Ignition switch, what happen when Left or Right is selected

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

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

"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.

"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

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.

"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.

"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.

"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.

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!

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.