto? (also a question, not a quiz)

I would have sworn there was an excellent article on avweb.com, I thought it was
by Deakin. I can't find it.

Anyway, an impulse coupling is a device to assist starting. At low RPM
(starting), the spark is retarded, and the impulse coupling spring spins the
magnetic fields faster during part of the rotation to produce a stronger spark.
Usually only one of the two magnetos has an impulse coupling, by convention,
usually the left. When you pull the prop through by hand you can hear the
"sproing!..." as the impulse coupling spins around. You'll hear it twice per
revolution of the crankshaft.

Dave

[Guillermo]

I was wondering what is exactly the function of the impulse-coupled magneto.
I've read about them a little bit on the web, but I'd like somebody to
explain it in simple terms.

thanks

guillermo

"Tauno Voipio" > wrote in message
...
> Greg Farris wrote:
> >
> > In article >,

> > says...
> >
> >
> >>Assuming just one magneto with an impulse coupling, the other magneto is
> >>shorted and the starter is engaged (by electrical relay, generally...I'm
not
> >>aware of any airplane starter that runs the current straight through the
mag
> >>switch).
> >>
> >>Note also that this is true only of systems that use an impulse coupling
for
> >>starting.
> >>
> >>Of course, you could have just posted that information, rather than
turning
> >>into a pop-quiz. ;)
> >>
> >>Pete
> >>
> >>
> >
> >
> > What's the logic of grounding the non impulse-coupled mag during
> starting?
> > (Not a "pop quiz" - I'd like to know).
> >
> > G Faris
> >
> >
>
> The non-impulse magneto will fire earlier, if at all, and it may
> back-fire the engine, as the cranking speed is slow.
>
> --
>
> Tauno Voipio
> tauno voipio (at) iki fi
>

[RST Engineering]

Just running down the list in my head, I am going to take the opposite
position. Most aircraft have BOTH mags with an impulse; a small minority
have only one with an impulse.

Jim


>>Most aircraft have only one mag with an impulse

[Dave Butler]

RST Engineering wrote:
> Just running down the list in my head, I am going to take the opposite
> position. Most aircraft have BOTH mags with an impulse; a small minority
> have only one with an impulse.
>
> Jim
>
>>>Most aircraft have only one mag with an impulse

Not sure to whom you are responding. I don't see that exact text in my posting,
but I did say something like that.

....but I defer to your greater experience.

Dave

[RST Engineering]

I'm not saying I am right, just that off the top of my head without going
down the list with a pencil and paper, it seems that most (if not all)
Cessnas from the 120 to the 210 have both mags impulsed, most Pipers also,
and that is 90% of the fleet right there.

However, I'm also willing to be corrected by my greasywrench colleagues who
are more into engine work than I.

Jim



"Dave Butler" > wrote in message
news:1116429660.228058@sj-nntpcache-5...
> RST Engineering wrote:
>> Just running down the list in my head, I am going to take the opposite
>> position. Most aircraft have BOTH mags with an impulse; a small minority
>> have only one with an impulse.
>>
>> Jim
>>
>>>>Most aircraft have only one mag with an impulse
>
> Not sure to whom you are responding. I don't see that exact text in my
> posting, but I did say something like that.
>
> ...but I defer to your greater experience.
>
> Dave

[Greg Farris]

In article >,
says...
>
>
>Just running down the list in my head, I am going to take the opposite
>position. Most aircraft have BOTH mags with an impulse; a small minority
>have only one with an impulse.
>



Well, I certainly can't contest that, but Lycoming did issue a service
instruction some years ago, indicating that motors with two impulse coupled
magnetos should have the right one replaced with a plain magneto.
Implementation at owner's discretion.

[Bill Zaleski]

Almost all of the make/models that you mention below have an impulse
mag on the left side only.


