DG_pilot
Posts: 810
Joined: Tue Sep 07, 1999 10:21 am

Mag Turned Off?

Sun Jul 28, 2002 5:38 am

Whenever I switch a mag off for one reason or another, what "actually"
happens? The engine loses some RPM, but what exactly happens as far as the
timing, firing, and total power produced..and why?

Thanks...
 
Sinlock
Posts: 1631
Joined: Fri Dec 22, 2000 12:55 am

RE: Mag Turned Off?

Sun Jul 28, 2002 6:37 am

I take it you are doing a Mag check when you do this?

I'll put this in simple terms.

When your at run up speed any were from 1700 to 2300rpm (It depends what type of engine)

You turn the key from BOTH to LEFT (Same thing for the RIGHT mag)

This will ground out the Right mag so it sends no power to the TOP spark plugs in the cylinder.

This should cause a drop of about 75rpm +/- 25rpm


The reason for the drop is because only one spark plug is burning the fuel/air mixture and it burns not as efficiently.

If the LEFT mag is bad the engine will stop.
If the RIGHT mag is bad there will be no drop.
Vice Versa

These numbers are not solid and are only used as pointer.
If you want to know in more detail ask your Flight instructor or an A&P at your airfield.
 
shaun3000
Posts: 439
Joined: Thu Mar 14, 2002 4:10 pm

RE: Mag Turned Off?

Sun Jul 28, 2002 6:37 am

I'll use a 172 as an example. 172s have four cylinders. Each cylinder has two spark plugs, one for each mag. So you have a total of 8 sparkplugs. So when you have both mags on, you have both spark plugs firing, giving you more even ignition thus an increased RPM. When you turn one mag off, then you only have one spark plug firing in each cylinder which decreases the RPM as the fuel isn't burning as evenly which reduces power.

As I understand it, that is how it works. If I messed something up, please feel free to correct me.  Smile
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Mag Turned Off?

Sun Jul 28, 2002 9:05 am

The engine will not stop if one mag is bad and you turn off the other. The engine still has at least one other cylinder, unless you are running a lawnmower engine, which only has one mag. The engine will become very rough and grumbly, but will not die.
 
flyf15
Posts: 6633
Joined: Tue May 18, 1999 11:10 am

RE: Mag Turned Off?

Sun Jul 28, 2002 10:44 am

Flightsimfreak, an engine (such as what is found on a 172) only has two mags. The magneto is what produces the electrical charge firing the spark in the spark plugs. If a mag has gone out, all spark plugs on that mag will no longer function.
 
Sinlock
Posts: 1631
Joined: Fri Dec 22, 2000 12:55 am

RE: Mag Turned Off?

Sun Jul 28, 2002 11:49 am

F15, Thanks for the help.

FlightSimFreak,
I would think that I know what I'm talking about.
Being the fact that I work on a fleet of aircraft including 5 C-152s, 3 C-172s, 2 PA-23s, a C-421,and a B-18.
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Mag Turned Off?

Sun Jul 28, 2002 2:54 pm

its ok flightsim freak.
u rae a lil wrong but u do know more than the average bear. hehe.
but he is a mechanic so he does no his stuff. there are 2 mags for each cylinder u see and when u switch mags its changing all of the mags for each cylinder.
 
vc10
Posts: 1339
Joined: Thu Nov 15, 2001 4:13 am

RE: Mag Turned Off?

Sun Jul 28, 2002 11:21 pm

I think you are all correct because of the following :-

1] If the selected Magneto itself is unserviceable then the engine will stop
2] However if the selected Mag is OK, but there is a fault with one of it's associated plugs or harness then the engine will run rough as one of it's piston's is not working.

That is my opinion, but feel free to shoot me down

little vc10
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Mag Turned Off?

Mon Jul 29, 2002 12:45 am

nah look.
each cylinder has 2 spark plugs. and each spark plug in the cylinders is connected to a different magneto. so in case one mag fails u have another to keep some engine power rather than none.
 
pmk
Posts: 609
Joined: Wed May 26, 1999 10:07 am

RE: Mag Turned Off?

Mon Jul 29, 2002 1:27 am

There are only 2 Magnitos, not eight on a four cylinder engine!!!!

Peter
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Mag Turned Off?

Mon Jul 29, 2002 10:41 am

People seem to think that I know nothing about aviation because of my username... I've been flying for a while now, and I have 55 hours in a Beech Skipper. Also, I've had a broken spark plug in my previous car before, it was not generating a spark, but the engine did not die. That's why I said what I said. I find it hard to believe that an engine would fail if one cylinder was not firing.
 
flightsimfreak
Posts: 698
Joined: Sun Oct 29, 2000 9:36 am

RE: Mag Turned Off?

Mon Jul 29, 2002 10:45 am

You are correct... I was thinking in terms of one spark plug, not the mags. Sorry about that, honest mix up.
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Mag Turned Off?

Mon Jul 29, 2002 1:27 pm

no i know there are only 2 mags. but they both power each cylinder.
 
DG_pilot
Posts: 810
Joined: Tue Sep 07, 1999 10:21 am

RE: Mag Turned Off?

Tue Jul 30, 2002 6:20 am

I already knew that each magneto provided spark to its respective set of spark plugs, but why exactly does it run rough and so on? Is it because the fuel is not being burned as efficiently or what? Any idea how much total power is lost when running on one mag? I've ran on one mag in flight before due to a rough engine, and there seemed to be just a little power loss.
 
cosync
Posts: 635
Joined: Sat Nov 17, 2001 7:53 pm

RE: Mag Turned Off?

Tue Jul 30, 2002 11:48 am

sometimes it doesnt burn all the fule evenly across the cylinder. like it will push one side of the piston down a tiny bit harder than the other.
 
MagicMan_841
Posts: 177
Joined: Sat Jan 19, 2002 8:12 am

RE: Mag Turned Off?

Wed Jul 31, 2002 3:35 am

The magnetos use engine rotation to power the spark plugs.

If you turn it on OFF the spark plugs will stop firing and the engine will quit.

If you put it on LEFT or RIGHT, one of the two spark plugs per cylinder will stop firing, but the other will still do sparks, thus explaining the small loss of RPM.

Why two spark plugs per cylinders ?? -- best firing and security.

If you turn in on OFF and it doesn't stop, there's a problem with the electric system (that's why you check the OFF position during the engine shutdown checklist and also during run-up)

M@g!¢  Yeah sure

Who is online

Users browsing this forum: No registered users and 22 guests