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Mag Turned Off?  
User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Posted (11 years 12 months 3 days 13 hours ago) and read 896 times:

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...


15 replies: All unread, jump to last
 
User currently offlineSinlock From United States of America, joined Dec 2000, 1637 posts, RR: 2
Reply 1, posted (11 years 12 months 3 days 12 hours ago) and read 880 times:

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.



My Country can beat up your Country....
User currently offlineShaun3000 From United States of America, joined Mar 2002, 445 posts, RR: 0
Reply 2, posted (11 years 12 months 3 days 12 hours ago) and read 879 times:

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


User currently offlineFlightSimFreak From United States of America, joined Oct 2000, 720 posts, RR: 0
Reply 3, posted (11 years 12 months 3 days 9 hours ago) and read 858 times:

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.

User currently offlineFlyf15 From , joined Dec 1969, posts, RR:
Reply 4, posted (11 years 12 months 3 days 7 hours ago) and read 853 times:

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.

User currently offlineSinlock From United States of America, joined Dec 2000, 1637 posts, RR: 2
Reply 5, posted (11 years 12 months 3 days 6 hours ago) and read 844 times:

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.



My Country can beat up your Country....
User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 6, posted (11 years 12 months 3 days 3 hours ago) and read 828 times:

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.


User currently offlineVc10 From United Kingdom, joined Nov 2001, 1406 posts, RR: 16
Reply 7, posted (11 years 12 months 2 days 19 hours ago) and read 815 times:

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


User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 8, posted (11 years 12 months 2 days 17 hours ago) and read 808 times:

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.


User currently offlinePmk From United States of America, joined May 1999, 664 posts, RR: 2
Reply 9, posted (11 years 12 months 2 days 17 hours ago) and read 807 times:

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

Peter


User currently offlineFlightSimFreak From United States of America, joined Oct 2000, 720 posts, RR: 0
Reply 10, posted (11 years 12 months 2 days 7 hours ago) and read 779 times:

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.

User currently offlineFlightSimFreak From United States of America, joined Oct 2000, 720 posts, RR: 0
Reply 11, posted (11 years 12 months 2 days 7 hours ago) and read 778 times:

You are correct... I was thinking in terms of one spark plug, not the mags. Sorry about that, honest mix up.

User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 12, posted (11 years 12 months 2 days 5 hours ago) and read 767 times:

no i know there are only 2 mags. but they both power each cylinder.

User currently offlineDg_pilot From United States of America, joined Sep 1999, 856 posts, RR: 2
Reply 13, posted (11 years 12 months 1 day 12 hours ago) and read 750 times:

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.

User currently offlineCosync From Mexico, joined Nov 2001, 556 posts, RR: 0
Reply 14, posted (11 years 12 months 1 day 6 hours ago) and read 729 times:

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.

User currently offlineMagicMan_841 From Canada, joined Jan 2002, 182 posts, RR: 0
Reply 15, posted (11 years 12 months 15 hours ago) and read 716 times:

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


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