On Wed, 18 May 2005 09:00:21 -0700, "RST Engineering"
> wrote:

>I'm not saying I am right, just that off the top of my head without going
>down the list with a pencil and paper, it seems that most (if not all)
>Cessnas from the 120 to the 210 have both mags impulsed, most Pipers also,
>and that is 90% of the fleet right there.
>
>However, I'm also willing to be corrected by my greasywrench colleagues who
>are more into engine work than I.
>
>Jim
>
>
>
>"Dave Butler" > wrote in message
>news:1116429660.228058@sj-nntpcache-5...
>> RST Engineering wrote:
>>> Just running down the list in my head, I am going to take the opposite
>>> position. Most aircraft have BOTH mags with an impulse; a small minority
>>> have only one with an impulse.
>>>
>>> Jim
>>>
>>>>>Most aircraft have only one mag with an impulse
>>
>> Not sure to whom you are responding. I don't see that exact text in my
>> posting, but I did say something like that.
>>
>> ...but I defer to your greater experience.
>>
>> Dave
>

[RST Engineering]

Perhaps those Lycoming powered that came off the production line after the
service bulletin advising to make that switch, but prior to that date they
were double impulse mags.

Let's take just two examples with which I am intimately familiar, since I
just did a mag rebuild on both of them. The Continental O-470L with Bendix
magnetos has dual impulse mags. (Cessna 182 et al)

The Continental O-300D with Bendix magnetos has dual impulse mags. (Cessna
172 et al)

I suppose we can go down the list of every aircraft that was ever made and
start a table. I'm not going to waste my time doing that.

Jim




"Bill Zaleski" > wrote in message
...
> Almost all of the make/models that you mention below have an impulse
> mag on the left side only.
>
>
> On Wed, 18 May 2005 09:00:21 -0700, "RST Engineering"
> > wrote:
>
>>I'm not saying I am right, just that off the top of my head without going
>>down the list with a pencil and paper, it seems that most (if not all)
>>Cessnas from the 120 to the 210 have both mags impulsed, most Pipers also,
>>and that is 90% of the fleet right there.
>>
>>However, I'm also willing to be corrected by my greasywrench colleagues
>>who
>>are more into engine work than I.
>>
>>Jim

[Peter Duniho]

"OtisWinslow" > wrote in message
m...
> That's a good thing to pay attention to. You can also check it at
> idle before shut down by briefly turning the switch to "OFF" and
> see if the engine starts to shut down.

Emphasis on "at idle", of course. :)

[tom418]

To drum up business for A&Ps, via A.D. 78-09-07 :)
"Guillermo" > wrote in message
...
> I was wondering what is exactly the function of the impulse-coupled
magneto.
> I've read about them a little bit on the web, but I'd like somebody to
> explain it in simple terms.
>
> thanks
>
> guillermo
>
> "Tauno Voipio" > wrote in message
> ...
> > Greg Farris wrote:
> > >
> > > In article >,
>
> > > says...
> > >
> > >
> > >>Assuming just one magneto with an impulse coupling, the other magneto
is
> > >>shorted and the starter is engaged (by electrical relay,
generally...I'm
> not
> > >>aware of any airplane starter that runs the current straight through
the
> mag
> > >>switch).
> > >>
> > >>Note also that this is true only of systems that use an impulse
coupling
> for
> > >>starting.
> > >>
> > >>Of course, you could have just posted that information, rather than
> turning
> > >>into a pop-quiz. ;)
> > >>
> > >>Pete
> > >>
> > >>
> > >
> > >
> > > What's the logic of grounding the non impulse-coupled mag during
> > starting?
> > > (Not a "pop quiz" - I'd like to know).
> > >
> > > G Faris
> > >
> > >
> >
> > The non-impulse magneto will fire earlier, if at all, and it may
> > back-fire the engine, as the cranking speed is slow.
> >
> > --
> >
> > Tauno Voipio
> > tauno voipio (at) iki fi
> >
>
>

[OtisWinslow]

"Peter Duniho" > wrote in message
...
> "OtisWinslow" > wrote in message
> m...
>> That's a good thing to pay attention to. You can also check it at
>> idle before shut down by briefly turning the switch to "OFF" and
>> see if the engine starts to shut down.
>
> Emphasis on "at idle", of course. :)
>

Yes. At idle. And I should have added that if it starts to shudder and
die don't turn it back on. When I do it it's just a brief switch to the
OFF position and back on.

[[email protected]]

>Yes. At idle. And I should have added that if it >starts to
shudder and
>die don't turn it back on.

It WILL start to shudder and die if you turn it off, unless there's a
bad P-lead connection or mag switch. You want it back on so you can
shut the engine off with mixture, not ignition.
I get really annoyed when students go to "Off" during the runup
and mag testing. We try to get them to hold the key firmly, not by its
edges, so that it won't inadvertently snap over to "Off" when they
select "R." They almost always immediately turn it on again (instead of
letting it spool to a stop and restarting) and the exhaust makes an
almighty BANG and I have to look closely at that muffler and piping
during inspections to find the cracks that often result.
That was the big joke when we were young, right? Turn of the
auto's ignition and back on again to get the big boom from the exhaust
system. Doesn't work anymore with fuel injection. Saves exhaust
systems, no doubt.

Dan

[RST Engineering]

I get annoyder when they go the other way and grind the starter bendix. I'm
not sure which one is more expensive to fix.

{;-)

Jim


> I get really annoyed when students go to "Off" during the runup
> and mag testing.

[Morgans]

"Guillermo" > wrote in message
...
> I was wondering what is exactly the function of the impulse-coupled
magneto.
> I've read about them a little bit on the web, but I'd like somebody to
> explain it in simple terms.

A regular non impulse magneto has the engine rotation directly attached to
the rotating part of the magneto, so at low cranking speeds, it may not be
going past the windings fast enough to generate a sufficient spark.

An impulse magneto has the generating part of the magneto designed just the
same as the regular magneto, but the trickery comes in, at how the magneto
is coupled to the engine.

In the impulse type, when the engine is being turned very slowly, there are
little counterweights that do not allow the generating portion to turn, but
instead, the rotation winds up a powerful spring. When it gets all the way
wound up, and the time has come to make a spark, it releases, and spins the
generating gizmo fast as the devil, creating a powerful spark, which sends
your piston scurrying down it's travel. The next time around, the mag will
be going fast enough to not wind up the spring, and it will behave just like
a regular magneto.

The winding up of the spring also serves to delay the spark until the piston
has barely gone past top dead center, rather than well before top dead
center. That is the reason you shut off the other regular mag; so it does
not possibly fire early, and make the starter have to work against the
attempted backwards combustion.

The impulse magneto is why rotating a prop can be so dangerous. If it has
the mags turned off, but the impulse mag has had it's ground path fail, it
will still wind up the spring. No matter how slowly the prop turns, if it
has a sufficient fuel air mix left in the cylinder and the impulse snaps
over center, the windings will send the spark on the way, thus starting the
engine.

By the way, at OSH, in the HB show area, the young men and women (Aviation
Explorers) helping you park your plane are NOT ALLOWED to touch your prop,
due to safety rules, because of this characteristic.

Clear enough? Ask if you need more clarification.
--
Jim in NC

[John Galban]

RST Engineering wrote:

>
> I suppose we can go down the list of every aircraft that was ever
made and
> start a table. I'm not going to waste my time doing that.
>

My old straight tailed 172 had dual impulse mags. Made for very easy
handpropping, which was often required because of the cheesy 20 amp
generator.

Most of the later model Lycoming powered Cessnas and Pipers that I've
seen have the impulse mag on the left only. Piper started doing this
in the Cherokee model right after they quit using shower of sparks.
Standard start procedure was : key switch on the left mag and hit the
starter button.

John Galban=====>N4BQ (PA28-180)

[nrp]

Assuming lawyers drive every technical decision nowdays, I wonder why
they did that (recommend switching to single impulse mags)?

[George Patterson]

Guillermo wrote:
> what is a non-impulse coupled magneto? (also a question, not a quiz)

As has been explained elsewhere, an impulse coupling is a feature of some
magnetos which enhances the spark and retards the timing at low revolutions. A
"non-impulse coupled magneto" does not have an impulse coupling.

To give an example, the left magneto on the Lycoming O-320-B2D has an impulse
coupling. The right one does not. When the ignition switch is placed in the
"start" position, the left mag is hot and the right one is grounded out. If the
left magneto fails for any reason, it is impossible to start the engine with
either the starter or by hand-propping.

With that engine, the right magneto is a non-impulse coupled magneto.

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.

[OtisWinslow]

My point was that if it gets past the point where it's going to keep
running, just leave it off. A quick flick of the switch to OFF and back ON
it
will certainly quit firing and lose a few rpm, but it won't die. I've been
doing these as long as I've been flying (the 70s) and I think it's
important to know the mags aren't hot. It's part of my shut down.
You should simply use any method you feel comfortable with to
check the P leads.


> wrote in message
ups.com...
>
> It WILL start to shudder and die if you turn it off, unless there's a
> bad P-lead connection or mag switch. You want it back on so you can
> shut the engine off with mixture, not ignition.
>
> Dan
>

[[email protected]]

Exactly. I learned to fly in the early '70s and they didn't
teach us to look for a hot mag. I learned that when upgrading some
years ago. Doing this, I found that one of our 172's mag switches would
allow the mags to keep firing if the key was forced against the "Off"
stop, and upon investigation found an AD against any of those switches
that did that (76-07-12). Bet there are still a lot of them out there.
As far as the discussion about a single impulse mag rather than
two: Lycoming may have recommended a change to one to get rid of the
impulse coupling on one mag. The springs in these things have been
known to fail, and with two mags one might fail and not be readily
noticed if the pilot is a careless twit who doesn't do a decent runup;
if the other spring then failed as well the timing on both would go to
near zero BTDC. Power output would be low indeed. Corrosion has been a
problem in engines seldom flown or flown on very short flights, and the
springs, being a high-carbon steel, corrode quickly. The Bendix dual
mag (one housing, one gear and impulse coupling, two mags) had an AD
against it to replace that spring with a better one, since its failure
could and did cause accidents.

Dan

[nrp]

DT - Good explanation. I'd think a broken impulse spring would cause
the timing to lag so much that the runnup would make it obvious
something is grossly wrong. Maybe not?

[Icebound]

> wrote in message
ups.com...
> Exactly. I learned to fly in the early '70s and they didn't
> teach us to look for a hot mag. I learned that when upgrading some
> years ago. Doing this, I found that one of our 172's mag switches would
> allow the mags to keep firing if the key was forced against the "Off"
> stop, and upon investigation found an AD against any of those switches
> that did that (76-07-12). Bet there are still a lot of them out there.


Well, I certainly had occasion to turn the key to "OFF" on a 172 and hand it
to my instructor in the right seat while the prop kept spinning. We
reported it (and it was returned "fixed") at least 3 times on that aircraft
before somebody finally managed to isolate and *really* fix the problem.

Otherwise the RPM drop for each mag was consistent, so both mags were
"grounding", but not in the OFF position.

[Wade]

OtisWinslow wrote:
<snip>
>
> That's a good thing to pay attention to. You can also check it at
> idle before shut down by briefly turning the switch to "OFF" and
> see if the engine starts to shut down.

I've been taught this shtudown procedure, too.
But, what is the magic about turning them BOTH off, simultaneously?

If, at idle, you turned off the left mag, and noted an rpm drop,
then turned off the right one, and also noted an rpm drop,
then doesn't that confirm the same thing as turning
both of and feeling for the engine to start to shut-down?

Although, maybe it's just me remembering my student fubar
of once turning off both mags during runup, then turning
them back on, and KA-BLAM!

[Morgans]

"Wade" > wrote

> I've been taught this shtudown procedure, too.
> But, what is the magic about turning them BOTH off, simultaneously?

As someone else said, it catches a faulty "off" position, in the switch.
--
Jim in NC

[Dave Butler]

Wade wrote:

> I've been taught this shtudown procedure, too.
> But, what is the magic about turning them BOTH off, simultaneously?
>
> If, at idle, you turned off the left mag, and noted an rpm drop,
> then turned off the right one, and also noted an rpm drop,
> then doesn't that confirm the same thing as turning
> both of and feeling for the engine to start to shut-down?

The procedure you suggest does not test the OFF position of the switch itself.
These cheesy switches have been known to fail that way.

[George Patterson]

Wade wrote:
>
> If, at idle, you turned off the left mag, and noted an rpm drop,
> then turned off the right one, and also noted an rpm drop,
> then doesn't that confirm the same thing as turning
> both of and feeling for the engine to start to shut-down?

No. Turning off the left mag verifies that that position of the switch will
ground out the left mag. Turning off the right verifies that the switch will
ground out the right mag in that position. You can only verify that "off" really
grounds out both mags by turning the switch off.

George Patterson
"Naked" means you ain't got no clothes on; "nekkid" means you ain't got
no clothes on - and are up to somethin'.